trends in tillage practices in relation to sustainable crop production with special reference to...

• Soil & !illa8 e .

- ~ K e s e a r c n ELS EVI ER Soil & Tillage Research 30 (1994) 245-282

Trends in tillage practices in relation to sustainable crop product ion with special reference to temperate

c l imates

R.Q. Cannell *'a, J.D. H a w e s b

aDepartment of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0404, USA

bUnited States Department of Agriculture, Soil Conservation Service, Richmond, Virginia 23229- 5014, USA

(Accepted l December 1993)

Abstract

The concept and some definitions of sustainable agriculture are reviewed. Most of these definitions include economic, environmental and sociological aspects. The finite area of land emphasizes the need for consideration of soil conservation and of soil quality in relation to sustainability. An important element of soil quality is rooting depth. Therefore loss of soil by erosion is a dominant factor in long-term sustainability. The effects of tillage on soil parameters in minimum data sets that have been proposed to describe soil quality are reviewed. Soil organic matter may be one of the most important soil quality characteristics in relation to tillage because of its influence on other soil physical, chemical and biological properties. Conservation tillage practices can increase the organic matter content, aggregate stability and cation exchange capacity (CEC) of the topsoil. However, bulk density and penetrometer resistance are also increased, especially with zero tillage. Al- though such soil quality parameters may form a basis for describing some of the consequences of particular tillage practices, they do not provide a basis for predicting the outcome in terms of crop growth and yield. This is both because critical values of soil quality parameters have not been defined and because in some soils biopore formation in zero or minimally tilled land can modify the soil for water movement and for root growth and function.

The effects of tillage on crop growth and yield in long-term experiments are reviewed. The review only includes experiments in North America, Europe and New Zealand that have lasted 10 years or more to allow for seasonal variation in weather, possible progressive changes in soil conditions and the learning phase often experienced when new tillage methods are used. While there is a good deal of variation in the results of these tillage experiments some patterns have emerged. In long-term experiments, yields of maize in Europe and the US and soybeans in the US have been similar after ploughing and no-

*Corresponding author.

0167-1987/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0167-1987 ( 94 ) 08026-V

246 R.Q. Cannell, .I.D. Hawes ~Soil& TiUage Research 30 (1994) 245-282

tillage, especially on well-drained soils. In Europe, yields of winter cereals have also been similar after traditional and simplified tillage but yields of spring cereals have sometimes been less after direct drilling than ploughing.

Trends in tillage practices are reviewed. Conservation tillage in the US is increasing and is used on about 30% of cropland, including no-till on about 10% of cropland. This increase in use of conservation tillage is mainly attributed to the legal requirement for farmers who are in government price support programs to adopt conservation plans which may involve conservation tillage. However, the allowable rates of erosion in these plans are likely to be in excess of rates of erosion for long-term sustainability. Survey information on tillage practices needs to be considered in relation to predictions on suitability of conservation tillage based on experimental results. In the semi-arid prairies of Canada there is a trend toward fewer cultivation operations, but in eastern Canada the mouldboard plough is still the dominant tillage method. In Europe although erosion is less obvious it is be- lieved to be increasing, but minimum tillage is not widely used. This is because of the need to remove at least some straw for successful minimum tillage in sequential winter wheat and barley crops, but there are few economic uses for straw, and burning is illegal in many countries. In the more moist cooler conditions of Europe grass weed proliferation is another constraint, at least with present technology. So far, the overall success of conservation tillage has not been limited by the growing problem of genetic resistance of weeds to herbicides. Societal attitudes to the continued use of herbicides may pose longer-term problems for some conservation tillage practices.

Keywords: Conservation tillage; Direct drilling; Erosion; No-till; Soil organic matter; Soil quality; Tillage practices

1. Introduction

In recent years there has been heightened interest in the sustainability of agriculture. This issue is not new, and the history of the sustainable agriculture concept has been well described by Harwood (1990). Present day interest in sustainable agriculture has followed European interest in organic, biological, and ecological agriculture, regenerative agriculture in the USA, and the United States Department of Agriculture (1980) Report and Recommendations on Organic Farming. Out of this interest the concept of sustainable agriculture has emerged, and there has been considerable debate concerned with defining sustainable agriculture.

Some definitions of sustainable agriculture are broad. Many incorporate three components: the need for economic viability, environmental well being and so- cial aspects. Such a definition was adopted in legislation by the United States (1990) where "Sustainable agriculture is defined as: ...an integrated system of plant and animal production practices having a site-specific application that will, over the long-term: satisfy human food and fibre needs; enhance environmental quality and the natural resource base upon which the agricultural economy de- pends; make the most efficient use of non-renewable resources and integrate, where appropriate, natural biological cycles and controls; sustain the economic viability of farm/ranch operations; and enhance the quality of life for farmers/

R.Q. Cannell, ,I.D. Hawes / Soil & Tillage Research 30 (1994) 245-282 247

ranchers and society as a whole." Note the references to the long-term time scale. Dumanski et al. ( 1991 ) in an international forum also took a broad view, distin- guishing between sustainable agriculture and sustainable land management. They regarded the latter as a package of technologies that contribute to sustainable agriculture. They defined sustainable land management as follows: Sustainable land management combines technologies, policies and activities aimed at inte- grating socio-economic principles with environmental concerns so as to simulta- neously: maintain or increase production; reduce the level of production risk; achieve environmental stability, and not degrade soil and water quality; be economically vaiable; be socially acceptable. (This definition is a slight variant of the original (Dumanski et al., 1991 ) and was provided by Dumanski at the In- ternational Workshop of Sustainable Land Management for the 21 st Century, Lethbridge, Alberta, Canada, June, 1993 ). Other definitions focus more on specific production factors, including tillage methods. The references to economic viability in the definitions imply a short-term perspective as well as a long-term view, since inadequate short-term viability can also constrain future profit. Til- lage practices can have both short-term and long-term consequences, particularly in relation to soil erosion and soil structure, and thus are a key component of soil management strategies for sustainable land management, crop production and agriculture as a whole.

2. Effect of tillage on sustainability

The importance of tillage in the context of sustainability was clearly recognized at the first conference of ISTRO in Sweden in 1955 by Larsen (Van Ouwerkerk, 1985 ). He noted that the American farmer increasingly considers the soil not as a means to earn as much money as possible, but as a heritage of earlier generations, with the mission to hand it down to coming generations in the same condition. It is well-recognized that there is no universal recipe for tillage. This was very evident when the established cultivation techniques of Western Europe which depended on the mouldboard plough were introduced to the eastern United States where rainfall caused serious erosion on sloping land, and its removal from crop production, and in the Great Plains the plough contributed to the dust bowl in the 1930s. Thus in regions prone to erosion the desirability of conservation tillage techniques that leave crop residues on the soil surface and minimize soil disturbance has been established for many years. By contrast in some Western Euro- pean countries the heavy yields of cereal crop residues now restrict the adoption of certain minimal tillage methods (Cannell, 1985 ).

In this paper the terms zero-tillage, no-till, no-tillage and direct drilling are used interchangably, although the latter is rarely used in the USA. Conservation tillage is used in accordance with the definition of Mannering and Fenster (1983) to cover tillage practices that reduce soil and water loss compared to conventional tillage for a particular region. The debate about how much tillage is needed is long-standing. Results from early experiments re-inforced the debate (Sturte- vant, 1885; Russell and Keen, 1938 ). Until about 30 years ago the lack of suitable

248 R.Q, Cannell, J.D. Hawes / Soil & Tillage Research 30 (1994) 245-282

herbicides was the main practical constraint to simplifying tillage. That changed following the introduction of atrazine in 1959 (Moody et al., 1961 ) and paraquat in 1961 (Hood et al., 1963) (see Baeumer and Bakermans, 1973). Since then, the availability of a wide range of herbicides and new machines has increased the options for soil tillage and particularly created the opportunities to minimize soil disturbance.

2.1. Erosion and sustainabili ty

The finite quantity and the variable quality of the earth's land resources will set the limits of population that can be supported. It is commonly considered that the best land is already under cultivation, and that much of the potential arable land is of marginal value for agriculture because of steep topography, shallow soil or proneness to drought or excess water (Treitz, 1991; Lal and Pierce, 1991 ). Soil degradation by water and wind erosion and depletion of organic matter, with related consequences, including loss of nutrients, in many circumstances are among the main factors affecting the quantity and quality of land and thus the long-term sustainability of agriculture. As Dregne (1982) noted, soil erosion is a relentless process that is nearly impossible to stop, usually difficult to control, and easily accelerated by man. When erosion rates are modest, the consequences may seem unimportant. However, if the time scale under consideration embraces the ability of the land to sustain agriculture several centuries hence a different perspective is needed. Consequently Laflen et al. (1990) considered that "a sustainable agriculture is one where topsoil removal is very slow, near what might be called a 'geologic rate', and at the rate at which topsoil is formed from subsoil and near the rate at which bedrock and other subsoil materials are transformed into a satisfactory material in which a crop root can thrive". Some estimates for these parameters have been made. The geologic rate of erosion or rate of denudation in the Appalachians has been estimated at 0.04 mm year-1 (Hack, 1979). This is comparable with the rate of soil or saprolite formation of 0.004 to 0.04 mm year-~ for that region (Pavick, 1986; Velbel, 1986), suggesting that in that cli- max forest landscape in the long-term an equilibrium exists between weathering and denudation by erosion (Velbel, 1985 ). The rate of soil formation from un- consolidated sediments is more rapid (Hall et al., 1982 ), and Schumm and Harvey ( 1982 ) gave a range of the rates of soil formation from 0.02 to 0.5 mm year-

However, in much cropland erosion rates are in excess of these rates of soil formation and several times faster than natural rates of erosion (Schumm and Harvey, 1982). Soil erosion from cropland in the United States by water and wind has been estimated at an average rate of 16 Mg-1 ha-1 year-1 (equivalent to 1.0 to 1.5 mm year-1, depending on bulk density) with sheet and rill erosion by water accounting for about 55% of the loss (Soil Conservation Service, 1989 ). Not surprisingly, therefore, Laflen et al. (1990) expressed the view that current standards of acceptable soil erosion rates in the US, 2.2 to 11.2 Mg ha- 1 year- 1, are too high to support a sustainable agriculture if future lands needs are considered. These rates or soil loss tolerances, the so-called Tvalues for use in conjunc-

R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (1994) 245-282 249

tion with the Universal Soil Loss Equation, were derived at a series of workshops in 1961 and 1962 (Wischmeier and Smith, 1978). According to Logan (1982) the only consistent criterion used in establishing T values was favourable rooting depth, with the highest values for soils deeper than 1000 mm to the bedrock or sand or gravel, and the least where it is less than 250 mm to bedrock or 100 mm to a claypan. Thus the best soils would be lost at a faster rate than poor soils. In 1987 it was estimated that 22 and 18% of cropland in the US were being eroded by water and wind, respectively, at values equal to or greater than T (Soil Con- servation Service, 1989).

The effects of different climate, soil and cultural practice variables on the long- term average rates of soil losses by sheet and rill erosion can be predicted by the Universal Soil Loss Equation (USLE) model (Wischmeier and Smith, 1978 ). Some recent modifications have resulted in the Revised Universal Soil Loss Equation (RUSLE) (K. G. Renard, personal communication, 1993 ). The USDA Soil Conservation Service plans to introduce the RUSLE in 1994. The soil loss equation is A =RKLSCP, where A is the computed soil loss per unit area. R estimates the likelihood of erosive rainfall events in a particular climate. This value varies with the geographic location of the field where soil loss estimates are made. As amount and intensity of rainfall increases, R increases. K describes the inherent erodibility of the soil. L is the slope-length factor, defined as the horizontal distance from the origin of overland flow to the point where the slope gradient decreases enough that deposition begins or runoff becomes concentrated in a defined channel (Wischmeier and Smith, 1978). S describes the steepness of the slope. Steepness is expressed in vertical distance divided by horizontal distance times 100. P, the support practice factor, describes the effect of such practices as contour tillage and stripcropping. The support practice factor in the RUSLE model is the ratio of soil loss with a specific support practice to the corresponding soil loss with upslope and downslope tillage (K.G. Renard, personal communication, 1993). C reflects the effect of cropping and management practices on erosion rates. Tillage increases the C factor value and the amount of soil erosion by burying crop residues that are left from a previous crop. The T value denotes the maximum rate of soil erosion that can occur and still permit crop productivity to be sustained economically (Wischmeier and Smith, 1978 ). Conservation tillage practices, where crop residues are left on the soil surface or concentrated in the upper layer can greatly diminish soil loss by erosion. However, from a detailed analysis of North American data it is evident that for a given percentage of ground cover the reduction in soil erosion by rainfall from zero-tilled land compared with ploughed land is very variable (G.R. Foster, personal communication, 1993 ). Some examples illustrate the magnitude of the effects. In experiments in Missis- sippi comparing no-till planting, where crop residues remained on the soil surface, with ploughing, soil losses from water erosion were reduced by 85% (from 17.5 to 2.5 Mg ha- ~ year- ~ ) (McGregor et al., 1975 ), but on gently sloping sites, where erosion in not a severe problem, erosion rates after no-till treatments are little different than with ploughing (Lal et al., 1989b). An example of using the RUSLE to predict the average soil losses for a typical soil by adopting different

250 R.Q. Cannell, .I.D. Hawes /Soil & Tillage Research 30 (1994) 245-282

tillage practices is shown in Fig. 1. As the slope of the land increases, the need for more residue cover to reduce erosion to an acceptable rate becomes more evident. In this example with a 4% slope, all tillage methods would keep erosion within an acceptable range, as indicated by T, but for the steeper slopes no-tillage would be needed.

Soil erosion affects crop yield mainly by reducing nutrient supply, water infiltration and soil water holding capacity, but the effects in the US vary widely (Langdale and Shrader, 1982 ). For example, on Ultisols in the Piedmont, yield losses on eroded soils have ranged up to about 60% (Langdale et al., 1979). By contrast in an 18 year study in Iowa on deep loess Mollisols with favorable physical and chemical properties no significant differences in maize grain yield were found between sites with different amounts of topsoil erosion (Alberts and Spomer, 1987 ). In Mississippi, on loess soils where fragipans are found at varying depths, reflecting past erosion, soyabean yields declined where the soil thick- ness above the fragipan was less than 600 mm (Rhoton, 1990). In Wisconsin where the depth above a red clayey residuum (450-950 mm) also reflected past erosion, erosion level had no effect on early season soyabean growth, but did affect maximum plant height, although not grain yield (Andraski and Lowery, 1992 ). In Ontario, Battison et al. (1987) found that maize yield on depositional sites exceeded that on non-eroded sites, which in turn were on average about 60% heavier yielding than eroded sites, on a range of soil textures and slopes. On the Canadian prairies wind erosion losses can be severe, and the long-term sustainability of agriculture is dependent on arresting wind and water erosion and the loss of soil organic matter from excessive tillage (Lafond et al., 1992). The importance of such erosion is evident from Verity and Anderson (1990) who found that the addition of 50 mm of topsoil on an eroded knoll resulted in about a 50%

150

g

~Q

100

50

~.,fJ///////.~ . 4%SLOPE 8%SLOPE

[ ] < 30% RESIDUE COVER

[ ] > 60% RESIDUE COVER

12%SLOPE

[] 30TO 60°/0 RESIDUE COVER

Fig. 1. A comparison of the predicted annual soil loss using the Revised Universal Soil Loss Equation with tillage systems involving different percentages of ground cover on 4, 8 and 12% slopes, for a Piedmont site in Virginia, where R = 175, K= 0.32, L = 46 m and P= 1. Residue cover of 30% is needed to meet the conservation tillage definition of CTIC ( 1992 ).

R.Q. Cannell, J.D. Hawes /Soil & Tillage Research 30 (1994) 245-282 251

yield increase of spring wheat. And in Alberta in a simulated erosion experiment where different thicknesses of topsoil were removed ( 50 to 200 mm) yield losses of spring wheat averaged 25% when 50 mm was removed and 80% when 200 mm removed; the losses in yield were not offset by irrigation (Larney et al., 1994). In Western Europe, where rainfall intensity is usually less than in North America, erosion may be less obvious. Nevertheless Mediterranean lands and the lowlands of northern and central Europe are among the areas at risk from erosion (Morgan and Rickson, 1990). Erosion in Northern and Central Europe affects mainly the sandy, loessial and chalky soils devotcd to continuous arable production and may be associated with the increased area of winter cereal crops, more mechanization, use of tramlines and consolidation of land into larger fields (Morgan and Rick- son, 1990 ). In Denmark, wind erosion can be a serious problem on sandy soils in the spring at sowing time; such soils comprise about 2 M ha or 60% of the total agricultural area (Hansen, 1989 ). In the fertile loess areas north of the Alps, char- acterized by loamy soils with low structural stability, several million hectares are threatened with soil loss and degradation (De Ploey, 1989). Cultivated land in this region is particularly at risk from water during winter and spring (Auzet et al., 1990), as are chalky soils on undulating land in England (Boardman, 1991 ). And in the hilly Apenincs region of Italy there is increasing risk of erosion on clayey soils (Chisci, 1989 ). Few studies on the effect of erosion on productivity of land have been made in Europe. In Germany, Becher et al. ( 1985 ) reported a 4% reduction in maize yield on a loess soil after l0 years of an average annual soil loss of 30 Mg ha- 1, and erosion-damaged slopes on an alfisol showed about 23% loss of yield of forage maize (Goeck and Geisler, 1989). Burnham and Mut- ter ( 1993 ) found no relation between cereal yield and soil depths above chalk, if adequate nutrients were applied perhaps because roots can extract water from the chalk below the topsoil (Gregory, 1989 ).

Various erosion control procedures are being considered or advocated in Eu- rope. De Plocy (1989) stated that efficient erosion control will depend on a fun- damental change of tillage techniques and/or soil protection, and found that direct drilling of winter wheat or winter barley in stubble eliminated rill erosion in Central Belgium. In France, no-till or minimum tillage procedures are also being considered for erosion control (Auzet et al., 1990; Boiffin and Monnier, 1991 ). In Denmark, reduced tillage, incorporation of straw and catch (cover) crops are appropriate strategies to reduce erosion (Hansen, 1989 ). In Italy, on clayey soils perennial crops can grcatly diminish run-off and erosion, and while minimum tillage reduced erosion in a Vertic Xerochrept, it did not on a Typic Udorthent, and crop yields were less (Chisci, 1989). In Switzerland, Maillard et al. (1990) found that mulch tillage for maize or sugarbeet without ploughing reduced soil erosion compared with ploughing from between 0.7 to 2.2 Mg ha-1 year-~ to about zero on slopes of 5 to 8%.

The off-site consequences from soil deposited downstream or downwind add to the importance of the direct effects of soil erosion on sustaining crop production. These off-site consequences can be costly for society (Morgan and Rickson, 1990). Sedimentation reduces the capacity of rivers and ditches, increasing the


risk of flooding, and shortens the life of reservoirs. There is expense in removing sediment from roads and drains. Sediment itself can be a pollutant when it carries adsorbed chemicals into reservoirs for example. Furthermore wind borne materials may constitute an additional health hazard. The combination of these effects may be sufficiently socially unacceptable for the associated land management practices to meet the definitions of sustainable agriculture.

3. Effects of tillage on soil quality

The case for sustainability to be considered in terms of production per unit of non-renewable resource, or per unit degradation of soil characteristics has been presented by Lal ( 1991 ). For soil, these characteristics represent quantity and quality components, respectively. This implies the need for some estimate of 'soil quality'. The Soil Science Society of America (1987 ) defined soil quality as an inherent attribute of a soil that is inferred from soil characteristics or indirect observations (e.g. compactibility, erodibility and fertility). Larson and Pierce ( 1991 ) defined soil quality as the capacity of a soil to function within the ecosys- tem boundaries and interact positively with the environment external to that eco- system. They pointed out that indicators of soil quality need to reflect the ability of soil to provide a medium for plant growth, to regulate and partition water flow through the environment, and to serve as an effective environmental filter. Parr et al. ( 1992 ) broadened the definition of soil quality to "the capability of a soil to produce safe and nutritious crops in a sustained manner over the long-term, and to enhance human and animal health, without impairing the natural resource base or harming the environment." This definition which has production, environmental and health components, is very close to some definitions of sustainable agriculture as a whole, which imply time scales that are long relative to human lifespan. Definitions of soil quality are further considered by Doran and Parkin (1994). Although there are no generally accepted criteria to evaluate changes in soil quality, several attempts have been made to identify the key biological, chemical and physical attributes (Larson and Pierce, 1991; Arshad and Coen, 1992; Visser and Parkinson, 1992). The possibility of identifying a minimum data set or a list of key attributes for monitoring soil quality has been proposed by Larson and Pierce (1991), Arshad and Coen (1992), and R.J. Papendick (personal communication, 1993 ). Several parameters, such as climate, slope, soil classification and texture can be regarded as site characteristics, changing very slowly, if at all. Other parameters can change with time, some quickly, others much more slowly (Arnold et al., 1990). The proposed minimum data sets include attributes to indicate both soil quality and changes in quality, for example when affected by soil management. The attributes have also been selected for ease of measurement. Several parameters were common to the minimum data sets proposed above (Table 1 ), including nutrient availability, pH, electrical conductivity, total organic carbon, plant available water content to the depth of rooting, bulk density and hydraulic conductivity. However, limiting values for an accept-

R.Q Cannell, J.D. Hawes ~Soil& Tillage Research 30 (1994) 245-282 253

Table 1 Soil attributes and their time changeability proposed for minimum data sets that estimate soil quality

Soil attributes Time changeability (y)

(a) Attributes common to Larson and Pierce ( 1991 ), Arshad and Coen (1992), R.J. Papendick (personal communication, 1993) and Doran and Parkin (1994) Nutrient availability a pH Electrical conductivity Soil organic carbon Plant available water content to depth of rooting Bulk density Hydraulic conductivity

0.1-10 0.1-1 1-10 1-10 3-5

<0.1-3 3-5

(b) Attributes common to two of the proposers in (a) above: Aggregate stability

Water - wet aggregates Wind - dry aggregates

(c) Attributes proposed by one source: Labile organic carbon Cation exchange capacity Exchangeable sodium percent Mineralizable nitrogen b Respiration Toxic compounds (including pesticides) Soil depth Heavy and trace metals Earthworms

aCEC could be inferred from exchangeable Ca, Mg, Na and K, soil organic matter and clay contents. bWould provide an indirect measure of microbial biomass (J.W. Doran, personal communication, 1993).

able range for par t icular pa r ame te r s have not been set. In some instances, such as bulk density, where there is not a good corre la t ion with root growth or water m o v e m e n t , the pa r ame te r s m a y only serve as warning indicators. However , in- fo rma t ion on bulk densi ty enables vo lumet r ic relat ionships to be de termined. Tillage can affect m a n y o f the a t t r ibutes in the p roposed m i n i m u m data sets, and these are reviewed in the following sections. However , considera t ion o f the min- i m u m data sets in relat ion to tillage is cons t ra ined by the fact that mos t experi- men t s did not measure all the parameters .

3.1. Soil organic mat ter

Soil organic m a t t e r can influence m a n y soil qual i ty proper t ies including aggregate stability. For mos t minera l soils, s tructural stabil i ty decreases when soil m a n a g e m e n t me thods reduce organic ma t t e r content (Green land , 1981 ). Such changes can lead to a de ter iora t ion o f soil workabil i ty. Thus, Larson and Pierce ( 1991 ) cons idered tha t soil organic carbon or soil organic ma t t e r is perhaps the

2 5 4 R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (1994) 245-282

single most important indicator of soil quality and productivity. Depletion of soil organic carbon is associated with an increase in soil compactability and deterio- ration in soil structure (Soane, 1990; Fig. 2) and the loss of nutrients. Soil organic carbon also affects water holding capacity and can influence the effective- ness of agro-chemicals. When old grassland is ploughed and cultivated, soil organic matter declines to a new equilibrium appropriate to the climatic conditions and the soil management regime, but it can take several decades to reach the new equilibrium (Mann, 1986 ). Prairie soils in North America have only been under cultivation for up to 250 years, and many for only about 100 years, and so are excellent models for studying the effect of change in land use or management on soil organic matter. In the North American prairies many measurements show that after 40 to 80 years of tillage and cropping the organic matter content of the top 150 mm of the soil declined by 30-60% before reaching a new equilibrium (Haas et al., 1957; Bauer and Black, 1981; Campbell and Souster, 1982 ). Bales- dent et al. ( 1988 ) found that when virgin prairie grassland in Missouri was cultivated, organic carbon reached a new equilibrium after about 30-40 years when an easily mineralized component was exhausted. However, a pool of stable organic matter of prairie origin persisted during 100 years of cultivation, and

1.7

Tilla c.le system Gievsol Combisol DD 4 years , o DO I g years e 0

\ SC I 9 years * • P I 9 years • c~

~I.6 J ~ ~ o P I' )

~ e -

~ 1.5

E x a

"~ 1.4

y = - 0 . 1 0 2 x + 2.07

r a = 0 .93 ~ *

1.3 I ] I [ [

4.0 S.O 6.0 7 .0

Organic matter ( X , w / w )

Fig . 2. Effect of differences in organic matter content (0-50 m m depth) resulting from various tillage systems in a long-term experiment on the compactability of disturbed samples of a gleysol and a

cambisol based on data by Ball et al. ( 1 9 8 9 ) , by Soane ( 1 9 9 0 ) . ( D D , direct drilling; SC, shallow cultivation; P, ploughing. )

R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (I 994) 245-282 255

amounted to about half the soil carbon at that stage (Fig. 3). Conversely when land is restored to perennial vegetation, carbon and nitrogen accumulate in the upper layer of the soil, probably because more plant material enters the soil each year, but may not reach an equilibrium for many decades (Jenkinson, 1988). Short-term grass-leys have a relatively small effect on organic carbon (Russell, 1973 ). Although many experiments have compared the effects of different conservation tillage systems, including no-tillage and varying amounts of 'minimum' tillage with conventional tillage (often mouldboard ploughing), relatively few have been continued for a sufficient period to observe the longer term effects on soil organic matter content. However, there are several notable experiments that have lasted for 25 years or more. In the southern Great Plains, Unger (1982) found that for a dryland cropping system for winter wheat that lasted for 36 years organic matter initially declined, but less rapidly with sweep tillage than with discing, and seemed to stabilize in the later years. He concluded that none of the tillage methods resulted in a condition that would adversely affect wheat production. In the northern Great Plains soil carbon (and nitrogen) losses in the upper 150 mm also were less with stubble mulch (sweep) tillage (27%) after about 25 years than with conventional tillage (usually mouldboard plough) (38%) compared with virgin grassland (Bauer and Black, 1981 ). In the central Great Plains, after 16 years at a site previously in native pasture the soil organic carbon content of the top 100 mm in the no-till soil was about 80% of the grass, compared with 60% for ploughing, with stubble mulch intermediate (FoUett and Peterson, 1988 ).

The question thus arises as to whether conservation tillage where crop residues are returned to the soil can increase organic matter in soils with a long history of cultivation by traditional methods. In the wet winter-dry summer climate of the

o

o l

v

~ 4

40

16

12

8

4

0 I 1888 1900

W H E A T

\

I I I I

1920 1940 1960 1980

Fig. 3. Changes in amount and origin of soil organic carbon accompanying the long-term cultivation of wheat in a fine, montmorillonitic mesic Udollic Orhraqualf, formerly virgin prairie soil. Open circles denote total carbon, and solid circles represent carbon of prairie origin with upper and lower points at different dates for the 0-100 and 100-200 mm depth samples, respectively. The straight, solid line shows the level of stable carbon (from Balesdent et al., 1988 ).

256 R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (1994) 245-282

Pacific Northwest intermountain region of the US after 44 years, organic carbon in the top 75 mm of soil was 32% higher and much more stratified in stubble mulch systems (disc, subsurface sweep) than with mouldboard ploughing after wheat and fallow sequences in alternate years (Rasmussen and Rohde, 1988 ). In Ohio, different tillage methods were used for up to 28 years on several soils types: a poorly drained silty clay loam after a mixed crop/grass rotation for 6 years, and on two better drained silt loams, one previously in grass for 6 years and the other in a corn-soyabean rotation for 6 years (Mahboubi et al., 1993 ). After 18 years on the poorly drained soil, organic carbon in the topsoil of the un-tilled land was unchanged, but after ploughing it declined by about 13% (Table 2). After 19 years, organic carbon in one of the silt loams soils had declined by about 11% without tillage, but by about 24% with ploughing. After 28 years, there was a slight further loss of organic matter after ploughing, a slight increase after chisel ploughing, and a 64% increase after zero-tillage. On the other silt loam (Crosby), there was little change in organic matter in the soil after 28 years of ploughing, but nearly three times the original value after zero tillage. In Kentucky, on a silt loam the content of organic carbon in the 0-50 mm layer in zero-tilled land was 70% more than in ploughed land after 10 and 20 years of treatment (Blevins et al., 1983; Ismail et al., 1994). In the south-eastern US in Alabama, at a site that had been conventionally tilled for more than 50 years, after I 0 years of no-tillage, organic matter increased in the surface layer but was unchanged after continued ploughing (Edwards et al., 1992 ).

In European and New Zealand experiments, where residues were usually removed, except for stubble, or burned, more organic matter has also been found in the topsoil of direct-drilled treatments than in ploughed land (Bakermans and De Wit, 1970; Ellis and Howse, 1980; Ball et al., 1989), but was less than in grassland retained from the start of the experiment (Fleige and Baeumer, 1974; Douglas et al., 1986 and Table 3; Home et al., 1992; Francis and Knight, 1993). In Australia, both burning stubble and tillage led to reduced organic carbon, with

Table 2 Organic carbon (%) in three Ohio soils after long-term tillage t r ea tments

Year Dep th Wooster silt loam Crosby silt loam Hoytvil le silty clay ( m m ) loam

Year 1 ( 0 - 2 2 5 ) 1.4 a 1.0 b 2.3 a

N T CP M P N T CP MP NT MP

Year 18/19 b ( 0 - 2 2 5 ) 1.3 1.I 1.1 - - - 2.3 2.0 ( 0 - 1 5 0 ) 1.5 1.2 1.2 - - - 2.7 2.0

Year 28 c ( 0 - 1 5 0 ) 2.3 1.5 1.0 2.7 1.2 0.9 - -

NT, no-till; CP, chisel plough; MP, mou l dboa rd plough. aFrom Dick ( 1983 ). bFrom Dick et al. ( 1991 ). CFrom M a h b o u b i et al. (1993) .

R.Q. Cannell, J.D. Hawes ~Soil& Tillage Research 30 (1994) 245-282 257

Table 3 Organic carbon content (%, w/w) of upper 25 mm of three soils growing grass or after different methods of tillage

Soil Treatment

Grassland Direct-drilled Shallow-tilled Ploughed

Clay (previously ploughed )" 4.5 2.9 Clay (previously grassland) a 5.8 4.9 Silt loam (previously ploughed) b 2.0 1.3

2.7 2.5 - 4.0 1.1 0.8

"From Douglas and Goss (1982). bFrom Douglas et al. ( 1986 ).

a 31% difference in the top 100 mm between the extreme management treatments of direct drilling with stubble retained and scarifying to 100 mm after burning stubble for l0 years (Chan et al., 1992). On ploughed land, the organic carbon content is usually similar down to the depth of ploughing or cultivation due to mixing, and sometimes below about 100 mm the content may exceed that in uncultivated or shallowly tilled land (Boone et al., 1976; Dick et al., 1991; Rasmussen, 199 lb; Chan et al., 1992 ). The current concerns about effects of atmospheric greenhouse gases on possible global warming broaden the possible importance of the effects of different tillage methods on soil organic carbon, since it is the largest terrestrial carbon pool (Post et al., 1990). Kern and Johnson ( 1993 ) have predicted that if conservation tillage is adopted in the US on 76% of cropland by the year 2010 (compared with 27% in 1990) there would be a benefit equivalent to approximately 0.7 to 1.1% of the US fossil fuel emissions, due to a combination of sequestering carbon in the soil and the reduction in fossil fuel required for conservation tillage methods.

3.2. Nutrient availability, cation exchange capacity (CEC) and pH

It has been known for more than 20 years that after no tillage, the concentration of extractable phosphorus and potassium increases in the surface layers of the soil, and pronounced gradients of these nutrients develop (Bakermans and De Wit, 1970). Many reports confirm this (Blevins et al., 1977; Hodgson et al., 1977; Follett and Peterson, 1988; Lal et al., 1990; Home et al., 1992). This is to be expected with no-tillage where phosphorus and potassium fertilizer are placed at or just under the soil surface as these nutrients move only slowly in the soil. Also, where residues are not incorporated into the soil the nutrients in the shoots are also deposited on the soil surface as plant residues decompose. Similar gradients of extractable phosphorus and potassium have been found after shallow tillage or stubble mulching in comparison with mouldboard ploughing (Ellis and Howse, 1980; Rasmussen 1981 ). On commercial farms it may not be practicable to use particular tillage methods continuously, and these nutrient gradients can be quickly changed (Lal et al., 1990).

258 R.Q. Cannell, J.D. Hawes /Soil & Tillage Research 30 (1994) 245-282

After continuing different tillage treatments for periods up to 28 years large effects on CEC have been found associated with the changes in organic matter content (Chan et al., 1992; Home et al., 1992; Mahboubi et al., 1993 ). However, on a poorly drained clay Lal et al. (1990) found a significant decrease in CEC of the 0-100 mm layer after 12 years of no-tillage compared with ploughing, but there was little effect on organic matter in this soil. It is well established that the pH of the surface layers of uncultivated soil often becomes acid more rapidly than in ploughed land (Triplett and Van Doren, 1969; Bakermans and De Wit, 1970; Ellis and Howse, 1980; Dick, 1983; Mahler and Harder, 1984; Lal et al., 1990; Home et al., 1992 ). The decline in pH in the surface layer of the untilled soil has been especially evident at high rates of nitrogen fertilizer (Blevins et al., 1983 ).

3.3. Bulk density

This is probably the most frequently measured soil quality parameter in tillage experiments. In European work, where potential problems with excess soil moisture, decreased aeration and compaction of the soil can be important, especially for crops sown in the spring, bulk density and porosity measurements have re- ceived particular emphasis. In numerous experiments bulk density was greater and porosity was less in the topsoil with zero-tillage, or shallow tillage than ploughing (for example Ellis et al., 1977; Pidgeon and Soane, 1977; Westmaas Research Group on New Tillage Systems, 1984; Ehlers et al., 1983; Kladivko et al. 1986; Mielke et al., 1986; Culley et al., 1987; Carter et al., 1990; Bruce et al., 1990; Chang and Lindwall, 1992; Edwards et al., 1992; Pierce et al., 1992; Mah- boubi et al., 1993 ). Where time trends of bulk density have been followed differences developed within the first few years, not changing thereafter (Pidgeon and Soane, 1977 ). In a few instances there have been no differences in bulk density in the topsoil between tillage treatments (Blevins et al., 1983; Chang and Lind- wall, 1989; Campbell et al., 1989; Braim et al., 1992b; Francis and Knight, 1993 ), but bulk density can be greater than in virgin grassland (Bauer and Black, 1981; Follet and Peterson, 1988). By contrast, in Australia on a duplex soil prone to consolidation under cultivation, Carter and Steed (1992) found a lower bulk density under direct drilling than conventional cultivation.

3.4. Hydraulic conductivity

Different methods of tillage have had varying effects on soil hydraulic properties. Some of the differences may be attributable to management factors, such as removing crop residues from the soil surface, but the formation of biopores can be a major factor. The importance of earthworm channels was clearly shown by Ehlers (1975 ) who found that in a loess soil in untilled land many earthworm channels reached the soil surface and could transmit free water to a depth of 1800 ram, whereas in the tilled land, the channels in the Ap Horizon were ineffective in water transmission. And Douglas et al. (1980) found that the saturated hydraulic conductivity at the interface between topsoil and subsoil in a clay was

R.Q Cannell, J.D. Hawes ~Soil& Tillage Research 30 (1994) 245-282 259

Table 4 Experiments comparing saturated hydraulic conductivity or equilibrium infiltration rate after zero- tillage, minimum tillage or ploughing

Authors Soil texture Location Duration ofexperiemnt (y)

(a) Where rates have been similar after different tillage methods: Blevins et al. (1983) silt loam Kentucky, USA 10 Mielke et al. ( 1986 ) silt loam Kentucky, USA 11 Mielke et al. (1986) silt loam Illinois, USA 6 Mielke et al. ( 1986 ) loam Nebraska, USA 13 Heard et al. ( 1988 ) silty clay loam Indiana, USA 10 Chang and Lindwall (1989) clay loam Alberta, Canada 10 Lal et al. (1989b) clay Ohio, USA 12 Lal and Van Doren ( 1990 ) silt loam Ohio, USA 25 Lal and Van Doren (1990) silt loam Ohio, USA 25 Sauer et al. (1990) sand Wisconsin, USA 5 Chang and Lindwall (1992) loam Alberta, Canada 8 Home et al. (1992) silt New Zealand 10 Pikul et al. ( 1993 ) silt loam Oregon, USA 27

(b) Where rates have been greater after ploughing: Lindstrom et al. ( 1984 ) clay loam Minnesota, USA 10 Lindstrom et al. (1984) loam Minnesota, USA 3 Mielke et al. (1986) clay loam Minnesota, USA 11 Mielke et al. (1986) clay loam Minnesota, USA 6 Mielke et al. (1986) silty clay loam Nebraska, USA 6 Mielke et al. (1986) silt loam Nebraska, USA 6 Heard et al. ( 1988 ) silt loam Indiana, USA 5 Douglas and Goss ( 1987 ) a clay England 7

(c) Where rates have been greater after zero-tillage than ploughing: Culley et al. ( 1987 ) silt loam Ohio, USA 4 Mahboubi et al. ( 1993 ) silt loam Ohio, USA 28 Mahboubi et al. (1993) silt loam Ohio, USA 28

aAbove topsoil/subsoil boundary.

greater in direct-drilled than ploughed land, and also associated with earthworm channels. In many instances, however, hydraulic conductivity did not differ between tillage treatments (Table 4).

3.5. Plant available water content

Chang and Lindwall ( 1989 ) after 20 years found no difference in plant available water holding capacity of the upper 120 mm between zero tillage, minimum tillage (herbicides and one sweep blade operation to 90 mm depth) and conventional tillage (heavy cultivation to 90 ram). On a loam soil, Chang and Lindwall (1992) also found no difference in water retention between those tillage treatments after 8 years. In the Ohio experiments on two silt loams (see above) the available water holding capacity in the upper 150 mm of the root zone was greater


in the 28th year after no-tillage than chisel or mouldboard ploughing (which were similar), but not between the crop rows where traffic occurred (Mahboubi et al., 1993 ). On a poorly drained clay after 12 years available soil water was similar in untilled and ploughed land (Lal et al., 1989a). In a 12 year experiment in the Central Great Plains of the US 9% more water was stored in the fallow period after zero tillage then where conventional blade tillage was used, and the difference extended to 3 m depth (Smika, 1990). In a 12 year tillage/crop rotation experiment on a loam (orthic dark brown chernozern) in Saskatchewan, Brandt (1992) found more available soil moisture to 900 mm depth in spring (total spring soil moisture in the profile minus value at harvest) for wheat after zero tillage than after conventional tillage with a cultivator, in 9 out of 36 comparisons, and no decreases during the course of the experiment. Also in Saskatche- wan, Lafond et al. (1992) found that under stubble cropping (but not after fallow) zero and minimum tillage (one pre-seeding operation) increased soil water to 120 mm depth by 6% over conventional tillage (2-4 operations at 5-10 mm).

3.6. Aggregate stability

Water-stable aggregates in the upper few mm of soil may improve germination and seedling establishment by reducing surface crusting and erosion, and by al- lowing water and air into the soil. Many reports show that the proportion of more stable aggregates in the upper 25-30 mm is greater in zero-tilled land than after ploughing after 2-10 years, even on weakly structured soils (Boone et al., 1976; Carter, 1992), but less than in grassland (Tomlinson, 1974; Douglas and Goss, 1982; Home et al., 1992). The positive influences of conservation tillage practices on aggregation can be more evident where stubble is retained rather than burned (Chan et al., 1992). Chisel ploughing can be intermediate between no tillage and ploughing (Mahboubi et al., 1993 ).

Schjonning and Rasmussen (1989) measured the topsoil wet aggregate stability of a fine loam in Denmark for each of the last 13 years of an 18 year experiment. Aggregate stability decreased when the soil was ploughed annually and all residues were removed from the field, and shallow tine cultivation to c. 100 mm depth and especially rotovating to only 50 mm depth damaged the soil structure less than ploughing. The increase in aggregation in the surface layers after repeated conservation tillage can be quickly lost by periodic ploughing (Mannering et al., 1975 ). The effects of tillage on aggregate stability may be enhanced by the type of crop being grown, for example soyabeans (Kladivko et al., 1986). Excep- tions to the positive effect of repeated no-tillage on aggregate stability compared with ploughing have been reported by Ball et al. ( 1989 ) and by Lal et al. (1989a) for poorly drained soils.

Consideration of the effect of conservation tillage on these parameters of soil quality leads to some favourable and some seemingly less favourable conditions. Organic matter content in the surface layer increases, especially after zero-tillage. Associated with this increase frequently there are measureable increases in aggregate stability of the upper few mm, and increase in mean size of aggregates both


after shallow tillage and zero tillage. Also there are several reports of increased CEC in the surface layer. Bulk density, however, has usually increased in the topsoil, often with increased penetration resistance, after zero and shallow tillage, though there are a few exceptions. The increase in bulk density is often associated with better trafficability. Mostly the effects of tillage on hydraulic conductivity have been slight, but in some instances the rates have been reduced under no- tillage. However, the development of continuous vertical biopores has been of over-riding significance in several reports. Available water content, determined from laboratory measurements of water retention characteristics, has generally been unaffected by tillage, but in field measurements, available water has been greater after zero tillage, especially in semi-arid areas where mulch has favoured retention of precipitation. Soil pH has declined more rapidly in the surface layers of untilled than tilled land, and phosphorus and potassium have accumulated in the surface zone.

4. Influence of soil quality with different tillage methods on crop growth and yield

While soil conditions are clearly important, and individual soil quality parameters can be changed by tillage practices, plants are the best integrators of the effects of soil conditions, mediated through the growth and function of the roots. Crop growth and yield are the result of the combined influence of the genetic make-up of the plants and the environmental conditions, both climatic and edaphic. In the following section some influences of tillage practices on the performance of roots and crop yield are considered. Root growth has been measured in only a few experiments and rarely over a sequence of years and reports mainly relate to situations where crop growth was limited by a particular tillage treatment. By contrast there have been many experiments comparing the effects of different tillage methods on crop yield, but many of these have been short-term lasting only a few years. Because of the variations in weather between years and the possible cumulative effect of changes in soil conditions, consideration of crop yield is confined to experiments that have lasted 10 years or more. Also, there are numerous examples of tillage experiments where, in the first year or years, crop growth and yield after direct drilling was less than with ploughing (Elliott et al., 1977; Ball et al., 1989; Caneill and Bodet, 1991; Maillard and Vez., 1994). This may be due to lack of familiarity with the new procedures and/or the different soil conditions in the early stages of the experiment. Indeed Maillard and Vez (1994) described the early phase as 'Tapprentissage" of tillage without ploughing.

4.1. Root growth and function

More abundant roots have been observed in the surface layers of untilled or shallowly cultivated soils (Bakermans and De Wit, 1970; Barber, 1971; Drew and Saker, 1978; Bauder et al., 1985; Rasmussen, 1991 ). This effect may reflect the higher bulk density, and often greater soil strength of the surface layers (Pid-

262 R.Q Cannell, J.D. Hawes ~Soil & Tillage Research 30 (1994} 245-282

geon and Soane, 1977 ), since greater mechanical impedance can restrict elongation of root main axes and stimulate branching of lateral roots (Goss, 1977 ), but may also be partly due to stimulation of root branching in zones with greater concentrations of phosphorus (Drew, 1975 ). Repeated zero-tillage can increase earthworm populations (Schwerdtle, 1969; Barnes and Ellis, 1979) and the number of continuous vertical earthworm channels (Ehlers, 1975; Westmaas Re- search Group on New Tillage Systems, 1984), as well as the number of smaller biopores, 1.5-2 mm in diameter, left by decaying roots (Goss et al., 1984; West- maas Research Group on New Tillage Systems, 1984). These channels can re- main intact in untilled land in successive years, and be reoccupied by roots. For example in a non-swelling silt loam loess, there were more roots in untilled than ploughed land below 200 mm, even though the bulk density and penetration resistance were greater in zero-tilled land (Ehlers et al., 1983 ). Also in cracking clay soils, where the bulk density was greater in uncultivated than ploughed land (Douglas and Goss, 1987), undisturbed fissures in the direct-drilled plots also facilitate penetration of roots. These examples signify the difficulty of using bulk density and penetrometer resistance as indicators of soil quality for root growth. Spring barley roots in sandy loams grew more slowly after direct drilling and shallow tillage (Ellis et al., 1977; Braim et al., 1992b) with lower grain yields than with ploughing, perhaps due to slower uptake of nitrogen (Braim et al., 1992a).

In spite of the greater concentrations of phosphorus and potassium in the surface layer and lower concentrations in the zone below after zero-tillage compared with ploughing, the uptake of these nutrients usually has not been adversely affected (Shear and Moschler, 1969; Triplett and Van Doren, 1969; Cannell and Graham, 1979). Indeed increased uptake of phosphorus and potassium sometimes has been found in zero-tilled maize (Singh et al., 1966), possibly due to less fixation than where nutrients are incorporated with ploughing, or to more moisture in the surface layers due to mulching. Root growth of winter wheat and maize in poorly drained soils can be adversely affected by eliminating tillage (El- lis and Barnes, 1980; Lal et al., 1989a).

4.2. Effects on crop yields

United States In the corn belt and mid-Atlantic region of the US several long-term experi-

ments give strong indications of the likely success of zero tillage and other methods of conservation tillage. In Ohio, Dick et al. ( 1991 ) reported results for four experiments on two well-drained silt loams, an imperfectly drained silt loam and a poorly drained silty clay loam, that had lasted for 25 years, starting in 1962. These probably include the longest continuing tillage experiments in the world. Results for two other poorly drained soils in Ohio also have been reported, an experiment lasting 11 years (1962-1972) (Van Doren et al. (1976), and a 12 year-experiment (1975-1987) (Lal et al., 1989a). In Kentucky, progress of an experiment on a well-drained silt loam, starting in 1970 has been reported by Blevins et al. (1977 ) and (1983 ) and Ismail et al. (1994). In Maryland, three


experiments on silt loams, two in the Coastal Plain, starting in 1973, and the other in the Piedmont, starting in 1974, are in progress (Bandel and Meisinger, 1993). In Indiana, Griffith et al. (1988) gave results for a 12 year experiment ( 1975-1986 ) on a poorly drained silty clay loam. All of these experiments compared no-till with mouldboard ploughing, but in Indiana chisel ploughing, shallow discing and ridge planting also were evaluated. Maize was grown at all sites, mostly continuously, but sometimes in rotation with soyabeans.

On the well-drained soils in Ohio, Kentucky and Maryland, the yields of no-till maize have generally been greater than with ploughing. In Ohio, the advantage for no-till was evident throughout the 25 years, and tended to become more pronounced as the experiment proceeded (Dick et al., 1991 ). In Maryland, on the coastal plain sites, the maize yield advantage for no-till also tended to increase with time (Bandel and Meisinger, 1993). In Kentucky, the maize yields on average were similar in the first 10 years after no-till and ploughing, but often were greater after no-till in the second half of the experiment (Ismail et al, 1994 ). In both Kentucky and Maryland, in the early phase of the experiments, at low rates of nitrogen fertilizer maize yields were greater after ploughing, but in the later phase the no-till treatments were more responsive to nitrogen. On the poorly drained soils that are slower to warm in the spring, yields of maize and soyabeans have usually been less with no-till. In Ohio, the average yield of maize over 25 years was 10% less than after ploughing, but the difference diminished in the last l0 years (Dick et al., 1991 ). On another poorly drained clay there was no yield difference between tillage treatments (Van Doren et al., 1976), but Lal et al. (1989a) reported yields of maize and soyabeans that were 10% and 6% less, respectively, after no-tiU than after ploughing. In Indiana, yields of continuous maize over 12 years were on average 9% less with no-till than with ploughing, with chisel and ridge systems giving intermediate yields (Griffith et al., 1988 ). When grown in rotation with soyabeans, oats and alfalfa or grass, the yields of maize and soyabeans on these poorly drained soils were less affected by reduced tillage (Van Doren et al., 1976; Dick and Van Doren, 1985; Griffith et al., 1988 ). When Dick and Van Doren ( 1985 ) grew Phytophthera root rot resistant soyabean cultivars on a poorly drained soil yield differences between the no-tillage and ploughed treatments were essentially eliminated.

Canada In Ontario, Vyn and Raimbault ( 1993 ) found in a 15 year experiment ( 1976-

1990) with continuous maize on a silt loam that yields after no-tillage were lower in every year than after autumn ploughing, averaging 16% less. Maize yields after spring ploughing and autumn chisel ploughing averaged 9% and 5% less, respectively, than autumn ploughing. However, shorter term experiments showed little difference between maize yields after no-till and ploughing when grown in rotation with soyabeans, wheat, barley or alfalfa (Vyn et al., 1994), similar to the earlier experience in Ohio and Indiana (Van Doren et al., 1976; Griffith et al., 1988 ). In the semi-arid conditions in Saskatchewan, Brandt ( 1992 ) reported results for a 12 year experiment on a loam soil in which zero-tillage and conven-


tional cultivation using spikes or sweeps were compared in continuous cropping or fallow- cropping rotations. Crop yields after zero-tillage exceeded those after the conventional treatment in 7 out of 36 comparisons, associated with increased spring soil moisture and adequate weed control. Yield decreases with zero-tillage were usually associated with poor weed control, especially of Avenafatua, or Hor- deum jubatum. It was considered that the cost of weed control versus the value of the increased yield is likely to be a major factor influencing adoption of zero- tillage. On a clay loam in Alberta, Lindwall and Anderson (1981) reported results for an experiment started in 1968, that is still in progress and giving similar results (C.W. Lindwall, personal communication, 1993 ). Wheat yields in the first 9 years were 9% greater with the chemical fallow treatment than with blade tillage (80-100 mm) , but a herbicide/fall blade treatment was the best yielding.

Europe In Belgium on a loess soil, ploughing treatments at 300 mm or 150 mm were

compared with direct drilling over a 15 year period (1967-1982 ), in a rotation of winter wheat, sugarbeet, spring cereals, winter beans or maize (Frankinet et al., 1979; Frankinet and Rixhon, 1983 ). Yields after direct drilling were slightly greater for winter beans, similar for winter wheat and spring oats, but 15% less for spring barley and maize, and 20% less for sugarbeet. In Denmark, in an 18 year comparison ( 1968-1985 ) of ploughing (200 ram), rotovating (30-50 mm or 100 mm) or stubble cultivation ( 100-120 mm) on a coarse sand, a moraine clay and a marine clay, yields of spring barley, spring oats and winter wheat were decreased with reduced tillage (Rasmussen, 1991 a). In a 13 year experiment (1975-1987) on sandy loam on marine sediments, yield of spring barley was reduced after rotovating (50-100 mm) compared with ploughing (200-250 mm ), but yields of spring oats and wheat or winter wheat were unaffected (Rasmussen and Anderson, 1994 ). In both these experiments there were more weeds without ploughing. Over a 12 year period ( 1974-1985 ) yields of continuous spring barley on a coarse sandy soil, two sandy loams on moraine deposits and a sandy loam on marine sediments were less after rotovating than after ploughing; shallow ploughing or discing gave a small increase in barley yield on the coarse sandy soil, but a slight decrease on the loams (Schjonning, 1986a).

In France, Caneill and Bodet (1991) reported results for 3 experiments comparing ploughing (250 ram), cultivation ( 150 mm) and direct drilling in a winter wheat-maize rotation but with each crop grown each year. One on a moder- ately well-structured loam lasted 20 years (1971-1990), and two on weakly structured loess soils from 1971 to 1982 and 1972 to 1981. Crop residues were chopped and spread. Overall yields were little affected by simplifying the tillage operation, provided rainfall was not excessive, that the ground was left in a suitable condition after the preceding crop, and that the machinery could create a suitable environment for the seed, particularly for maize. In Germany, Maidl et al. ( 1988 ) compared ploughing (270 ram), cultivation ( 150 mm), rotovation (120 ram) and rotary harrowing (80 mm) for 12 years (1975-1986), growing cereals. Compared with ploughing, yield was reduced by all treatments, especially

R.Q. Cannell, .I.D. Hawes / Soil & Tillage Research 30 (1994) 245-282 265

after the shallow procedures. Four experiments on loess soils are still in progress at Grttingen (W. Ehlers, personal communication, 1993 ). These include one that started in 1966, comparing ploughing and zero tillage, and three comparing minimum tillage with ploughing that started in 1973. On the zero-tilled treatment the straw was removed by baling. Average yields on these soils have been similar for the different treatments, but grass weeds have sometimes become a problem on the non-ploughing treatments (Ehlers and Claupein, 1994). Eichhorn et al. ( 1991 ) and Tebrugge et al. ( 1991 ) reported results for another experiment on a loess soil that started in 1981 and is still in progress (F. Tebrugge, personal communication, 1993 ). Yields of winter wheat, grown eight times, were similar after direct drilling and ploughing (250 mm) , and yield of spring barley, grown three times, was greater with direct drilling, on average 15%. Successful direct drilling depended on short chopping of the straw, but weed control was not a problem when glyphosate was used.

In Switzerland, Vez ( 1977 ) compared over l 0 years ( 1967-1976 ) ploughing (250 mm) with direct drilling ( 1967-1971 ) or in the later years a drill combined with a rotary harrow ( 100 mm) for continuous winter wheat on a well structured sandy clay loam; the straw was removed. The yield of winter wheat was about 15% greater after the direct seeding. In a 15 year experiment ( 1972-1986 ) on a loam, winter wheat and maize were grown in alternate years after ploughing (200- 250 mm) or rotary harrowing (50-100 ram) combined with a seed drill. The wheat straw was removed and the maize residues chopped (Maillard and Vez, 1988 ). The yield of winter wheat was similar after both treatments, but maize yielded about 15% more after shallow tillage. In further work Maillard and Vez ( 1993 ) reported the results of the first 20 years ( 1969-1989) of an experiment on a silt loam, that is still being continued (A. Maillard, personal communication, 1993). The effects of mouldboard ploughing (200-250 mm) , chiselling (250-300 ram), cultivation ( 100-150 mm ) and rotary harrowing (70-100 mm) were compared for a rotation of winter wheat, maize, winter wheat, oil-seed rape. The straw was removed after the wheat, but chopped for the other crops. On average the yields of all the crops after the non-ploughing methods were as good as after ploughing, and maize tended to be the best without ploughing. Without ploughing winter wheat was less affected by Pseudocercosporella herpotricoides, but there was greater incidence of slugs, Sclerotinia on rape and Ostrinia nubilalis on maize. Nevertheless, Maillard and Vez (1993 ) concluded that the needs for extra plant protection were offset by lower tillage costs and the slightly better yields. In the United Kingdom, Christian and Bacon (1990) reported the effects of mouldboard ploughing ( 150-200 mm) , shallow tined cultivation (50-70 mm) and direct drilling on yields of winter wheat, winter barley and winter oilseed rape over l0 years on three soil types; a weakly structured silt loam (a Typic Hapludalf) and two clay soils (Typic Haplaquepts). The crop residues were removed by burning. On average, yields of winter wheat and barley were similar after all treatments, but oilseed rape yielded significantly more after direct drilling than ploughing because of better establishment and uniformity of growth. In these experiments slugs were also more evident on direct-drilled winter wheat,

266 R.Q. Cannell, ,I.D. Hawes ~Soil & Tillage Research 30 (1994) 245-282

and annual grass weeds ( Alopecurus myosuroides, Poa annua and Poa trivialis ) increased with both direct drilling and shallow tillage, especially on clay soils. In Scotland a long-term tillage experiment lasted from 1968 until 1991 (Ball et al., 1993 ). Until 1982 spring barley was grown after mouldboard ploughing (180- 200 mm) , deep mouldboard ploughing (300-350 mm) , chisel ploughing (300 mm) and direct drilling on imperfectly drained loams, with half the replicates on a cambisol and half on a gleysol. Straw was removed, sometimes incompletely, leaving short stubble. On the cambisol more nitrogen was needed for yields of spring barley after direct drilling to equal those after ploughing, but on the gleysol even with extra nitrogen, yields were less after direct drilling, probably due to high soil strength and poor aeration. Deep ploughing was no advantage, however, and chisel ploughing encouraged grass weeds. From 1983 winter barley was the usual crop and the deep ploughing and chisel ploughing treatments were replaced by direct drilling and broadcasting plus rotovation. Over the period 1983-1986 long-term direct drilling was the best yielding treatment, with the other treatments similar, on both soils (Ball et al., 1989). In the final period 1987-1991, yields under long-term and short-term direct drilling were similar, but lower than under ploughing, mainly due to problems associated with straw residues, grass weeds ariel seedbed compaction (Ball et al., 1994). Soil aeration, strength and structure were more favourable under ploughing than direct drilling. Bulk density and soil strength did not show any long-term progressive changes in the long-term direct-drilled treatment. Direct-drilled wheat, barley and oats are more sensitive to drainage than when grown after ploughing. In a 10 year experiment on a clay where direct drilling and ploughing were compared with and without drainage, on average yields of winter cereals were 10% less after direct drilling (Christian and Ball, 1994), but in a wet year there was a significant interaction between tillage and drainage. In that year the yield of winter oats on drained land was 7.2 Mg ha - ~ after ploughing and 7.0 Mg ha - ~ after direct drilling, but on undrained plots the corresponding yields were 6.1 and 3.4 Mg ha-~ (Cannell et al., 1986). This was largely due to poor seedling establishment in the anaerobic soil conditions.

New Zealand In a 10 year experiment ( 1978-1988 ) on an imperfectly drained silt loam over

clay loam, Hughes et al. (1992) found that seedling root elongation of forage maize was slower in zero-tilled than in ploughed ( 150-200 mm) land; seedling root growth after disc harrowing to 50-100 mm was on average intermediate. The differences were attributed to mechanical impedance since there were no substantial crop residues and soil temperatures were not lower in the reduced tillage treatment. Root length density was similar at later growth stages, but the zero-till roots explored the soil less thoroughly in the deeper zones. Yields of forage maize were reduced by 16% after zero-tillage, but not significantly after minimal tillage. The yields of forage from oats grown in the preceeding winter were similar after all tillage treatments. In two other experiments on silt loams over 10 years ( 1978- 1988 ), one well-drained (Umbric Dystochrept), the other slow draining (Udic

R.Q. Cannell, J.D. Hawes /Soil & Tillage Research 30 (I 994) 245-282 267

Ustochrept), yields of winter wheat in twelve experiment years averaged 11% more, but spring barley yields, in 7 years, on the poorly drained site averaged 6% less without tillage (Francis and Knight, 1993 ).

Overall, these long-term European and New Zealand experiments show that after direct drilling or shallow tillage yields for winter cereals and oil-seed rape have usually been similar to those after ploughing if straw is removed and where drainage is adequate. Yields of maize have mostly been as good or better. Yields of spring barley have sometimes been reduced after direct drilling or shallow tillage. Sandy soils, low in organic matter, and clay soils containing non-swelling clay minerals compact readily and lack characteristics that naturally restore soil structure, are unlikely to suit repeated zero or minimal tillage in humid areas (Cannell et al., 1978; Ehlers and Claupein, 1994).

5. Adoption of soil tillage practices

5.1. United States

For many years nationwide surveys of tillage practices in the US have been obtained from county level Soil and Water Conservation Districts and USDA agencies, and published by the National Association of Conservation Districts Conservation Technology Information Center (CTIC). In these surveys conservation tillage practices to protect against water erosion are defined as any tillage and planting system where at least 30% of the soil surface is covered by residue after planting. Where soil erosion by wind is the primary concern, systems that maintain at least 1100 kg ha- 1 of flat, small grain residue equivalent on the surface during the critical wind erosion period are considered to be conservation tillage. The surveys by CTIC are helpful in indicating the broad geographical patterns in adoption of different tillage methods and can show trends over time. However, the surveys are not designed to identify the tillage methods used in relation to particular landscape features, such as soil type or slope. Additional information is now available from the National Resources Inventory (NRI), collected every five years or so by the USDA Soil Conservation Service. The NRI provides resource data from scientifically selected random sample sites. There were nearly 300 000 sample sites in the 1987 survey, the first time that informa-i tion on tillage practices was collected (Soil Conservation Service, 1989). The latest survey by CTIC ( 1992 ) shows that conservation tillage was used on 31%i of the cropped land, an increase from 26% in 1989, representing an increase of 7 M ha (from 29 to 36 M ha) (Table 5). The increase in conservation tillage has been mainly at the expense of land where the least amount of crop residue was left on the soil surface (Table 5). The Soil Conservation Service (1989) estimated that in 1987 conservation tillage was used on about 28% of the cropland, thus showing reasonable agreement between the two surveys. In 1992, however, 69% of the planted area still used tillage methods that did not leave sufficient residue to be regarded as conservation tillage.

268 R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (I 994) 245-282

Table 5 Summary of tillage practices in USA, 1989 and 1992. (Figures for different tillage practices are percent of total planted area, with millions ha for all crops for each practice in parentheses)

Year Total Conservation tillage Other tillage planted practices area ( > 30% residue or 1100 kg ha - I ) (M ha)

No-till Ridge-till Mulch Total 15-30% < 15% till cons. till residue residue

1989 (all crops) 113 5 1992 (all crops) 114 10 Corn (FS) 31.4 14 Corn (DC) 0.3 34 Soybean (FS) 22.4 15 Soybean (DC) 2.4 52 Small Grain (autumn) 20.7 5 Small Grain (spring) 15.0 3 Grain Sorghum (FS) 4.9 6 Grain Sorghum (DC) 0.2 25 Cotton 4.8 2 Forage Crops 3.2 9 Other Crops 9.2 2

(6) (11)

1 (1) 20 (22) 26 (29) 25 (29) 49 (55) 1 (1) 20 (24) 31 (36) 26 (29) 43 (49) 3 22 39 25 36 Tr 16 50 12 38 l 23 39 24 37

< 1 11 63 14 23 Tr 23 28 32 40 Tr 22 25 33 42 1 22 29 29 42 1 20 46 27 27 1 5 8 8 84 - 13 22 24 54 Tr 9 l l 20 69

FS, full season crop; Tr, trace; DC, double crop. (From CTIC, 1992).

The CTIC survey recognizes three main categories of conservation tillage; no- till, ridge-till and mulch-till. In both no-till and ridge-till the soil is left undisturbed from harvest to planting, except for nutrient injection. However, unlike no-till, weed control with ridge-till may involve cultivation as well as herbicides. The ridges are re-built during cultivation. With mulch-till, the soil is disturbed prior to planting and weed control is by herbicides and/or cultivation. Within the area adopting conservation tillage, mulch tillage was the most widely used approach, covering nearly 24 M ha in 1992, and accounting for about two-thirds of the area of conservation tillage (Table 5 ). Nevertheless between 1989 and 1992 the area of mulch tillage only increased by a relatively small amount (22 to 24 M ha; Table 5 ), but the area of no-till land increased to 11.4 M ha, doubling the area using no-till practices since 1989. In 1992 no-till methods were used on 10% of the planted crop land in the US (Table 5 ). The degree of adoption of no-till practices contrasts markedly with the previous decade. Between 1972 and 1980 the area of no-till crops changed little, totalling only about 2-3 M ha in any single year (Lessiter, 1981 ), and in 1987 was used on 3 M ha (Soil Conservation Service, 1989).

The 1985 Food Security Act (FSA) and the 1990 Food, Agriculture, Conser- vation and Trade Act (FACTA) (United States, 1990) called for a "substantial reduction of soil erosion" on soils determined to be "highly erodible" using the USLE model. The average amount of soil loss allowed by law is approximately 2.5 times the T value of a given soil. On steeper slopes, no-till may be required,

R.Q. Cannell, .I.D. Hawes /Soil & Tillage Research 30 (1994) 245-282 269

while on the flatter slopes, farmers may be able to do more tillage and still comply with the provisions of the Act (Fig. 1 ). Penalties for non compliance with the FACTA may be substantial if a farmer participates in commodity price stabili- zation programs. All program payments are contingent upon reducing soil erosion. So there is considerable financial incentive for decreasing tillage. Even with a reduced yield, farmers may be forced to consider using no-tillage, at least on fragile land. Thus US government programs probably have been the major factor influencing the increased adoption of conservation tillage in the last few years.

The importance of the different tillage methods depended on the crop being grown. Conservation tillage was most important for 'double crops'. (Double crops are second crops in a particular growing season. They are often planted slightly later and have somewhat lower yields than single or full season crops. ) More than half of the double crop maize, soyabeans and sorghum used conservation tillage, with more than one-third using no-till methods (Table 5 ). However, the largest area of no-tiU crops were full season maize and soyabeans with 4.3 and 3.3 M ha, respectively, accounting for about 15% of each crop and two thirds of all the no- till crops (Table 5 ). For these two species (full season and double crop) about 9 M ha were grown without tillage, accounting for nearly 80% of all the no-till area (Table 5 ).

More than 60% of maize in the US is grown in Iowa, Illinois, Nebraska, Min- nesota, Indiana and Wisconsin. In those states conservation tillage was used on nearly 40% of the maize area, including no-tillage procedures on 12%, similar proportions to the national figures (Table 5). Five of those states had near or above average rates of water erosion in 1987 (Fig. 4) and Minnesota had above average wind erosion (Fig. 5 ). In Nebraska, ridge tillage was used for 15% of the maize to facilitate irrigation on land with 0.5 to 1% slopes (E.C. Dickey, personal communication, 1993). Minnesota used ridge tillage on 5% of its maize crop, perhaps to aid soil warming. Regionally conservation tillage for maize is most important in Appalachia, where the terrain is undulating. In Kentucky, Tennes- see and Virginia nearly 60% of the maize was grown using conservation tillage, including more than a third without tillage. All of those states had above average rates of water erosion in 1987 (Fig. 4).

Almost two thirds of the full season soyabeans in the US are grown in Illinois, Iowa, Minnesota, Indiana, Missouri and Ohio. Conservation tillage was used on about 45% of the full season soyabeans in those states, including 17% without tillage, somewhat higher proportions than nationally (Table 5 ). Use of ridge tillage for soyabeans was only significant in Minnesota. As with full season maize, conservation tillage is proportionately most important in Appalachian states, and also in Iowa, where it is used on more than half the full season soyabeans. Ap- proximately two thirds of double crop soyabeans are grown in eight states Arkan- sas, Missouri, North Carolina, Illinois, Kentucky, Tennessee, Kansas and Vir- ginia) with five being in the southeast, where double cropping is more feasible due to the longer growing season. In many of these states conservation tillage was used for more than 90% of the double crop soyabeans. The most intensive use of conservation tillage for double crop soyabeans is also in Appalachia, and in In-

270 R.Q. Cannell, J.D. Hawes ~Soil& Tillage Research 30 (1994) 245-282

ESTIMATED SOIL LOSS BY WATER EROSION

. " . . . . , . . .

iiiiiii i !i!!ii ii!iiiiiii ii!i!iiiiiiiiiiiiiiiii!i!iii!iiiiiiiii!ili!!i .................. ............... • " . ' . ' . " • - • • ' . ' . " - • • • ' . ' . ' . ' . " . o - • . . , " . o . . . . . , o • • • - . • . o - - • . . . . - . •

A¥4N'anO 6OII I . ~a P f Y4mr % LRe than 8 ~ . pet Heat~e

9 tO 12 Mg. per Ho¢lnro 13 to ,~0 MB. Par HBIlt,,re

• 20 to 86 Mg. pgr I-legtarl

R

Fig. 4. Estimated soil losses by water erosion in the United States in 1987 (Soil Conservation Service, 1989).

diana and Illinois. In these states in 1992 double crop soyaheans were mostly grown without tillage. The area of double cropped soyabeans is likely to continue to increase in the southeast of the United States, because of advantages of time- liness in planting and moisture conservation in this region. For small grain crops that are planted in the autumn, mostly wheat, conservation tillage was used on only 28% of the area (Table 5 ), and on only about 20% of the crop in the three states with the largest area of winter wheat, Kansas, Oklahoma and Texas. These states are all prone to wind erosion (Fig. 5 ). Overall, no-till was little used for small ga in crops, but there were more than 100 k ha in each of the states of Illinois, Missouri and Ohio (CTIC, 1992 ). Conservation tillage was used slightly less for spring-seeded small grain cereals than autumn sown wheat (Table 5). Spring cereals, however, were grown without tillage on about 200 k ha in North Dakota, 80 k ha in South Dakota and 70 k ha in Montana, but these represented only about 4% of the spring cereals in these states (CTIC, 1992).

5.2. Canada

In the prairies, summer fallowing and mulch tillage are widely used as respon- sible farming methods, but a recent survey in Alberta showed a strong trend toward continuous cropping, and a reduction in the number of annual cultivations from between four and five to between two and three (Haigh and Haigh, 1992 ).

271

. ' . ' .

R.Q. Cannell, J.D. Hawes / Soil & Tillage Research 30 (I 994) 245-282

ESTIMATED 801L LOSS BY WIND EFiOSION

Awr~mu c ~ . LQaO ~or year .i.i. LuQ than 8 MG. par Flectare i

I tO 13 IdG paw HBGturo <~ 13 to ,~0 MB per Hmotsro • 20 to ~ Mg. pqlr I-IogtAnl

Fig. 5. Estimated soil losses by wind erosion in the United States in 1987 (Soil Conservation Service, 1989).

However, direct seeding and single cultivation options were not popular; nor was total chemical fallowing. While this survey showed that concern for soil quality was the most important motivating factor in adopting conservation tillage, the farmers had little awareness of government sponsored soil conservation initia- tives. In eastern Canada the traditional tillage system is mouldboard ploughing in the autumn followed by secondary tillage in the spring (Vyn et al., 1994). In spite of concerns about soil erosion and costs, the traditional systems still pre- dominate in Ontario. In 1990, mouldboard ploughing was used on 67% of the land, reduced tillage and mulch tillage (the latter a conservation tillage method with at least 30% of residue cover) on 14% each, and no-tillage on only 5% of the crop area (Lammers-Helps, 1990); 17% of the winter wheat was planted no-till. Simplified tillage is little used in Atlantic Canada (Carter and Kuneluis, 1990 ).

5.3. Europe

In spite of satisfactory yields in long-term experiments from several European countries, direct drilling has not been adopted on any significant scale. In Britain, direct drilling was used on a small area in the late 70s and early 80s, and for about 3% of the cereal crops in England and Wales in 1985/86 (Christian and Ball, 1994). Following a ban on straw burning due to it being socially unacceptable, direct drilling is unlikely to be used because of the difficulty of drilling and seed-

272 R.Q. Cannell, J.D. Hawes ~Soil & Tillage Research 30 (1994)245-282

ling establishment in the presence of straw residues (Graham et al., 1986) and ploughing is of increasing significance again (Ministry of Agriculture, Fisheries and Food, 1992). Survey results in 1989 showed that 90% of fields were ploughed (Cussans et al., 1990). Apart from problems with straw residues, Christian and Ball (1994) stated that the main reasons why farmers have been reluctant to change from traditional methods were systems reliability, weed control, soil drainage status and compaction. Similarly in Germany the mouldboard plough has a long tradition, being highly effective in burying and killing annual weeds and volunteer crops, thereby providng a clean surface for seeding (Ehlers and Claupein, 1994). Although experiments on some soils have demonstrated the similarity of yields with no-tillage and ploughing in the absence of straw residues, problems with straw residues preclude the use of zero-tillage. In Switzerland difficulties with direct-drilling have also precluded its use (A. Maillard, personal communication, 1993 ).

6. Other considerations

Conservation tillage is commonly thought of in terms of soil and water conservation (Mannering and Fenster, 1983), but reduced tillage procedures also can conserve energy and economize on capital and labour (Patterson et al., 1980; Tebrugge et al., 1991 ). These and other aspects of conservation tillage have been reviewed for many temperate regions by several authors in Carter (1994). Al- though long-term experiments can indicate the soil type/crop species/climatic combinations where reduced tillage is likely to give satisfactory results, these experiments represent only part of the overall situation. Experiments are sited in particular places for various reasons, including experimental convenience, inves- tigator interest and the importance of the soil type in the particular region. Short- term experiments may have pointed to circumstances where reduced tillage is less likely to be successful and would be hard to justify continuing. Various attempts have been made to classify the suitability of soils for different tillage methods (Galloway et al., 1977; Cannell et al., 1978; Canarache, 1987; Heinonen, 1991 ). Inevitably the classifications are based on limited information for some soils. Also the classifications relfect the constraints of the then current levels of technology. Particular constraints, which may be worse in humid areas, are straw residue management (a special problem where heavy yields of straw are produced such as in Europe), the need to relieve soil compaction caused by wheel traffic and problems of weed control. It is relevant to speculate if these constraints will continue to limit the use of tillage sytems that otherwise may be beneficial. While direct drilling of sequential winter cereal crops is not practiable in the presence of large amounts of straw from cereals that have been conventionally combine- harvested, the grain stripper combine leaves standing straw, and double crop soyabeans are being successfully zero-tilled into the standing straw after harvesting winter cereals in the mid-Atlantic and Appalachian regions of the US. The straw yields there are normally less than in Europe, however.


Although long-term experiments on poorly drained soils indicate that maize yields are likely to be less with no-till, modifications such as ridge-tillage (Grif- fith et al., 1990), the use of trash removers on planters and strip tillage may aid soil warming in spring (Vyn et al., 1994), and so extend the range of soils that can be no-tilled. Herbicide technology is a key to the success of conservation tillage, and many long-term experiments, especially in Europe, have shown that grass weed populations are likely to increase. Weeds of particular concern are Alopecuris myosuroides, Apera spici-venti, Bromus spp. and the broadleafed species Gallium aparine (G.W. Cussans, personal communication, 1993 ). Further- more, resistance of weeds to previously effective herbicides has been found in monocotyledons, including Alopecuris myosuroides and in dicotyledonous species (Saari et al., 1990; Moss and Cussans, 1991 ). However, new technological developments such as crops with genetic resistance to herbicides, whether by traditional breeding such as in soyabean, or by genetic transformation, such as glyphosate resistance that has been achieved in maize, soyabean and cotton will enhance the possibilities of sustaining conservation tillage. Rotation of crops increases the possible herbicides that can be used. While rotation of tillage methods between ploughing and conservation tillage can increase the ease of weed control, gains in soil quality are quickly destroyed. Other approaches may be possible. Winter annual cover crops can create a weed suppressing mulch into which spring crops of maize (Vaughan et al., 1992) and sugar beet (Maillard et al., 1990; Ehlers and Claupein, 1994) can be planted. The nitrogen economy of these cropping systems can be improved by catch crops which can take up nitrogen and reduce leaching, or by using nitrogen-fixing legumes (Wagger, 1989; Shipley et al., 1992 ). In addition the presence of cover crops during the winter reduces the risk of erosion. In humid areas such as Europe where direct drilling or shallow tillage are not practicable, tillage to incorporate straw residues has been found to increase soil organic matter in long-term experiments (Schjonning, 1986a and b), and may be combined with catch crops undersown in the preceeding cereal (Rasmussen, 1991 a and b). Nevertheless, societal concerns about the possible effects of chemicals on environmental quality (especially water contamination) and food safety, add to the difficulties of having sustainable crop production systems that can achieve heavy yields. The dependance of no-till systems on herbicides is already attracting media attention, even in the knowledge that soil erosion may be reduced (McMurray, 1993). Where conservation tillage may be legally required for farmers to receive government subsidies, the societal gains in erosion control will have to be weighed against the need for agrochemicals. These pressures will further encourage, as they already have, the development of herbicides which are needed at low rates and degrade rapidly. Research will continue to identify technological options that can be adopted in agriculture, and specifi- cally in tillage practices. For example procedures have been established that use artificial intelligence in farm planning for addressing problems of soil erosion (by using appropriate tillage and crop rotations), pesticide leaching and surface run- off of nutrients (Stone et al., 1992 ), with the intent of minimizing environmental impacts and achieving sustainable agricultural practices. The duration of the


modern era of tillage experiments is short in comparison with the time scale for sustainability of crop production, especially in relation to the time required to compensate for soil loss by erosion. Only a few long-term tillage experiments are in progress. It is to be hoped, however, that sufficient well-monitored long-term tillage experiments can be maintained or established in important regions, with measurements of soil erosion where relevant, to increase information on long- term effects of soil tillage on sustainability.

7. Conclusions

1. Soil erosion in parts of many countries is excessive in relation to long-term sustainability.

2. Conservation tillage, where crop residues are retained, can reduce erosion rates.

3. Soil quality parameters can form a partial description of the consequences of tillage practices but can not be used to predict the outcome.

4. Crop yields from long-term experiments, as integrators of the effects of soil quality and climatic factors on crop growth, are the best estimates of the expected effects of different tillage practices.

5. Conservation tillage practices are being readily adopted in the soyabean/ maize areas of the US and there is increasing interest in conservation tillage for wheat in the prairies in Canada. In eastern Canada, autumn ploughing predominates.

6. Minimum tillage is not widely used in Europe even though experimental results have been favourable for winter cereals and maize in many places. This is because of problems of controlling grass weeds, difficulties in disposing of crop residues in rotations and regions where small grained cereals are predominant, and the need to remove effects of soil compaction due to traffic.

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