soil tilth—the desired soil structural state obtained through proper soil fragmentation and...

-sou 8 Tillage Research

Soil & Tillage Research 43 (1997) 7-40

Soil tilth-the desired soil structural state obtained through proper soil fragmentation and reorientation

processes

A. Hadas * Institute of Soils and Water, Agricultural Research Organization, Volcatzi Center, p.0. Box 6,

Bet Dagan, 50250, Israel

Accepted 8 May 1997

Abstract

Soil tilth is a term used to describe a given soil structural state and its direct and indirect effects on the physical, chemical and biological processes occurring in the soil. Although the concept dates back to the initiation of agricultural activity, neither a clear definition nor a useful, quantitative index has been developed for it. Therefore, the term expresses a range of meanings from a simple, descriptive reference of a given soil structural state, to very complex, qualitative indices, which are sometimes, correlated to crop yields, The treatise presented here attempts to: (a) evaluate the interactions between changing soil structural states and the affected soil physical and mechanical properties; and (b) assess the possibilities of matching crop requirements and expected yields with the potential for controlling soil environment by tillage-induced soil structure modification. It is shown that an attempt to satisfy seasonal variations of phenological and climatic affected crop needs by tillage is very difficult. Tillage-induced soil structure instability, seasonal variability of weather condition and mechanized field activity may cause soil structure deterioration and impairment of soil physical conditions. As a result, the ability to control soil structure diminishes as the crop grows. Thus, the ‘desired’ soil structural state attained is a practical compromise solution, which matches expected future crop needs, weather conditions and soil structure changes with soil-tilling capabilities. Several ‘desired’ soil structural states may be considered for a given set of future crop needs, expected variations in climate and soil conditions. The objective is to evaluate and improve our current understanding of the interactions mentioned above, and to enable a knowledgeable, quantitative selection of a ‘desired soil structure’ management. Several potential research directions are indicated, which may lead to the improved understanding of the processes controlling: (a) the soil structure states and soil fragmentation; (b)

* Corresponding author. Tel.: + 972-3-9683288; fax: +972-3.9604017; e-mail: vwhads@votcani

0167-1987/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO167-1987(97)00033-O

8 A. Hadas/Soil & Tillage Research 43 (1997) 7-40

the interdependencies between the natural soil structure state prior to tillage and the stability of the various soil structural states produced; and (c) the crop’s root system response to the initial state and seasonal changes in soil structural states. It is shown that the current specifications of ‘desired soil structural states’ and means to attain them are empirically based rather then being based on estimated probable future climatic variations, seasonally changing crop needs and deterioration rates of soil structure. 0 1997 Elsevier Science B.V.

Keywords: Soil structure; Structure stability; Root systems; Crop stress factors; Soil fragmentation

1. Introduction

The shift from food gathering to controlled crop production introduced a basic cultural revolution, replacing a multi-speciated, balanced ecosystem with a monospeci- ated crop. This change required: (a) eradication of all other species from the field; (b) a higher degree of control of soil environmental conditions; and (c) uniform establishment and ripening of the crop (Whitney, 1925). To meet these requirements, various tillage practices, developed by trial and error, became the major agro-practices in use (Kuipers, 1984). The soil structural states resulting from tillage and affecting crop response are loosely referred to as ‘soil tilth,’ a term defined as, “the physical conditions of soil as related to its ease of tillage, fitness as a seedbed and its impedance to seedling emergence and root penetration” (Soil Science Society of America, SSSA, 1996). This definition purposely avoids the importance of interrelationships between porous, struc- turally stable ‘soil tilth’ and successful crop yields. Therefore, it eludes a quantitative reference to present and future soil conditions and properties conducive to crop production. Soil ‘tilth’ or tillage-induced soil structural state is the combined result of the various processes which determine the soil conditions affecting crop growth, e.g., free water intake and moderate water retention; optimal aeration and exchange of gases, low soil resistance to root penetration and proliferation; erosion resistance, stable soil structure, good biological activity, and a stable basis for vehicular traction (Slipher, 1932; Yoder, 1937). These factors concur with good tillage practice goals, namely improvement and maintenance of air, water and thermal regimes in the soil; proper seedbed preparation; management of organic residues and manure, fertilizer incorporation and placement; weed eradication; breaking up of hard pans, plough soles and compacted layers; and shaping a suitable soil surface relief, and facilitating mechanized field operations (Slipher, 1932). Tillage practices are commonly accepted as “the practices by which soil and its structure are mechanically manipulated to attain predesignated goals. ” Soil structure has been defined as soil particle assemblies (Soil Science Society of America, SSSA, 1996) “arrangement of the solid phase of the soil and its pore space located between its constituent particles” (Marshal and Holmes, 1979) or “the spatial heterogeneity of the different soil constituents or properties of the soil” (Add&cot and Dexter, 1994). Due to the hierarchical differentiation between different size scales observed in soil structure (Tisdall and Oades, 1982; Hadas, 1987a; Dexter, 198813) soil structure in the following discussion will be defined as “the spatial heterogeneity of the arrangements of the soil solid constituents, the enclosed pore spaces and the derived soil properties” (e.g., water and air contents, heat capacity, water, air

A. Hadas / Soil & Tillage Research 43 (1997) 7-40 9

and thermal conductivities, compressibility, and shear and tensile strength). The soil structural state is then defined as “ the instantaneous exhibition of processes related to soil strncture-dependent attributes.” Intensified mechanization of tillage and cropping practices has led to declining soil structure stability, generally accompanied with impairment of water-resource quality and soil fertility. Yoder (1937), Malsted (1954) and Kay (1992) have related these deleterious effects of tillage to the interdependencies among short- and long-term transitory states of soil structure, soil fertility, soil organic matter contents and crop yields.

Variations in crop needs, which the root system provides throughout the growing season, necessitates soil structure management practices oriented toward root development and activity. Obviously, it is essential to determine specific crop needs at specific developmental stages, and try, whenever possible, to modify soil structure accordingly. The specific objectives of this presentation are: to review crop requirements; to assess the interrelationships between tillage-induced soil structural states and root system needs; to define conceptual and theoretical knowledge gaps; to evaluate the limitations of current practices in producing soil structures conducive to proper root activity and crop yields; and to outline future research needs.

2. Soil environment and crop requirements

Crop needs for water and nutrients are provided for by root system development and its complex, dynamic interactions with diurnal and seasonal variations of soil-water, air and thermal regimes, and soil impedance to root development (Dexter and Woodhouse, 1985; Hamblin, 1985; Glinski and Lipiec, 1990; Smucker and Aiken, 1992; Snyder, 1992). Recognition of spatial and temporal architectural responses of root systems to changing soil factors, as being very difficult to measure and quantify in situ, has led to modelling efforts in an attempt to facilitate prediction of crop responses. Empirically-derived models relate integrated gross seasonal crop response to climatic and soil factors (e.g., annual precipitation, tillage treatments, annual irrigation quotas, schedules and amounts, and the mode of application of fertilization). Two general models, one based on the ‘Law of Diminishing Returns’ and the second on von Liebig’s ‘Law of Minimal Returns,’ are given in Eqs. (1) and (2), as suggested by Paris (1992) and Sinclair and Park (19931, respectively. In these equations, kj and ai are constants related to the growth-yield factor, Xi; and f<. . . ); g(. . . >. . . n(. . . > denote seasonal, empirically determined and fitted response functions of soil regimes and growth-related factors. The b-m Y,ax is the maximal yield observed:

Y= Y,,,Pi[l -k, exp(-aiXi)] (1)

Y = Min{ f( water), g (tillage) . . . , n(fertilizers)} (4 These models do not account for particular crop needs nor do they follow the

temporal course of conditions affecting these needs. They are also insensitive to short-term, tillage-induced changes in soil environmental conditions. Instead, they yield an empirically based mean-weighted response to fluctuating soil conditions, depending

10 A. Hadas / Soil & Tillage Research 43 (1997) 7-40

on the fitted yield response functions. It has been shown (Zaslavski, 1968) that fluctuating soil conditions may cause the average response of the yield [R,(f)] to differ from the observed yield [Z&f,,)] in response to the average value of these conditions, f,,, i.e., R,,(f) Z R (f,,).

Biosystems, e.g., crops, integrate their interactions with the weather and soil environment in nonlinear, time- and size-scale-dependent modes that follow short-term, sequential summation of increments. The observed mean resultant vegetative and reproductive yields diminish or increase, dependent on the rates of change of their response functions to the temporal and spatial variabilities of soil and climatic factors as shown by Eq. (3) (Zaslavski and Buras, 1967; Zaslavski and Mokady, 1967) where F, are the crop growth affecting factors, f, are the standard deviations from the mean of factors F,, A is the area covered by the crop and Err is the error introduced by neglecting the derivatives:

Y, = Y(F,,F,,... F;,) + 1,‘2!pij(( a’Y/aFjaF,)i(~S,)pijdA}/A + Err (3)

of higher order then two. For i = j, the integral term equals the variance riii of Fi, for i <j, it equals the product of the respective standard deviations of Fi, Fj and pij the coefficient of correlation between Fi, Fj. Since experimental fields are spatially variable to some extent, it is obvious that most reported crop response functions carry an unaccounted for deviation from the real mean response to soil and environmental growth affecting factors. Moreover, the magnitude of this deviation is unknown. Any variability in a soil resource (e.g., water, fertilizer, crop stand, seedbed aggregation) will be reflected in yield variations (Zaslavski and Mokady, 1967; Hadas et al., 1985).

Recent, more detailed models, which predict crop development and yields on a daily based summation of crop responses to climatic and soil conditions have been summa- rized (Hanks and Ritchie, 1991). In these models, simulated short-term changes in soil structure and resultant changes in physical properties and processes (e.g., soil impedance or water regime) are estimated. However, their effects are accounted for only in part (Jones et al., 1991; Snyder, 1992) since the empirically based relations were obtained under controlled conditions and an error may be introduced when they are extrapolated from a plant to the field scale. Crop response reflects the integrated function of root system activity in uptake of water and nutrients, subject to changing soil-structure dependent soil-environment factors and regimes, and extending from the plant to the field scale. Obviously, if soil-structure-dependent factors are to be controlled and matched to crop needs by appropriate tillage, it is important to continuously monitor and quantify crop responses to varying soil structure states using in-situ nondestructive methods.

2.1. Soil water regime

The soil water regime reflects the fluctuations in the given soil water balance:

(Pr+Ir-R)-(E+U)=T+As (4)

where Pr and Ir are the amounts of water applied by precipitation and irrigation, respectively, R is water lost by runoff, E and U are water losses by evaporation and

A. Hadas / Soil & Tillage Research 43 (1997) 7-40 II

deep seepage from the root zone, respectively, 1’ is water uptake by roots, and AS is the change in soil water storage in the root zone. The term (Pr + Ir - R) is the amount of water infiltrating the soil. Crop water requirement and response to the soil water regime have been discussed extensively in a general manner (Unger et al., 1981; Taylor, 1983; Hanks, 1983, 1991; Campbell, 1991; Smucker and Aiken, 1992; Baker et al., 1992); in relation to soil physical properties and the processes involved (Marshal and Holmes, 1979; Hillel, 1982; Hamblin, 1985); in relation to water flow toward and uptake by plant roots, root-soil contact and root-system patterns (Gardner, 1960, 1968; Cowan, 1965; Cowan and Milthorp, 1968; Herkelrath et al., 1977a,b; Hamblin, 1985; Lafolie et al., 1991; Smucker and Aiken, 1992); and in relation to tillage-induced soil structure and its deterioration caused by traffic and weather conditions (Amemiya, 196.5; Keller, 1970; Chancellor, 1976; Dexter, 1977, 1988a,b; Klute, 1982; Unger and Kaspar, 1994; Bathke et al., 1992; Horn et al., 1994). In fragmented soil, water retention slightly diminishes, hydraulic water conductivities increase, and root systems become more proliferous (Unger et al., 1981; Hillel, 1982; Hamblin, 1985). Infiltration increases and soil water evaporation decreases are controlled to a great extent by the structural state of the top soil, which is modified by shallow tillage, whereas total root-zone water storage and drainage can be slightly modified by tilling the subsoil or by mole drains (Bathke et al., 1992; Jalota and Prihar, 1990; Olsson et al., 1995). As soil structure deteriorates (crusting, increased bulk density) with time, climate and field activity, infiltration decreases and evaporation increases. The amending effects that tilled deep soil layers have on crop root systems, and its activities also diminish or are reversed (Barley and Greaten, 1967; Taylor and Ratliff, 1969a,b; Dexter, 1978, 1988a; Bathke et al., 1992; Unger and Kaspar, 1994). Under limiting water conditions, prolific root systems can tap soil water for longer times and at greater rates than sparser systems, and may ultimately lead to diminished yields or risk the crop’s survivability (Cowan, 1965; Hadas et al., 1980; Dexter, 1988a). In well-structured soils or successfully tilled subsoils, available soil water storage of 200 mm can be attained while maintaining about 10% air filled macropores (Olsson et al., 1995). Obviously, ‘tailoring’ tillage practices to suit root development, root activity and crop requirements for water and nutrients demands continuous quantitative monitoring of the dynamic root soil system interaction with tillage modified soil conditions and weather constraints.

2.2. Aeration regime

Oxygen distribution in soil depends on the existence of continuous air-filled pores, cracks, and root channels open to the atmosphere. Critical values of continuous air-filled porosity vary between 0.1 and 0.25 cm3 cme3, owing to great variability in soil structure-dependent air-filled porosity, pore connectivity, cracks and old root channels. Under conditions of adequate water and nutrient supplies, an active root system requires about 10 g O2 mm3 which corresponds to 10% air-filled porosity (Mayers and Barrs, 1991). Owing to the wide ratio between oxygen diffusivities in air and water-l: lo-” -any water-filled crack or pore, or a water film covering roots may drastically reduce oxygen transport in the soil profile, while oxygen diffusion into water-filled aggregates is limited to a few mm (Greenland, 1968; Glinski and Stepniewski, 1985; Drew, 1992).


Roots will proliferate readily into large pores, drained and well-aerated root channels, and biopores, whereas root penetration into water-filled soil volumes of finer pore sizes that drain at high suctions is limited or inhibited by aeration deficiencies. Therefore, the deeper the tillage-affected air-filled porosity extends, the deeper the active root system will penetrate. Demand for oxygen increases with soil temperature, root density and microbiological activity. If high oxygen demand conditions prevail next to the soil surface, more deeply located active roots become oxygen stressed, especially in non- drained wet soils, resulting in yield reductions (Box, 1991; Drew, 1992; Olsson et al., 1995). Air and water share the same pore space and an increase in water storage reduces air content and transport. Aeration stress can be alleviated by deep tillage, which forms larger pores in the subsoil, by promotion of earthworm activity, or by mole or tile drainage, if required (Dexter, 1988a; Box, 1991).

2.3. Soil thermal regime

Growth activity of root systems and crop canopy meristems near the soil surface respond nonlinearly to soil temperature changes. Threshold, optimal and maximal temperatures, which correspond to activity initiation, maximal growth and growth interruption have been observed for adventitious roots, sheaths and reproductive meris- terns next to the top soil layer (Voorhees et al., 1981; Jones et al., 1991; Kaspar and Blant, 1992). The soil thermal regime depends on the partitioning of radiation energy at the soil surface, and on soil thermal properties (heat capacity and thermal conductivity). The latter depend on the soil constituents (minerals, organic matter content), soil layers, water content and soil structure (van Duin, 1956; van Wijk, 1963; de Vries, 1963; van Wijk and Dirksen, 1963; Hadas, 1977).

Tillage-induced variations in soil structure changes thermal properties of affected soil layers. In a tilled, layered soil, the ratio between air and soil heat fluxes differs from that of a homogeneous soil. During warming periods (morning, spring), a tilled or mulched topsoil layer (low heat capacity and thermal conductivity) is warmer than an untilled layer, at and near the surface. Conversely, it is cooler below a certain depth, shallower than or equal to that of the non-tilled soil. Reverse trends are observed during cooling periods (evening, autumn). Greater diurnal surface temperature amplitude and time phase shifts are observed in tilled than in non-tilled soils. Crops grown on tilled soils are subjected to more extreme temperatures at and near the soil surface. This can result in increased risks of thermal scorching at noon or frost during early morning, delayed root development on wet, cool soils, but enhanced emergence and development on dry, bare areas (van Wijk and Dirksen, 1963; Mock and Erbach, 1977; Voorhees et al., 1981).

2.4. Soil salinity

The soil solution contains dissolved salts, the concentration and constitution of which bring about crop yield reductions due to: (a) species sensitivity to osmotic stress, or ion toxicity; (b) deterioration of soil-structure stability, water and air transport characteristics; and (c) soil surface sealing which reduces water infiltration, increases soil erosivity and impairs aeration (Shalhevet and Kamburov, 1976; Mass and Hoffman,

A. Hadas / Soil & Tillage Research 43 (I 997) 7-40 13

1977; Hoffman, 1981). The deleterious effects of soil salinity on soil physical properties and crop performance can be partly alleviated by mole or tile drainage, deep tillage to fragment the subsoil and additions of soil amendments. The operational efficacy of these measures in amending soil structure and reducing soil salt loads depends on the coarseness of soil fragmentation and on the spatial placement of amending materials (Shalhevet and Kamburov, 1976; Hoffman, 1981; Addiscot and Dexter, 1994; Jayawar- dane et al., 1995; Summner, 1995).

2.5. Nutrient deficiencies

Nutrient deficiencies observed in crops are attributed to: depletion of nutrients due to uptake, leaching, fixation or precipitation from the soil solution, poor fertilizer application and distribution, impaired root uptake activity caused by either improper root-soil contact (coarse, aggregated, tilled soil), restricted aeration, and root proliferation (in spite of good soil-root contact) due to traffic-induced soil compaction. Fertilizer placement in bands can reduce fixation of nutrients and increase their availability, but it concurrently increases spatial variability and risks of yield reductions as compared with broadcast application and mixing in of the added fertilizers (de Wit, 1953). The combination of tillage-induced soil fragmentation and proper fertilizer placement usually results in an appropriate aggregate size distribution and alleviates nutrient deficiencies, provided other potential induced stresses are prevented (e.g., aeration, temperature or salinity).

2.6. Soil mechanical impedance

Soil mechanical impedance to seedling emergence or root penetration is a major soil physical restraint to stand establishment and root system development (Bathke et al., 1992; Unger and Kaspar, 1994). ‘Bulk’ restriction of root elongation, root branching and deformation by fragipans and ploughpans, and impedance of seedling emergence by surface seals leads to: (a) impaired root system architecture and crop stand (Abdalla et al., 1969; Bowen, 1981); (b) reduced water and nutrient uptake (de Willingen and van Noordwijk, 1987; van Noordwijk and de Willingen, 1991); (c) increased risks of crop subjection to stresses and unbalances in plant hormonal coordination and growth regulation (Lachno et al., 1982; Jackson, 1994; Tardieu, 1994) and (d) ultimately leads to retarded crop development and reduced yields.

The pressure exerted at the tips of elongating roots varies between 0.1 and 1.2 MPa, and is species dependent (Taylor and Ratliff, 1969a,b). Root elongation rate decreases as soil impedance increases from a small threshold impedance, and ceases completely at a maximal impedance value which is plant species specific and dependent on soil ambient conditions (Bengough and Mullin, 1991). Hyperbolic or exponential correlations between soil density, structural state, and water content dependent soil resistance to penetration, on the one hand, and root elongation and branching on the other hand, have been reported (Taylor and Ratliff, 1969a,b; Bowen, 1981). The greater the water transpiration demand and the lower the plant capability to meet it (low water availability, sparse root system density, temperature or aeration stresses), the lower will be these limiting soil impedance values, as can be seen in Pig. 1 (Greaten, 1986; Dexter, 1987).


g 30

SOIL IMPEDANCE (MPa)

Fig. 1. Root elongation rates as function of transpiration rate in mm day 1 and soil impedance to penetration (Gupta et al., 1990).

Tillage operations induce soil cracking and fragmentation, break hardpans, plough soles and surface seals, improve water intake and redistribution, reduce evaporation from seed beds, and reduce soil impedance to root penetration and seedling emergence (Hadas et al., 1980; Bowen, 1981). Tillage operations are usually limited to depths of 0.3 m or less and although their effects on early crop establishment and development are known, their effects on deep root penetration are partially or completely unknown (Voorhees et al., 1981; Tardieu, 1991, 1994; Unger and Kaspar, 1994).

2.7. Combined effects of soil environment factors

Attempts to modify soil conditions or alleviate crop stresses by changing soil structure may, at times, lead to unforeseen results. Tillage is defined as “modification by mechanical means of soil structure and relief, and the physical and mechanical properties of the soil, for the expressed purpose of promoting crop production” (Bathke et al., 1992). Clearly, this vague definition combines the expressed goals of soil structure modifications to the expected variations in the soil environment and resultant potential crop responses. Yet, it does not render any quantitative indices for assessing the structural states resulting from the tillage operations. Some examples may illustrate the complexity of the combined effect of tillage-effected soil conditions on crops. Formation of a fine, ridged field aimed at draining the seedbed and warming it up to facilitate earlier sowing (e.g., in higher latitudes and with a short growing season) subjects seedlings to greater risk of freezing. The same mode of field preparation under semiarid warm conditions may subject the seedlings to midday sun burns and water stress because of poor soil water-root contact. A high transpiration rate reduces the ability of roots to elongate (Fig. 1) so that total root elongation decreases and water and nutrient uptake, which depends on new root segments, can be reduced (Gardner and Ehlig, 1962). Tilled, fractured soil increases root elongation rate because of reduced resistance, but it may also reduce soil-root contact, resulting in an unchanged or diminished water and nutrient uptake. In moist subsoils, the range of matric potential within which roots


SOIL RESTRICTIVE CONDITIONS 0.651 1

95% PENETRATION

50% PENETRPiTlON

0 10 20 30 40 5b Bo WATER CONTENT (% w/w)

Fig. 2. Soil conditions restrictive to root development for a silty clay soil (Typic Chromoxerert), based on data from Snyder and Hadas (1991). As an example, at total porosity of 0.50 (v/v) and water content of 25% (w/w), a root will operate either under the limiting air filled porosity criterion of air filled porosity of 0.12 (v/v) or soil impedance to root penetration, limiting 95% of the roots from penetrating the soil.

are able to grow and function is limited. To improve aeration and/or reduce soil strength is also difficult since water has to be drained first to facilitate improved aeration. At high bulk densities, small changes in water content lead to large changes in soil resistance to root elongation (Fig. 2). Reported ranges of some soil-structure-dependent factors and environmental, climatic conditions that cause stresses in crops and that can be partially alleviated by tillage are listed in Tables 1 and 2, respectively.

Table 1 Threshold, optimal and critical values of various stress factors affecting crop development and yields

A. Water stress (MPa)

Threshold

0.85-0.90 (Sat.) 0.60 (available water)

Optimal Critical

- 1.50 - 1.50

B. Aeration

Oats Cotton Corn, sunflower

C. Temperature (“(2)

Wheat 2-5 Corn 10 Cotton 15

D. Mechanical impedance (MPa)

20-30 26-30 25-33

0.10-0.25 (air-filled porosity) 15(~grn-‘s-~) 25 ( pg m-’ SK’) 25-30 ( pg m-* s-l)

40-46 40-45 45

Cotton Corn Barley Sorghum

Threshold impedance 0.4-0.6

50 (%) 1.0 0.9 1.3

0 (%) of elongation rate 2.5 1.6 3.7 2.0


Table 2 Potential stress factors affecting crop stands and yields under various climatic conditions, the severity of which can be partially or fully alleviated by proper tilth

Climate Cold Temperate warm

A. Seed-bed and seedling establishment DV Moisture scarcity (tillage) (Tillage and/or mulch, conserving mulch)

Low temperatures Moderate to high temperatures (as above) High mechanical impedances, surface self-sealing (tillage and/or mulch)

Moist Low temperatures Moderate to high temperatures (as above) Aeration-temporal (drainage, tillage and surface relief modifications)

Wet Low temperatures Aeration (need for drainage) Soil structural subsidence (mulch and crop’s residue incorporation)

B. Root-beds and subsoils

DV Water scarcity (deep tillage and mulch) Mechanical impedances (as above)

Moist Aeration (temporal)-(drainage) Wet Aeration (need for drainage)

Recent publications surveying these complex, dynamic interactions between crop needs, root system activity and the tillage-effected soil environmental conditions lead to the conclusion that lack of integrated studies preclude improvement of the current understanding of this intricate system (Bathke et al., 1992; Smucker and Aiken, 1992). Moreover, although tillage has been practiced since ancient times, the ability to quantify and predict the outcome of a given practice on root system activity and architecture, and on crop development is very limited. To close these apparent knowledge gaps, attention must be focused on the development of new methods to quantify and predict tillage-induced changes in soil structure, resultant soil properties, and corresponding root system and crop responses.

3. Root system response to soil physical conditions

Root behaviour in soil depends on the balance between the genetically coded potential and the response to actual field conditions. Being more complex than that observed in the laboratory, this behaviour is neither fully understood nor predictable (Bathke et al., 1992; Unger and Kaspar, 1994). Root growth is confined to pores, cracks, old root channels, interaggregate voids of similar or greater diameter then the root tip, and to aggregates sufficiently pliable to allow movement, deformation and penetration. Roots, concentrated in these spaces, competed for resources. The severity of this competition, the resultant loss in water and nutrient uptake efficiency, the type of stress, the timing of its onset and its severity and duration, and ultimately, the resultant yield losses it causes depend on: the age and spatial density distribution of the roots (Caldwell, 1987; Gregory, 1987; Fitter, 1991a,b); their exudates (Bar-Joseph, 1991; Marschner, 1991); the crop stand density, size and uniformity (Hadas et al., 198.5). Root

A. Hadas/Soil & Tillage Research 43 (1997) 7-40 11

clustering in cracks indicates sub-volumes of soil which the roots did not explore for resources, and the degree of competition between them (Cowan, 1965; Arya et al., 1975; Hadas et al., 1980, 1985; Jakobsen and Dexter, 1987; Dexter, 1988a; Bruckler et al., 1991; Lodgson and Allmaras, 1991).

Uptake processes comprise flow of soil solution to roots and exchange of nutrients for root exudates. Both depend greatly on soil hydraulic properties next to the root, root soil contact and root elongation rates to maintain the needed water and nutrient uptake rates. A root segment entering a new soil volume takes up soil solution at a decreasing rate as the soil next to it dries, so that flow rate and root-soil contact decrease (Gardner, 1960, 1968; Cowan, 1965; Cowan and Milthorp, 1968; Herkelrath et al., 1977a,b). At first, roots exploit soil volumes next to the soil surface, but as that region dries out, the roots elongate downwards into deeper soil layers, to draw water (Hadas et al., 1980). Wetting dried upper soil layers revives root growth in these layers and the root system center of activity shifts to these rewetted root zones. Root density and activity-depth fluctuations depend on transpirational demand, availability, storage, depletion and replenishment frequency of soil water, and the ensuing fluctuations in aeration, soil temperature and soil resistance to root penetration (Figs. 3 and 4a,b,c) (Shumaker and Smucker, 1981; Stone et al., 1983; Dexter, 1987; Smucker and Aiken, 1992).

The ability of the root tip to exert pressure is the net result of the balance between the root cell’s osmotic and turgor pressures and the external soil stress (Greaten, 1986; Dexter, 1987). A more elaborate model was suggested by Hettiaratchi (1990); it included root cellular biomechanics and soil machine mechanics, soil strength which is governed by the microstructure of the soil skeleton, and soil deformation as influenced by roots. Model-based calculations show that a varying root apex geometry enables roots to penetrate compacted soil. The model is, however, incomplete because of the great difficulties in quantifying the evolution of the components of soil stress imposed by developing roots, and in synchronizing the cyclic modes of root growth stages. No attempt has been made, so far, to introduce a hierarchy of modes of soil structure and strength into this model, nor has it been tied to a probabilistic model of root penetration into aggregated beds (Dexter, 1978; Hewitt and Dexter, 1978, 1984). Such combinations

61 = . I

n = I n . i 04

0 0.1 0.2 0.3 014 0.5 0.6 0.7 0.8 0.9 MATRIC POTENTIAL (MPa)

Fig. 3. Root water uptake activity as function of soil matric potential. The points reflect the envelop of maximal root activity at different soil matric potentials. Adapted from Hadas et al. (1980).


8 COTTON ROOT ACTIVITY 8 COTTON ROOT ACTIVITY

- . . . . I.6 DEPTH (m)

WHEAT ROOT DISTRIBUTION g 100

0.3 0.4 0.5 0.6 0.7 0.8 0.9 DEPTH (m)

NO-TILL --E-

2 YEARS -t-- YEARLY

- NO-TILL

2 YEARS

:.6

Fig. 4. Cotton root water extraction activity as a function of tillage treatment and time after emergence: (a) after 3 1 days; (b) after 77 days, and (c) in the third year when the crop is wheat. The tillage treatments were: no-till; second year after deep moldboard ploughing (to approx. 0.40 m), and deep mouldboard ploughing every year. Adapted from Hadas et al. (1980).

offer great potential for improvement of the current understanding of the interrelationships between root system development and existin, a natural or tillage-induced soil structure.

The ability of roots to penetrate pliable soil pedons, to move aside aggregates or to buckle and circumvent them (Dexter, 1978, 1987; Hewitt and Dexter, 1978, 1984; Misra et al., 1986; Whitley and Dexter, 1984) depends on soil depth, aggregate impedance to penetration, root/aggregate diameter ratio and root inclination from the vertical. The greater the impedance, and/or root aggregate diameters ratio, or the deviation from the vertical, the greater the tendency of the root to buckle, deflect, thicken and branch into many laterals. The more fragmented the soil is, the more branched the root system becomes and the less root-soil water contact there is (Abdalla et al., 1969; Dexter, 1978; Lodgson et al,, 1987). To optimize root system capacity to take up water and nutrients, necessitates matching root-soil contact, elongation rates and branching hierarchies to crop needs and soil structure-dependent physical and mechanical properties. In practical terms, this means matching modes of soil fracturing to tillage operations in


order to produce a structural state conducive to the desired root system development. Dexter (1978) and Hewitt and Dexter (1978, 1984) have developed probabilistic models relating root elongation and branching patterns to the spatial sizes and strengths of aggregate distributions. Although these models have never been validated under field situations, the required matching could be attained, provided the joint probabilities of observed root development responses to measured soil structure characteristics (peds and aggregate sizes, strengths and spatial distributions), and the soil bed hydraulic properties are known. No such attempt that has been made, hitherto, to match measured soil structure indices determined in laboratory beds (Grant et al., 1985) or tilled field plots (Hadas and Shmulewich, 1990) to root system development patterns.

Such an attempt, if carried out, should provide an insight into root system response to tillage operations as well as a powerful tool to ‘tailor’ root systems to crop needs (Greaten, 1986). Current models of root development ignore soil structure effects to a great degree, and barely predict spatial distributions of root activity in detail although some account for inter-root competition. These effects are indirectly incorporated into the models by ‘fine tuning’ the outputs with root data observed in the field or the laboratory (Huck and Hillel, 1983; Hoogenboom and Huck, 1986; Box, 1991; Pellerin et al., 1991; Tardieu, 1991; Klepper, 1992). Recent developments in modelling efforts introduce more realistic estimates of soil conditions, root elongation and branching, soil-root contact and water uptake, yet none reliably yield a root distribution, optimized to varying soil environment and crop requirements.

Tillage practices, aimed to affect root responses and yields, produce site-specific, nonhomogeneous soil structure, of which the fragment sizes and degree of mixing can not be predicted. Moreover, tillage operations induce, as a tradeoff for the necessary traction, soil conditions which inhibit root development, e.g., soil compaction. Extreme experimental difficulties are encountered in monitoring all the spatial and temporal changes of the modified soil-structure characteristics and root-system responses (Unger et al., 1981; Taylor, 1983; Eck and Unger, 1985; Glinski and Lipiec, 1990; Bathke et al., 1992). Furthermore, these destructive soil and root sampling methods preclude long-term, in situ, types of observations of soil structure-root interactions. Use of nondestructive, slightly perturbing methods, e.g., minirhyzotrons, may improve our ability to observe root behavior, and to monitor and validate current and future root models.

4. Soil profile modification, fragmentation and soil tilth

Soil tillage drastically changes soil structure and at the same time destabilizes it. The induced changes in soil structure evolve, and the derived physical properties deteriorate in the course of time, because of climatic variations and human activities. Purpose-oriented tillage operations usually induce several other effects at the same time, not necessarily the desired ones (e.g., changed soil surface relief, coarser seedbed, increased soil temperature variations, soil compaction).

Tillage, by definition, aims to produce, by mechanical manipulation of soil structure, weed eradication and residue burial, the best possible soil environment for seed germination and emergence, root elongation and activity, resulting in conditions conducive to optimizing crop yields under the prevailing conditions. Tillage implements are


designed to loosen the soil, and mix and reposition its structural units. The traction forces required by the tractor stress the soil, causing structural collapse and compaction (Bowen, 1981; Voorhees, 1992).

The initial soil volume to be tilled is usually rather dense, and the aggregates, clods or blocks are neatly packed close to one another. Each is made of small units, usually clustered together to form larger units, which in turn cluster into larger ones and so on (Tisdall and Oades, 1982; Hadas, 1987a,b; Dexter, 1988a). Soil aggregates, clods and blocks, can be defined as soil units which exhibit internal cohesion forces greater than the inter-units adhesion forces. The internal cohesion and external adhesion are greatest within and between the smallest units.This hierarchical order stems from the various soil solid constituents, binding materials, and modes of unit formation. The smaller the units are, the better packed or compacted, and the more homogeneous in their properties they are. Hierarchical ordering has been reported for: water stability of particles (Tisdall and Oades, 1982; Oades and Waters, 1991); unit size-strength relation (Dexter et al., 1985; Hadas, 1987a); unit size-heat capacity, water retention and thermal and water conductivities relations (Amemiya, 1965; Hadas, 1977; Horn et al., 1994); and gas diffusion (Glinski and Stepniewski, 1985). Water retention characteristics of aggregated beds of different aggregate sizes, and equal or differing bulk densities (Fig. 5a,b) and conductivi-

MATRIC POTENTIAL [kPa]

3 0.057 ‘,,‘**’ ,/““’ “““’ “““’ “‘1 1 10 100 1000 10000 1’00000 MATRIC POTENTIAL [kPa]

Fig. 5. Water retention characteristics of aggregated beds (a) the same bulk density BD (0.96 + 0.03 Mg md3) and aggregates of various mean diameter, d (mm); (b) the same mean diameter (d = 0.18 mm) and different bulk densities.


d=3.1; BD=0.96

d=0.72; BD=0.96 ~.-* d=0.37; BD=0.96

3% d=0.18; BD=1.43 = 1E-061:

0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 WATER CONTENT [m h 3/m A 31

Fig. 6. Water conductivities (cm/d) of aggregated bed as functions of aggregate mean diameter, d; bulk density, BD; and volumetric water content, (m’ mm3). The left side values in the legend are the aggregate mean diameters and the right ones the bulk density.

ties to water (Fig. 6) reflect the volume-weighted means of these properties. It is obvious that tillage-induced soil structure states may differ in their physical properties because of differing aggregate size distributions being associated with the same total porosity, or same size distribution with differing total porosities. Furthermore, the same tillage operation may produce different soil structural states, especially at or near the soil surface, and therefore have different effects on the water balance and aeration, (Marshal and Holmes, 1979; Hamblin, 1985).

Tillage improves infiltration, and the coarser the structure is, the higher will be the initial infiltration rates. These rates will be maintained provided the soil surface structure withstands slaking and raindrop impacts and does not disintegrate due to air entrapment or has no compacted, impervious layer below it (Bolt and Koening, 1972; Bowen, 1981; Hillel, 1982; Voorhees, 1992; Unger, 1993).

Tillage-produced ‘soil mulch,’ a structured soil layer, reduces soil water evaporation in a similar manner to plant residues mulch, provided its aggregate sizes range between 8.3 and 5.0 mm diameter and its depth is between 5 and 10 cm (Hadas, 1974; Jalota and Prihar, 1990). Due to the clear access of the top soil layers to tilling, and to the great effects it may be expected to have on crop establishment and soil water balance, greater attention is given to seedbed preparation rather than to soil structure modification in deeper soil zones.

Breaking shallow or deep traffic-compacted ploughsoles or hardpan layers, with an aim to improve soil water regimes and prepare root beds, produces cloddy, coarse structured soil. In order to change the coarse top layer and produce a seedbed, further tillage passes are needed. The finely structured seedbed has to be stabilized against destruction by raindrop impact into a sealing crust. These tillage passes may indirectly affect by compaction the partially prepared root bed.

22 A. Hadas / Soil & Tillage Research 43 (19971 7-40

4.1. Seedbed conditions and preparation

The topsoil layer, finely tilled to ensure soil conditions promoting fast seed gehna- tion, emergence, and uniform stand establishment is defined as the ‘seedbed.’ Various soil physical indices serve to define ‘preferred soil seedbed structure,’ e.g., shear strength, infiltration rates, porosity, sorptivity, weighted median aggregate diameter (Hadas and Russo, 1984b; Tennant and Hamblin, 1987; Braunack and Dexter, 1989) and the procedures required to attain it (Hadas et al., 1978; Braunack and McPhee, 1991).

Soil structure of a layer purposely produced as a seedbed, under either no-till or conventional practice, is a compromised solution, aimed to minimize risks of crop stand failure under anticipated future weather conditions, soil structure changes and resultant variations of soil environmental conditions, while minimizing energy and labor inputs. For each case, there are several possible solutions, derived by trial and error, because of climatic uncertainties and soil structure instability. Under warm, arid conditions, establishment of a stand requires good seed-soil water contact, and rapid water uptake by the seed in order to avoid the effects of rapid soil surface water evaporation and resultant hardening of rain-impacted soil surface structure into an impenetrable crust (Hadas and Stibbe, 1977). Priority is given to forming a fine, stabilized structured bed that ensures fast stand establishment. Under wet, cold conditions, improved aeration and drainage and enhanced soil warming are sought and attained by producing a porous soil structure. But faster soil warming due to lower heat capacity and thermal conductivity means greater surface temperature variations and lower heat flux and storage in the soil, which increase risks of night freezing.

Obviously, generalization of seedbed preparation practices would necessitates a delicately balanced, compound probabilistic approach, accounting for: (a) weather variations; (b) known probabilities of forming the right soil structure by given tillage practices; (c) deterioration rate of this structured soil, affected by traffic and weather conditions (Dexter, 1979); (d) the resultant changes in water, air and heat regimes around the germinating seed; and finally, (e) the probability of stand establishment. Such an approach eludes us because of our current incomplete knowledge and our inability to assess these probabilities, partly owing to lack of standardized characterization of the structured bed and its derived properties (Kuipers, 1984; Lal, 1991). Instead, current tillage practices are based on empirically proven long-term sustainability of agro-sys- terns under specific socioeconomical constraints. Therefore, at a given location, one or more tillage practices exist which are aimed at attaining the same goals (Hadas et al., 1978; Kuipers, 1984). Some will, and others will not, minimize tillage inputs; the former involve tilling just the 0.05 m top layer, the latter treat the soil well below the seedbed layer by tilling the soil down to 0.25 m or even deeper and finely tilling the top 0.05 m, thus, indirectly producing a coarsely structured layer in the upper part of the rootbed, which will affect the water and air transport and balance at deeper soil layers.

4.2. Root bed modification

The upper part of the rootbed is formed by primary tillage operations, usually carried out before winter time. Thus, allowing the coarse structure produced to break down into


finer structure by rain, wetting drying or freezing/thawing cycles. Seedbed preparation is carried out at a later time, e.g., spring season. The ability to till and manage the deeper soil volume of the root bed (to depths exceeding 0.5 m) is limited mechanically and economically. Consequently, these operations are performed when extreme amelio- rative measures are needed. Deep tillage increases the volume fraction of macropores that drain at - 10 kPa matric potential. Olsson et al. (1995), recommended the maintenance, by deep tillage, of effective air-filled and available soil water storage volumes comprising 10% and 20%, respectively, of the soil volume. Such cognizant optimization of the soil environment is theoretically attainable under field conditions for the upper part of the root zone, whereas control of interactions between the soil environment and the root system by tillage of deeper soil layers is currently very limited. Instead, reliance is placed on naturally formed soil structure inhomogeneities in the root zone in attempts to minimize deep soil compaction.

Modification of deep soil layers characterized by induced fracturing increases root density in these layers but roots are clustered in the fractures and are unable to penetrate the dense clods and draw stored water from them. Thus if soil modification increases yields, this benefit should be attributed to deeper air and water penetration and redistribution through large pores and cracks in the deeper zones of the soil profile, rather than to increased soil water storage. (Olsson et al., 1995). Reported effects of long-term deep tillage operations on root development and yields are inconsistent. These effects depend on site, crop, climate, implements used and their depth of operation, and the degree of fracturing and mixing achieved.

Furthermore, although root activity and water use were often improved, and deep tillage effects lasted several years, these effects also led to increased yield variability and did not add viability or provide an economic advantage attributable to these tillage practices (Stibbe and Hadas, 1977; Hadas et al., 1980; Unger et al., 1981; Eck, 1986; Jakobsen and Dexter, 1987; Campbell et al., 1988; Eck and Winter, 1992; Unger, 1993; Pikul et al., 1993; Cannel1 and Hawes, 1994; Nicoullaud et al., 1994; Merrill et al., 1996).

Systems aimed at soil and water conservation, labor and energy savings (i.e., no or minimal tillage) are characterized by producing denser soil, equal or slightly or sometimes significantly higher organic matter contents, and improved water infiltrabil- ity, due to bioactivity, as compared with those observed on conventionally tilled areas (Ehlers, 1975; Pidgeon and Soane, 1977; Dexter, 1988a,b; Mahabubi et al., 1993). They tend to be wetter, less aerated, slower to warm in the spring and early summer, and at times exhibit retarded crop growth and yields (Allmaras and Nelson, 197 1, 1973; Ellis et al., 1977; Russel, 1977; Ganzer and Black, 1978; Voorhees et al., 1981; Ehlers et al., 1983; Allmaras et al., 1986). Conventional practices have been found to be slightly better than direct drilling when yields, crop residue management and weed eradication were considered (Ball et al., 1994) for northern, cooler climates, not for warmer climates. In subtropical and tropical regions conservation tillage practices have fre- quently proven better.

Not all roots penetrating deep soil layers are active in water uptake. Root density distribution diminishes exponentially with depth, and root activity &pen& on root elongation rates, soil water content and root-soil contact. Tillage-induced soil fracturing


or compaction affects the distribution of these factors but not equally on all soil types. Root density distributions vary between soils and tillage practices but show low field-to-field variability on a given soil type. This pattern is invariant except on vertisols, where root distribution varies with rainfall between years because of varying aeration conditions (Nicoullaud et al., 1994). Major questions arise concerning the degree of fragmentation needed to store water, and the root density required to provide crop needs under newly tilled conditions. De Willingen and van Noordwijk (1987) suggested a range of root density of 2-6 cm me3 as the optimum, based on equating ‘crop-investment’ in root tissue to marginal water uptake needed. This estimate matches reported root densities under corresponding conditions (Jordan and Miller, 1980). The idea of ‘crop resources investment-partitioning’ is not new; amounts of pholosynthates invested in root formation and maintenance are estimated at 40-80% of those found in above ground parts. If exudates are added, the total may well exceed 100% (Boef-Tremblay et al., 1995). Scarcity of data dictates that greater research efforts are needed in this area (Fitter, 1991a,b). Such data are needed before suggestions can be developed concerning the achievement by tillage of desired root density-depth distributions, and the appropriate soil unit size and depth distributions. If ‘tailoring’ the root system through soil structure control could reduce crop photosynthates invested in roots, it offers a potential to greatly increase crop yields.

4.3. Soil fragmentation

Tillage implements exert an external stress onto the soil, causing it to fail in several different modes (brittle, shear, plastic, compressive), depending on initial soil conditions (bulk density, water content and existing fractures or cracks). The extent, mode and fineness of soil failure determine the quality of the soil structure produced. Although tillage implements have been in use for generations, their mode of operation in soil is only partly understood. The operation mode of tined implements, in contrast to that of mouldboaxd ploughs, is theoretically predictable with respect to the energy required and the form of the general failure planes produced. These predictions are based on simple soil mechanical principles, and have been validated in laboratory soil bins (Payne, 1956; Koolen, 1977; Godwin and Spoor, 1977; Spoor and Goodwin, 1978; McKeys, 1985). The forward movement of the implement in the soil causes the soil to fail, first by block displacement forward, sideways and upwards along the major failure planes which extend from the implement tip to the soil surface. Secondary fissures are formed in the sliding block, which in turn, is further fragmented by the implement shank or bottom.

Soil fragmentation is also caused by internally induced stresses (swelling/shrinkage during wetting/drying cycles) or volume changes forced by freezing water ‘lenses’ during soil freezing/thawing cycles (Dexter, 1988a,b). These processes are utilized to break down coarse soil structure, formed by primary deep tillage operations performed prior to winter or rainy seasons.

Application of classical soil mechanics principles suggests that soil fragment sizes produced by tillage are independent of shear and cohesion, and depend only slightly on the angle of soil internal friction (Payne, 1956). Alternatively, it has been suggested on the basis of observations carried out mainly in soil bins (with only a few in the field)

A. Hudas/Soil & Tillage Research 43 (1997) 7-40 25

that soil is loosened by shear, which overcomes soil cohesion and overburden stress (Stafford, 1981; McKeys, 1985). Field soil which is periodically compacted and retilled is composed of neatly arranged clods and blocks. When tilled, these blocks are torn and moved sideways and upwards, but no apparent secondary failure zones, or secondary fragmentation could be observed in dry or slightly moist soil, containing 15% or more expandable clay and around 50% of the plastic-limit moisture content (Hadas et al., 1978; Hadas and Wolf, 1983). Recent observations have suggested other factors to be responsible for soil primary fracturing and fragmentation (Braunack et al., 1979; Utomo and Dexter, 1981; Hadas and Wolf, 1984; Wolf and Hadas, 1987; Snyder and Hadas, 1992; Hallett et al., 1995). An experimental procedure for studying interactions between soil and tillage implements and the forces causing fragmentation, and for providing some predictive capacity, has been developed but has not been applied (Koolen, 1977). Currently, data on tillage operations and resulting soil fragmentation are obtained from field and laboratory experiments (Dexter, 1977; Koolen, 1977; Hadas et al., 1978; Hadas and Wolf, 1983; Snyder and Hadas, 1992; Perdok and Kouwenhoven, 1994). Stresses applied to a soil concentrate at those cracks or pore tips oriented in the general direction of the applied stresses. Local failure occurs once the strain energy at the pore tip exceeds the local cohesive forces, and the pore or crack extends (Anderson, 1995). Many such local failures may occur before a complete failure (Hadas, 1987a; Hadas and Lennard, 1988). Complete failure occurs when the cracks extend across the stressed soil unit and the amount of energy released is proportional to the new surface areas produced. The stress distribution in a particulate material, e.g., soil or aggregated soil, varies around a mean, not necessarily following a normal distribution; some sub-volumes will be subjected to a lesser, others to a much higher, stress than the mean stress applied (Harr, 1977; Anderson, 1995). Soil-unit size distributions or scale-dependent inhomogeneous pore size distributions exist in soils, especially in structured ones, and it seems that due to nonuniform stress and pore-tip distributions, the probability of fracture failure depends on the joint probabilities of: a stress being greater than that of the local ‘surface tension’ at the pore tips (e.g., due to bonding or cementing of soil particles); the stress being applied to the ‘right’ pore, and of the available strain energy being sufficient to bring about a complete failure (Anderson, 1995). Soil structural units show an inverse hierarchical order; the strength of a unit increases as the volume of the units decreases. These relationships are described either by Weibull’s model (Braunack et al., 1979; Hadas, 1987a,b) or by the modified Weibull-fractal cube model (Perfect and Kay, 1995a,b) and were obtained by crushing aggregates and clods. There are no direct methods either to determine the strength of the soil units in situ, or to predict the expected degree of fragmentation. Instead, these properties are estimated by measuring the energy input via the tillage operation and determining the pre- and post-tillage aggregate size and surface area distributions produced at different soil water contents (e.g., Table 3) (Hadas and Wolf, 1983, Wolf and Hadas, 1987). Proper soil fragmeuta- “Lion will probably be best achieved by using knowledge of the natural spatial distribution of the soil units, and of the hierarchical order of their strength, to direct the stresses to the various weaker soil zones, thus causing stepwise failure in flaws, e.g., pores and cracks (Snyder and Hadas, 1992; Perdok and Kouwenhoven, 1994; Hallett et al., 1995). Due to scale-dependent inhomogeneities in pore sizes, and distributions of cracks, root

26 A. Hudas / Soil & Tillage Research 43 (1997) 7-40

Table 3 Energy input in tilling soils, En (k&I mm’) at different water contents, P,, 100 X (kg kg-’ ), geometric mean aggregate diameter, D (mm), and the geometric standard deviation, G,,

Soil Operation 4 PW D Gsd

Clay Mouldboard 22.09 20.293 6.7k2.1 144 17.4 Ploughing 15.9750.451 14.3 f 3.2 33 8.7 Ploughing 20.70 f 0.358 21.5 + 4.2 10 7.4 Ploughing 17.59+0.118 33.2k2.1 10 6.9

Sandy Ploughing 38.42 f 0.279 4.3 f 1.7 45 50.7 loam Ploughing 36.28 ItO. ll.lk3.4 5 5.6 Clay Ploughing (20 cm) 16.64 11.6k3.1 8 8.8

Ploughing (20 cm) 22.09 7.4+2.9 11 7.6 Ploughing (35 cm) 38.03 11.6k3.1 10 12.5 Ploughing (35 cm) 45.57 7.4*2.9 3 6.7 Subsoiling (35 cm) 29.94 11.6k3.1 9 14.6 Subsoiling (35 cm) 31.21 7.4 f 2.9 6 10.4

channels, and bonding materials, unconfined or under partial confinement, a cascading array of brittle failures would be expected. Such predicted, stepwise relations between applied energy release and soil fragment size, appear as periodic oscillations in the drawbar pull during tillage, and have been observed in field soil (Glancey et al., 1988, 1996).

Spoor and Goodwin (1978, 1979) showed that brittle failure can occur in wet clay samples once the soil is stressed beyond the ‘super-critical zone’ (in critical state-mechanics terms). A criterion for a tined implement to produce a desired soil structure by causing super critical failure in a soil was developed by Spoor (1975). Breakup of clods under tension then depends on the relative balance of strengths between the bulk soil and the initially formed individual clod (Hettiaratchi, 19881, but fragment size distributions cannot be predicted. If an implement imparts low-shear, high normal stresses to a confined soil (a deep layer), the soil will be compacted rather then fragmented, provided these stresses exceed the magnitude of the stresses previously applied at the same or higher water contents. If instead, high-shear, low normal stresses are applied, under no or low confining stress, the soil will break up into large clods with no apparent change in volume. In the first case, the soil will yield along the Roscoe surface, while in the second, it will follow the Hvroslav surface to the tension cutoff and will be fragmented (Hettiaratchi, 1988). These observations explain the low efficacy attained in fragmenting deep soil layers which are normally confined, wet and pliable. A finely fractured soil is attained by means of tine implements, operated at minimal soil confinement, in soil of the right water content, at a depth above the implement’s critical depth, and stressing the soil sideways and upwards (Spoor, 1975; Spoor and Goodwin, 1978, 1979; Stafford, 1981; Blackwell et al., 1987; Spoor and Fry, 1983). To till effectively below the implement’s ‘critical depth,’ the operation should be performed stepwise, with depth increments, and at soil water content in the range of 0.6-0.9 of the lower plastic limit of the soil for silty and clay soils, respectively (Stafford, 1979). Consideration of the facts suggests that because of cracks and faults formed in the soil by expansion and decay of roots and plant residues, freezing/thawing and drying/wetting cycles, it is possible by


minimizing soil compaction to promote production of finely structured soil beds and to conserve energy.

4.4. Soil structure stability-formation of crusts and compacted layers

When a soil structural state at equilibrium is perturbed by tillage, a destabilized, new structural state is produced. Its degree of instability will increase the finer, fresher and the larger new surface areas of the tilled soil. The freshly tilled soil structure generally starts changing toward a new equilibrium state, not necessarily the one desired for high yields. Under natural, rain drop impact, swelling/shrinkage, or external loads, the deterioration rate is greatest at the soil surface, and diminishes with soil depth. Soil surface aggregated structure usually disintegrates and ‘ vanishes’ during the same season (Keller, 1970; Dexter, 1977, 1988a,b), but cloddy structure of deeper tilled layers may persist through several seasons (Stibbe and Hadas, 1977; Hadas et al., 1980). Several mechanisms or processes induce changes in the new tilled soil structure. These are categorized as: (a) ‘natural’ processes, which include raindrop impact, cyclic wetting and drying, freezing and thawing, breakdown of aggregates by air entrapment and explosion, slaking, and loss of biobinders because of decomposition of roots, hyphae, and organic ‘cements’; and (b) soil compaction induced by tillage practices and vehicular traffic. The upper soil layers are affected by natural and traffic induced stresses, whereas deeper soil layers modified by tillage fail, either because of soil compaction, if these stresses penetrate to that depth (Horn et al., 1994; Horn, 1995) or through ‘creep,’ a very slow failure induced by overburden stresses and water content changes which cause localized swelling and drying as well as by soil water uptake by roots.

4.4.1. Suface seal or crust formation Crusts are formed when the aggregated bed at the soil surface disintegrates under rain

drop impacts, slacks by wetting and air entrapment, and is rejoined by the surface tension of water menisci during evaporation (Bolt and Koening, 1972; Keller, 1970; Bradforfd and Huang, 1992; Hillel, 1982). The crust formed consists of several layers: an upper, very thin, compacted one; an interim layer, the structure of which is slightly changed and which receives added deposits of loosened and entrained surface soil; and the lower one of intact tilled aggregates. This sequential order of layers is very effective in reducing: (a) air exchange and water infiltration, because of the crust’s density and very low conductivity to water, and free air porosity, and (b) its resistance to seedling emergence, owing to the high strength attained during drying (Hadas and Stibbe, 1977; Bowen, 1981; Hadas and Frenkel, 1982; Bradforfd and Huang, 1992; Mualem and Assulin, 1992; Morin and van Winkler, 1996). The deterioration rate of soil structure and the final degree of equilibrium of the crust formed, as well as the associated changes in hydraulic and mechanical properties, are usually related to soil constituents, soil type, soil strength, and tillage operations. Eventually, they affect and control various processes, e.g., runoff, infiltration, and seedling emergence (Bradforfd and Huang, 1992). Xnitial infiltration rates into a freshly tilled surface are rather high, but they fall exponentially as the crust forms and stabilizes. After some drying, the initial infiltration rate which had deteriorated is partially restored with successive rainfalls (e.g., Table 4)


Table 4 Initial, i,, final, i,, infiltration rates (mm h-l), and soil stability S, (mm-‘) into a sandy loam soil, for wetting and varying drying duration (days), after Morin and Byniamini (1977)

Drying duration ii lf ss

0 (wetting) 320 8 0.106 1 50 5 0.70 6 160 8 0.160 11 170 8 0.160

i = i, + (ii - i,) exp (- sS Rt) where R is rain intensity, f is time, and i is infiltration rate.

because shrinkage causes the crust to crack (Morin and Byniamini, 1977). The rate of and total seeds emergence through the crust, and crust penetration by adventitious roots are gravely affected by the plants’ water uptake, the point stress the shoot and roots exert on the crust, and the crust’s tensile strength and cohesion (Hadas and Stibbe, 1976; Goyal et al., 1980a,b, 1981, 1982; Bowen, 1981). A common tillage practice which improves water infiltration and seedling emergence through crusted soils is to break the crust with light implements such as rotary harrows. Alternatively, the soil surface may be covered with plant residues to try to prevent or delay the formation of the crust. Few studies have tried to deal with the mechanisms and processes of crust formation; the bulk of reports explain the observed phenomena, qualitatively, rather than quantifying them (Bradforfd and Huang, 1992).

In order to predict the formation of a crust of known characteristics there is a dire need to: (a) formulate in the sequential order of occurrence the processes involved in crust formation, by applying soil physical, chemical and mechanical principles; and (b) use probability based data sets pertaining to rainfall characteristics, and wetting and drying cycles between successive rain events (Bradforfd and Huang, 1992; Mualem and Assulin, 1992).

4.4.2. Soil compaction Tillage operations, carried out to alleviate previous soil compaction and soil resis-

tance to root penetration, and to improve the water balance of the crop, are traded off against the deleterious effects of soil compaction by traffic and implement passes (Hadas et al., 1983, 1988). Increases in soil bulk density are responsible for reduction in pore diameter and volume, reductions in infiltration, water redistribution and storage, and air exchange, and increases in resistance to root penetration (Bowen, 1981; Gupta et al., 1990; Unger and Kaspar, 1994). The higher the applied stress at the soil surface or the wheel contact pressure in a furrow, the larger the contact area for a given stress is. Consequently, the deeper, larger and denser the compacted soil volume will be. Shallowly placed compacted layers becomes detrimental to root development and inhibit the crop from reaching its potential development and yield. Furthermore, they induce greater spatial variability in water use across fields (Hadas et al., 1985, 1990). The effects of soil compaction on soil water and air transport properties are complex and can not be characterized by simple, unique functions of bulk density, water- or air-filled porosity. Instead, they are multi-valued functional relationships dependent on the way


they were derived. Aggregated beds are especially sensitive to compaction (Fig. Sa,bFig. 6). The ratio between shear and normal stresses at soil failure is correlated to these relations; the greater that ratio is, the lower the water and air conductivities will be for a given bulk density (Davidovski and Koolen, 1986; Gupta et al., 1990). Breaking up

these compacted layers by tilling them is a common practice, provided they are accessible to tillage implements and the operation is economically feasible. The deeper they are located, or the harder and denser they are, the more difficult it will be to alleviate their deleterious effects on soil conditions, and crop development and yield (Bowen, 1981; Unger et al., 1981).

Complete alleviation of compaction-induced crop stresses and impaired soil conditions by tillage operations is, however, impossible. Impaired crop stands, caused by a coarser seedbed, led to improper vegetative and reproductive crop development and lower yields have been reported on compacted, tilled plots compared to non-compacted tilled ones (Hadas et al., 1983, 1985, 1990).

A favorable soil structure must be stabilized, at least for its designated period of high efficiency, especially at the soil surface. Improvement and stabilization of a freshly tilled soil structure can be achieved in practice by increasing the organic matter content of the soil, mulching, adding amending chemicals, incorporating plant residues, and by use of proliferous root networks (Snyder and Hadas, 1992; Bathke et al., 1992; Kleinfelder et al., 1992; Horn et al., 1994; Jayawardane et al., 1995). Soil structure stability is generally attributed to soil constituents and a host of processes and mechanisms. But any attempt to define the role and importance of each of these constituents, processes and mechanisms in determining soil structure stability reveals a wide gap in our knowledge that necessitates future research efforts.

4.5. Soil structure indices-crop yield relations

Different tillage sequences and/or passage of different implements over a given field often produce the same ‘tilth.’ Likewise, a given practice at different times may yield myriad forms of soil structural states that are difficult to characterize and relate directly to crop yields (Hadas et al., 1978, 1980; Unger et al., 1981; Snyder and Hadas, 1992; Unger and Kaspar, 1994). Several attempts have been made to quantify ‘Soil Tilth’ and formulate a ‘Tillage Index’ (Colvin et al., 1984), or ‘Tilth Forming Processes’ or ‘Soil Tilth’ (Karlan et al., 1990) and a ‘Tilth Index’ (Singh et al., 1992). Different soil descriptors were incorporated into the various indices (bulk density, resistance to cone penetration, soil plasticity, depth of primary tillage, surface roughness, etc.), all selected for their ease of determination and availability of correlations to plant responses. Reported correlations between crop yields and ‘Tilth Index’ determined at different times of the year show great variability. This variability reflects the futility of correlating soil ‘tilth’ data collected at a few fixed times during a cropping cycle to yields. The task of continuously collecting pertinent data on soil-structure-dependent descriptors, soil physical and mechanical properties, and correlate them with the crop responses, integrated over the whole growing season with respect to fluctuating weather conditions and varying soil structure dependent soil environment, is formidable. Combining previous attempts to define ‘soil tilth’ and the conclusions of the previous sections leads to a


conceptual definition of ‘Soil Tilth’ as, “ tillage affected, quantifiable soil structural- state-dependent attributes governing and controlling a soil environment, favorable to crop production.” This broad definition means that many management practices will eventually produce a ‘desired tilth,’ or a given practice may yield many different, so-called ‘desired tilths.’ The choice of the most effective tillage practice to attain a given tilth may vary, not only between different agro-ecological environments, but also within the same ecological environment.

4.6. Characterizing soil structural state or tilth

The dual nature of the definition given for soil structure implies that characterizing soil structure or a structural state according to a single value descriptor, either the solid matrix or the evolved pore network (bulk density, aggregate mean diameter, porosity) may be valueless. Characterization of any structure-dependent soil property distribution (aggregate- and pore-size distributions, water retention) yields at least two or more parameters, specific to that distribution. Processes-oriented, structure-derived properties (e.g., water conductivity) require a more complex characterization, of the form of either a vector or a tensor if the soil is homogeneous or nonisotropic, respectively. Most methods currently in use for measuring soil properties are destructive in nature (e.g., core sampling); others induce soil structure changes during the measurements (e.g., infiltration test). However, if predictive methods are used, a measured validation by any of the above and other methods is a must. The current methodology of accumulating data on an intermittent basis yields valuable information with reasonable accuracy, assuming that the measured property changes continuously with time. This sporadic data collection has great inherent uncertainty in actually monitoring the processes under consideration and in the accuracy and pertinence of the data to the processes involved, uncertainty that may lead to misinterpretation of the results obtained. The choice of the soil properties to be determined and the methods to quantify them with respect to changing soil-structure attributes requires nondestructive or minimal destruction type methods that are sensitive enough to detect slight changes in soil structure. In addition, we must be able to monitor changing conditions and processes involved at the appropriate length, area or volume scales (e.g., for a process occurring near a root; the size scale will be few mm, whereas rainfall infiltration occurs on a scale of hectares) (Hamblin, 1985; Williams et al., 1987; Addiscot, 1993). In view of the tremendous amount of information that must be collected to characterize and quantify the ‘E Ii1 environment favorable to crop needs,’ the general tendency is to use models to h lp predict and continuously describe that changing crop habitat and its responses. Under these constraints, it is obvious that great efforts must be made to measure the needed properties continuously in the least destructive way and at several size scales, to enable validation of models at different size scales (Addiscot, 1993). Since tillage operations and subsequent structure deterioration change the soil surface level and internal porosity and material distributions, it is necessary to determine soil structure and related properties with respect to normalized levels or as a deformable matrix within moving material coordinates. This can be attained if the tilled layer is characterized by the

A. Hadas / Soil & Tillage Research 43 11997) 7-40 31

Table 5 Saturated hydraulic conductivity K,,, (m day ’ ) dependence on initial matric potential (MPa), void ratio, e (m3 rnm3) attained by collapsing aggregate bed (2-5 mm)

Initial soil matric potential (Mpa) e - 1.5 -0.5 -0.1 - 0.033 - 0.010

Soil type K sat

Sandy loam 1.14 8.7 12.4 10.3 0.91 6.7 6.3 8.1 0.73 1.05 0.36 0.02

Silty loam 1.06 1.7 0.7 0.6 0.13 0.11 0.89 0.8 0.3 0.21 0.07 0.24 0.74 0.031 0.0057 0.0025 0.0004 0.00009

Clay 1.1 8.3 6.7 4.6 0.92 0.98 0.88 0.31 0.072 0.0046 0.78 0.051 0.0008 0.00005

material coordinate m given in Eq. (5), where m is the amount of solids between the surface and depth, z, and E(Z) is the porosity at the depth z (Addiscot and Dexter, 1994).

m = lz[l - s( z)]dz (5) This approach is in essence similar to that proposed by Kim et al. (1993) to describe

the hydraulic regime of a swelling soil matrix. Since soil structure undergoes volume changes as it subsides to its ‘equilibrium state,’ it is interesting to examine whether the relation between soil water properties and the ‘voids ratio’ (the ratio between soil porosity and soil solids volume) are unique, single-valued functions, as assumed (Phillip, 1967). Data presented in Table 5, which suggest that the paths these changes follow and ,,e range of these structural changes, determine the observed values of the properties. In this case, there is no unique, single-value relationship, a fact that confirms the need for more research to develop new, innovative concepts and methods to characterize soil structural states and derived soil properties.

5. Needs for future research

Vast amounts of experimental and theoretical work have accumulated concerning tillage practices and crop systems. Yet, our knowledge of the cause-effect relationships between soil structure manipulation and crop responses above and below ground resembles an incomplete jigsaw puzzle. The lack of information on both specific and indirect effects of tillage operations on soil environmental conditions, root system development and crop response is due to: (a> a very small number of very detailed experiments of integrative nature; and (b) existin g. incomplete data sets which combine studies where sporadic noncontinuous sampling and measurements of soil, root, crop and weather data have been carried out. This research deficiency can be attributed to the complexity and the dynamic nature of such studies, characterized by the need to monitor a large number of parameters and their interactions continuously. Apart from this


insurmountable task of data collection, most current soil and root sampling methods are

destructive, and introduce methodological errors due to soil structure changes &ring

measurements. Furthermore, these parameters, which are measured spora&cally, differ

in their size and time scales, and vary greatly in their spatial variabilities. To overcome these major obstacles, nondestructive methodologies-even perturbing ones-must be developed for monitoring soil tilth variations and associated root responses to measured changes in soil environmental conditions. These requirements include methods for following procedures.

1. Root and pore observations: deduction of soil water properties from in situ observations of pore and crack arrangements and connectivity; measurements of root elongation rates, degrees of root branching, root-soil contact, aggregate size distributions and water uptake. Observations of soil structure changes will provide probabilities of tilth stability, and will facilitate predictions of changes in soil hydraulic properties. The use of images in a geometrical similitude approach, namely fractal geometry-based techniques, should be expanded to estimate degrees of soil fragmentation occurring in response to tillage operations, and the response of soil physical properties and root architecture to spatial variations in soil structure (Dexter, 1978, 1979; Fitter, 1991a,b; Rieu and Sposito, 1991a,b; Perfect and Kay, 1995a,b; Anderson et al., 1996).

2. Quantification in situ of the soil structural state, namely attempted estimation of the size, density and strength distribution of the soil units, prior to tillage operations. In other words, methods to estimate the density and length of existing flaws and cracks and to determine soil unit strength hierarchies and orders of size before tillage operations are carried out. High-resolution, sensitive ultrasound, X-ray, y-radiation, and nuclear mag- netic resonance imaging technologies might provide promising methods for such investi- gations. Another way to determine in situ arrangement of soil units in natural soil structure arrangement is to use on-line Fourier analysis of direct drawbar draught readings to determine their harmonics and wave lengths, which mark the size and strength distributions of fragmented soil units.

3. Owing to the current absence of such methods, modes of attaining new soil tilth conditions from known initial soil states can be tested empirically by tilling the soil at several different soil water contents, using different implement sequences, determining energy input, and distributions of fragment and pore sizes and new surface area. Analyzed data should yield probability type estimates of observed soil structure features and correlate them with soil physical properties as listed above (Snyder and Hadas, 1992; Wagner and Ding, 1993; Roytenberg and Chaplin, 1995). Another direct experimental approach to prepare predetermined soil structural states and observe resultant soil physical properties, root responses and crop development was suggested by de Freitas et al. (1996).

4. From data obtained from sequential tillage trials as described above, soil resistance to tillage, soil units lengths and aggregate size distributions can be calculated. Probabili- ties of attaining given aggregated beds can be computed for each given set of soil conditions, for each tillage pass and the related to energy inputs, agropractice purpose and implement sequence. This tedious, expensive, time- and energy-consuming methodology would be simplified if nondestructive methods were applied to reduce the need for observations of some parameters (e.g., soil unit sizes). Further improvement should arise


from theoretical developments to make possible the definition and quantification of soil fragmentation processes. These theoretical efforts should evaluate the use of developments in understanding behavior of fragmented materials under stress and their yield conditions (Anderson, 1995).

5. Soil structure deterioration and resultant temporal changes in derived physical properties should be examined and treated theoretically, similarly to the case of deformable porous materials (Phillip, 1967; Kim et al., 1993; Addiscot and Dexter, 1994).

6. Combined analysis of observed time-related changes of soil pore systems and root development patterns by means of incomplete fractal algorithms may lead to improvement of the current level of understanding of the complex, dynamic nature of the interactions between ‘soil tilth’ and root system. This would facilitate computation of ‘crop costs’ for root development under varying ‘soil tilths’ (Fitter, 1991a,b; Rieu and Sposito, 1991a,b; Perfect and Kay, 1995a,b).

Acknowledgements

The author would like to thank Dr. W.E. Larson of the Dept. of Soils, University of Minnesota; Dr. R.R. Allmaras, USDA-AR& St.-Paul, MN, Drs. R.M. Cruse and J. Swan of the Dept. of Agronomy, Iowa State University; and Dr. D. Or of the Dept. of Soils and Biometeorology, Utah State University, and the anonymous reviewers, for their helpful, notes, remarks and comments during the preparation of this article. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 1947-E, 1996; and the Dept. of Agronomy, Iowa State University where parts of this manuscript were written during the sabbatical leave of the author.

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soil tilth—the desired soil structural state obtained through proper soil fragmentation and...

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