J Sci Food Aqric 1991, 56, 247-257

SCI Agriculture Group Symposium The Scientific Basis to Sustainable Agriculture

The following are summaries of papers presented at a joint meeting of the Agriculture Group of the Royal Society of Chemistry, the British Society of Soil Science and the SCI Agriculture Group, heldat X I , 14/15 Belgrave Square, London SWIX8PS, UK, on 15 March 1951. The papers published here are entirely the responsibility of the authors and do not reflect the views of the Editorial Board of the Journal of the Science of Food and Agriculture.

Sustainable Agriculture and Soil Erosion

R J Rickson

Silsoe College, Silsoe, Bedfordshire MK45 4DT, UK

Soil erosion is a natural process. The environment is in equilibrium when rates of soil erosion are in balance with rates of soil formation. However, many present-day intensive agricultural practices and land uses incompatible with land capability (such as intensive arable production on shallow soils and on steep slopes) accelerate soil erosion rates and disrupt this fragile balance.

Loss of topsoil by rainsplash, sheet, rill and gully erosion results in: the reduction of effective rooting depth, soil water holding capacity and available water capacity for the crop; increased soil compaction and crusting; and declining levels of soil organic matter, trace elements and nutrients (especially N, P and K, which are preferentially removed by the erosion process). Through these changes, erosion affects the productivity or production potential of the soil and threatens sustainable production from the land, as evidenced by strong negative correlations between crop yield and cumulative erosion. Declining yields are associated with less crop cover to protect the soil from rainfall and runoff, causing erosion to accelerate further.

The situation is complicated when loss of productivity is masked by the introduction of new technology such as increased fertiliser applications, improved crop varieties or irrigation. Doubt has been cast as to whether these counter- measures are adequate substitutes for the lost soil or are simply ‘cosmetic interventions’. There is evidence to suggest that addition of inorganic fertilisers will not bring crop yields back to their pre-erosion level. Replacement of natural

241

248 SCI Agricult urc Group Symposium

fertility with inorganic fertilisers may maintain yields in the short term, but the cost to be paid in the future will be poorly structured soils, which are highly susceptible to further erosion, thus requiring forever increasing fertiliser inputs and their associated costs. Such dependency on expensive fertilisers may not be economically sustainable. There are also problems associated with where and at what rate the fertiliser should be added; erosion processes do not operate uniformly over a field. Attempting to sustain agricultural production with increasing rates of inorganic fertilisers may result in pollution by nitrates and phosphorus downstream.

Long-term research is required to represent the relationships between soil erosion and agricultural sustainability, and few scientific studies exist at present. However, attempts to quantify the threat to sustained agricultural production have been made. For example, Stocking and Pain (1983) developed a model which calculates the productive lifespan of a soil when subjected to erosion. The input parameters include depth of available productive soil, minimum soil depth for a particular crop, the estimated rate of soil formation and the predicted rate of soil loss. Calculations using the model show that some agricultural soils in the UK are eroding at a rate which implies their effective lifespan may be less than 20 years.

Agricultural production can only be sustained in the long term if the soil resource is conserved, so that soil loss is in balance with soil formation. Soil conservation measures, including erosion control, should be designed with the minimum of cost and inconvenience to the farmer.

Rejerence Stocking, Pain 1983.

Long-term Chemical and Biological Effects of Agricultural Practices

K E Giller" and S P McGrathb

"Wye College, University of London, Wye, Ashford, Kent TN25 SAH, UK 'AFRC Institute for Arable Crops Research, Rothamsted Experimental Station. Harpenden, Herts AL5 2JQ, U K

In a truly 'sustainable' agriculture the nutrient outputs, in the form of agricultural products and losses by leaching or to the atmosphere, must be balanced with nutrient inputs. If less chemical fertilisers are to be used, we need to increase the efficiency of nutrient cycling in agriculture. Ways in which we can do this are to increase the use of organic manures, both animal manures and sewage sludge, and to increase our reliance on biological N, fixation. But organic manures often contain heavy metals, and although legislation has been passed to protect our soils from contamination recent evidence suggests that further steps may be necessary to protect the fertility of our soils for the future.

A balance sheet has been constructed for heavy metals added and removed from

The scientijic busis to sustainable agriculture 249

a long-term field experiment which was established at the Woburn Experimental Farm in 1940. Addition of metal-contaminated sewage sludges ceased in 1961, and 25 years later over 80% of the metals which were added to soil remain in the topsoil (McGrath and Lane 1989). Thus, once heavy metals are added in organic manures, they will persist in soil far into the future.

Recent research has shown that heavy metals are highly toxic to soil microorganisms, and particularly to several important groups of microorganisms that are likely to be of increasing importance in maintenance of production with minimal inputs (Giller and McGrath 1989). In metal-contaminated plots the size of the microbial biomass was halved (Brookes and McGrath 1984) and nitrogen fixation by blue-green algae was absent for the first two months (Brookes et al 1986). Nitrogen fixation in clover was completely suppressed (McGrath et al 1988) due to toxicity to Rhizobium (Giller et a1 1989). Infection of clover roots by vesicular-arbuscular mycorrhizal fungi (VAM) was delayed, even when an inoculum of VAM was added (Koomen et al 1990). All of these effects have been seen in soil which was contaminated with heavy metals in sewage sludge more than twenty years ago, and which now has soil concentrations of heavy metals close to guidelines in the current CEC directive (86/278/CEC) and UK legislation (Statutory Instrument No 1263 1989). Similar effects have recently been reported in metal-contaminated soils in Sweden where the size of the microbial biomass, the activity of free-living nitrogen-fixing bacteria and the numbers of Rhizobium were all reduced (Martensson and Witter 1990).

There is currently pressure to revise the legislation in the UK to take account of effects on soil microbial processes described above, but we do not yet know which of the cocktail of heavy metals often present in sewage sludges is responsible for these effects. A cause for concern is the lack of regulations to restrict the use of land which is already contaminated with heavy metals for agriculture.

References Brookes P C , McGrath S P 1984 Effects of metal toxicity on the size of the soil microbial

biomass. J Soil Sci 35 341-346. Brookes P C, McGrath S P, Heijnen C 1986 Metal residues in soils previously treated with

sewage-sludge and their effects on growth and nitrogen fixation by blue-green algae. Soil Biol Biochem 18 345-353.

Statutory Instrument No 1263 1989 The Sludge (Use in Agriculture) Regulations. HMSO, London.

Giller K E, McGrath S P 1989 Muck, metals and microbes. New Scientist 124 31-32. Giller K E, McGrath S P, Hirsch P R 1989 Absence of nitrogen-fixation in clover grown

on soil subject to long term contamination with heavy-metals is due to survival of only ineffective Rhizobium. Soil Biol Biochem 21 841-848.

Koomen I , McGrath S P, Filler K E 1990 Mycorrhizal infection of clover is delayed in soils contaminated with heavy metals from past additions of sewage sludge. Soil Biol Biochem 22 871-873.

Martensson A, Witter E 1990 The influence on biological nitrogen fixing micro-organisms of various soil amendments in a long-term field experiment, with special reference to sewage sludge. Soil Biol Biochem 22 977-982.

McCrath S P, Lane P W 1989 An explanation for the apparent losses of metals in a long-term field experiment with sewage sludge. Enuiron Pollut 60 235-256.

McGrath S P, Brookes P C, Giller K E 1988 Effects of potentially toxic metals in soil derived from past applications of sewage-sludge on nitrogen-fixation by Trifolium repens L. Soil Biol Biochenz 20 415-424.

250 SCI Agricul/urr Group Symposium

The Maintenance of Soil Fertility in Organic Farming Systems-Some Observations from the Rotharnsted Long-term Experiments

A Edward Johnston

AFRC Institute of Arable Crops Research, Rothamsted Experimental Station. Harpenden, Herts AL5 2JQ, UK

Soil chemical properties which can be subsumed within soil fertility can be put into two categories. Some properties such as pH, P and K levels and soil organic matter or humus can be considered long-term aspects because they change only lowly over time. However. they provide the framework on which the short-term effects of nitrogen inputs and agrochemicals can be used to achieve any desired level of productivity up to the yield potential of the site. Some inputs widely used in current farming practices to maintain both soil fertility and productivity are not allowed in organic farming systems. I t is important to have a realistic assessment of the benefits and limitations of the permitted inputs.

In a number of experiments at Rothamsted, Woburn and Saxmundham, changes in soil fertility, as estimated by soil analysis, can be followed over long periods with treatments acceptable in organic systems. In a few cases effects of treatment on yield over 20 to 70 years can be assessed.

Maintenance of soil pH, for arable crops 6.5, for grassland 6.0 (in water), is a first essential, and using magnesium limestone maintains soil magnesium levels. When yields of arable crops are related to P soluble in 0.5 M NaHCO, they invariably reach a plateau at a P value which varies between crops but for any crop is independent of seasonal effects on yield. Where organic farming systems are started on soils with soluble P above the critical values, the problem will be to maintain existing values. For the same amount of total P added, some organic manures, such as sewage sludge, are less effective at maintaining soluble P than, for example, farmyard manure (FYM). The use of an inappropriate extractant, such as a dilute acid, can give a very erroneous estimate of P fertility where rock phosphates are applied to neutral or calcareous soils. When soils have the same amount of bicarbonate-soluble P derived from additions of superphosphate or FYM, then more of the P in FYM-treated soil is soluble in 0.01 M CaCI,. This may explain why P leaches from such soils. Potassium in most organic manures is water soluble and behaves like K in water-soluble fertilisers. Increasing the humus content of soil provides additional cation exchange sites. Most minerals release too little K to meet crop demand. The relationship between the increase in humus and the amount of organic matter added is independent of the type of organic matter added but is dependent on soil texture. The amount and type of organic matter together with its time of addition appreciably affects not only yields but also amounts of nitrate in soil at risk to loss by leaching.

Over a 72-year period the productivity of a Norfolk four-course rotation, in

The scientific basis to sustainahle agriculturr 25 1

which the only agricultural nitrogen input was from a 1-year clover ley or beans, was critically dependent on maintaining the P status of the soil. It was about 40 years before K deficiency was suspected, because the soil contained about 25% clay which released some K. Maintaining appropriate levels of readily soluble P and K in organically farmed soils is likely to be a major problem. Soil nitrogen supply reflects the organic matter status of soil. Ensuring that mineralisation will meet the nitrogen demands of the crop whilst minimising nitrate at risk to loss by leaching is a major concern to all.

Nutrient Balances in Organic Agriculture

C Stopes and L Philipps

Elm Farm Research Centre, Newbury, Berkshire RG15 OHR, UK

A seven-fold increase in the input of nitrogen fertilisers to UK agricultural land over the past 40 years has increased food production. However, it has also enabled a transformation of farming systems in those areas of the country now experiencing problems of nitrate and agro-chemical leaching to ground and surface waters. The implications of these large-scale but regional changes in farming systems are considered. The paper focuses upon the nitrogen balance achieved by organic farming systems, since this is the most significant nutrient where the approach to supply and utilisation distinguishes conventional from organic systems.

The organic system (as defined in Standards, verified by regular inspection and certified by approved bodies) aims to achieve a balance between the supply and demand of nitrogen through natural fixation by legumes included in a multi-annual rotation. The import of nitrogen on to the farm is restricted whether in the form of feed, manures or other fertilisers. The legume is usually included in the rotation as a grass/clover ley providing an economic return to this agronomically important fertility building phase through the inclusion of a livestock enterprise.

Organic farming systems depend upon attention to all aspects of farm nutrient management in order to minimise polluting and/or agronomically wasteful losses of nutrients. The approaches will be outlined and an overview of the nutrient balance of an organic mixed farm will be presented.

It is often considered that the greatest risk to the environment from the adoption of organic farming systems is that of nitrate leaching. However, when evaluating this risk, the leaching potential and proportion of land utilised by each phase of the rotation must be considered as well as the timing and quantity of manure applications. These features may result in losses that do not pose an environmental hazard. Other advantages that accrue to the organic farming system, which relies upon a sustainable balance of nutrient supply and utilisation, will be considered.

Pest and Disease Control in Organic Cropping Systems---The Long-term View

E K M Lennartsson

Henry Doubleday Research Association. Ryton-on-Dunsmore. Coventry CV8 3LG, IJK

The underlying principle of pest control in organic systems is to design and manage cropping systems so that they in themselves prevent the development of serious levels of weeds, insect pests and diseases. Use of extensive crop rotations provides efyective control against a wide range of pests. Emphasis is placed on using preventative measures as much as possible and minimising the need for curative solutions.

Soil jt2rtility tiiid orguni tnmures In organic systems soil fertility is maintained by rotating fertility building and exploitative crops and by using organic manures. The importance of incorporating manures such as livestock manures, green manures, crop residues and composts is in providing nutrients to support vigorous plant growth, but equally important is the provision of energy and nutrients for the soil’s fauna and flora. The process of organic matter decomposition can influence soilborne pests directly, or the effect can be of a complex nature, dcpendent on the activity of a range of organisms. Increased knowledge of the role of soil biological activity in relation to pests, pathogens and host plants is likely to lead to entirely new approaches to crop protection in future.

Species diversity In practice, benefits of species diversity are obtained mainly by using crop rotations. Here the mechanisms of pest control are well understood; pests and pathogens can be eliminated during prolonged periods without their hosts, and weeds can be controlled by cultivating at different times in the year.

Mixed species cropping and variety mixtures have been used to a very limited extent in temperate agricultural systems. Trials with variety mixtures of cereals have, however, confirmed the effectiveness of mixtures in restraining the development of some foliar diseases. Multicropping with different species has even greater potential. Experimental inter-cropping systems have been designed to control weeds and insect pests as well as foliar and soilborne plant pathogens. Further research is required on the mechanisms of pest control in these systems.

The value of species-rich field boundaries and ‘islands’ within fields to increase numbers of predators is becoming increasingly appreciated and scientifically substantiated. Encouragement of naturally biological control agents by careful manipulation of habitats is an approach of biological control which will become increasingly important.

Sound husbandry practice Pests and diseases can be controlled by using resistant varieties and sound hygiene

The scientific basis to sustainable agriculture 253

practices and by destroying alternative hosts. Cultivation, planting and harvesting can also be strategically timed so as to avoid pests. Current research on the forecasting of pest populations will have important implications.

Mechanical, biological and chemical control Technical developments have increased the opportunities for mechanical weed control, not only at periods between crops but also in established crops. There are now even machines which remove pests from plants. Physical barriers are being used to keep insects away, and future developments of barriers and traps will improve their effectiveness and ease of use.

Biological control with introduced organisms plays an important role in some cropping systems, notably in glasshouses. Current research is likely to result in the availability on the market of a larger range of effective and reliable biological control agents.

However, at present there is a tendency to use biological control agents in almost the same way as chemicals. There is a need to recognise that in sustainable systems the release of organisms will be but one of many techniques used to increase the ratio between natural enemies and pests. Further research should have this control approach as a major objective.

Some chemical control agents are approved for use in organic food production. However, chemicals should always be used as a last resort. It is likely that they will be used even less in future as sustainable cropping systems are successfully developed.

Is Biological Control the Route to Sustainable Agriculture?

K A Powell

ICI Agrochemicals, Jealott’s Hill Research Station, Jealott’s Hill, Bracknell, Berks RG12 6EY, UK

‘Events of the past decade have reinforced our view that biological control holds the answers to many problems of today’s agriculture, . . . control of plant disease is among the most significant ways to increase crop production, and biological control achieves this goal in a way that is economical and, most important, sustainable . . . The need for a sustainable agriculture will be met in part by wider use of biological control’. (Cook and Baker 1983).

Sustainable agriculture has been defined in many ways, including, by Cook (1990), as (among other things) meaning to a farmer the ability to continue farming. It is particularly this concern that I would like to address. We are unlikely to see a return to higher labour inputs into farming, hence pest, disease and weed control, while partially achievable by rotation, amendment and cultural practice, will still require effective treatment to achieve the yields needed to feed the world’s population. Can biological control meet the need for a sustainable agriculture?

254 SCI Ayriculiurr Group Sq’mpoPiuni

On past evidence which is described in this paper the answer must be no: there is little evidence for the successful application of biological control to major diseases, even less for weeds, and while there is some success for insects, the majority of agriculture depends on sensible application of agrochemicals for maintenance of yield.

In this paper I attempt to describe some of the reasons for lack of success in biological control, to discuss the potential to overcome some of the problems and to illustrate the argument with examples from work on plant pathogens. The conclusion 1 reach is that while biological control has some applications in specific niches it is unlikely to offer adequate control of major pests in broad-scale agriculture.

References Cook R J (1990) Quoted in: Conway K E & Power A G (1990) In: New Directions in

Biological Control: alternatives for Suppression Agricultural Pests and Diseases, eds Baker R R & Dunn P E. Alan R Liss, New York, pp 467--471.

Biological Control of Plant Puthogrns. The American Phytopathological Society, St Paul. MN.

Cook R J, Baker K F 1983 The Nature and Pructice

The Long-term Effects of Leakage from Agricultural Systems

Stephen C Jarvis

AFRC Institute ofGrassland & Environmental Research, Hurley. Maidenhead, Berks SL6 5LR, UK

Leakage from agricultural systems can take many routes with the removal or release of materials in solutions or as solids or gases to the wider environment. Such ‘leakage’ can take a wide variety of forms with impact at local, regional or global levels and with social/health and/or environmental consequences. Leaks usually occur as the result of various imbalances within particular production systems, eg between the total amounts of available nutrient and the potential for carbon fixed by photosynthesis, or between rate of supply, and the ability of soil to immobilise or inactivate materials. The processes and mechanisms involved are interactive. When, for example, a nutrient is in excess, blocking one route of escape may precipitate or exacerbate an alternative route. The extent of loss will be dependent upon amounts, forms and timing of inputs in relation to environmental conditions and the management of the production system.

The nitrogen cycle provides examples of all the above effects, and the possible extent and routes of escape of N have been much discussed recently (Jenkinson 1990) and have considerable long-term and far-reaching impact. For example, volatilisation of NH, results in interactions with atmospheric acidity, and, after deposition, soil acidification and disturbed nutrient balances in natural ecosystems. The major source of NH, in the UK is animal production systems: a recent estimate for the UK (Jarvis and Pain 1990) indicated that the emission from

The scientific basis to sustainable agriculture 255

agriculture was 186 kt N year-’. This is lower than previous estimates, and indicates that the strength of other sources needs more accurate quantification.

Another form of gaseous loss from the nitrogen cycle, ie N 2 0 through denitrification, also has profound, although different, effects. The atmospheric concentration of this trace gas is increasing substantially, and, because of its capacity to absorb long-wave radiation, N,O is contributing significantly to greenhouse effects and is also involved in reactions with ozone. Fertilised soils are a major source of N 2 0 , with the estimated contributions from grassland and tilled areas accounting for 84% of the emissions from the UK land surface. The emissions from agriculture are from 7 to 23 times greater than those associated with fossil fuels. Although any anaerobic soil with an excess of NO; is a potential source of N,O, it is difficult at present to provide an accurate description of the extent of and the factors controlling the exchanges of N,O between soils and the atmosphere.

A similar degree of uncertainty exists with another trace gas with similar, long-term atmospheric effects, ie CH,. Release of CH, represents an energy, rather than a nutrient, loss and arises from the activity of methanogenic bacteria, eg in the digestive tract of ruminants or in anaerobic soils. Agricultural emissions represent over 30% of the UK total with a calculated 0.98 Mt being released by cattle and sheep (Jarvis 1991). Again, because of the difficulties involved in monitoring fluxes, the extent to which agricultural systems are acting as sinks or sources is largely unquantified.

Many problems exist in gauging the long-term effects of leakage from agriculture, and not least of these is the availability of appropriate methods to monitor fluxes. In order to provide accurate models of the interaction of agriculture (whether based on fertiliser or alternative managements) with the environment and the role of agricultural soils as sources or as sinks, a much greater understanding of the processes involved is required.

References Jarvis S C 1991 Losses of methane and ammonia from glassland production systems. In:

Chemistry, Agriculture and the Environment, ed Richardson M L. Royal Society of Chemistry, London (in press).

Jarvis S C, Pain B F 1990 Ammonia volatilisatin from agricultural land. Proc Fert Soc 298 1-35 .

Jenkinson D S 1990 Leaks in the nitrogen cycle. In: Fertilisation and the Environment, eds Merckx R, Vereecken H & Vlassak K. Leuven University Press, Leuven, pp 35-39.

Farming and the Rural Economy-The Balance of Advantage

Eric S Carter

15 Farrs Lane, East Hyde, Luton LU2 9PY, UK

Over the past decade there has been a considerable amount of work on conservation at the interface between farming and the ‘natural’ environment and changes in

256 SCI Ayriculturc. Group Symposium

farming practice designed to reduce the impact of agriculture on wildlife and game species. There is a better understanding of the way in which wildlife responds to its environment and a much greater appreciation among farmers and landowners of the fact that the countryside and farming enjoy a symbiotic relationship.

For generations farmers have farmed the land and maintained the countryside in a form to which we have become accustomed. This was not the direct result of a master plan, but the outward manifestation of thousands of separate decisions made by generations of farmers and landowners, decisions made in relation to their personal assessments of land capability, market requirements, prices, costs and income. Not only was farming involved: landowners also managed land for sport and amenity, all of which influenced its appearance and wildlife.

In this country we now enjoy ample supplies of high quality food at very reasonable prices. In 1973 we spent less than E l on food in each &5 spent, and today it is little more than &1 in each &lo and the choice available is wider than i t has ever been. We now have the ability to produce surpluses and, being well fed, replace concern about quantity and continuity with concern about how food is produced, whether it is ‘pure’ and whether producing it is ‘destroying’ the countryside.

The agricultural issues most frequently mentioned in public opinion surveys as posing a threat to the environment are the use of fertilisers and agrochemicals. The role which pesticides play in protecting crops and providing produce free from pests and diseases and harmful contaminants is not appreciated.

The needs of wildlife species requiring special protection are catered for by Sites of Special Scientific Interest (SSSIs) which provide relatively secure bases for our plants and animals. But it is just as important to keep common species common as it is to ensure the survival of rare species. More people gain enjoyment from an abundance of common and conspicuous plants than from the conservation of obscure rarities.

Wildlife conservation will not be favoured through reduced inputs, set-aside or other schemes on their own. Conservation of wildlife and landscape requires commitment, understanding and planning so that it is integrated into the overall management of the farm or estate.

The agricultural industry will be under great pressure during the coming years. It will be expected to produce cheap, high quality food in a pollution-free accessible countryside, farmed without the use of chemicals, protected from development and providing a habitat for wildlife. There is little chance of meeting all these expectations. There must be a balance of advantage.

Without grant aid, farmers may find it increasingly difficult to fund on-farm conservation unless such activities are carefully integrated into farming practice. Should we farm all our land more extensively or concentrate our farming on a smaller, intensive area? Other land areas could be managed for environmental purposes or a mixture of modified farming and environmental uses. Such a move would reduce the total requirements for agrochemicals and other resources.

We are in a time of innovation and there is a need for some wide-ranging and unconventional thinking. We must strive for balance; but to attempt to balance what some see as outrageous by excessive, impractical demands will achieve

The scientific basis to sustainable agriculture 251

nothing. A balance is vital otherwise we may damage the industry through restrictions which owe more to misplaced anxieties than to solid scientific fact.

We need balance if we are to ensure a thriving, diverse, attractive, environmentally healthy, skilfully managed, sustainable, multipurpose countryside of quality with the flexibility to adapt to uncertain future needs.