Soil health strategies for organic agriculture

By Robyn Neeson, Alternative Farming Systems Officer, NSW Agriculture, Yanco

Organic farming's basic tenet is the creation of a healthy, fertile soil - on this base the rest of the farm agro-ecosystem is built. The concepts of the Living Soil and the Law of Return are fundamental principles of organic agriculture.

The 'aliveness' or dynamic nature of soil is intrinsic to organic agriculture. Organic proponents often equate the quality of soil with the level of health of plants and animals, and in-turn humans, living on that soil. Organic farming is primarily a soil building process. Relevant to this is the belief that without an understanding of the soil as a living, dynamic entity, and without an intimate working relationship with the soil, soil building will not occur, and a sustainable self-sufficient agro-ecosystem able to produce basic food requirements will be unattainable.

The Law of Return, in its simplest meaning, says that "all life forms must return at death what they took from their resources during their life time." (Farmer, 1977). This statement recognises the cyclic nature of the earth's natural processes. Refusal to recycle biological wastes back into the soil deprives microbial decomposers of their food supply, which in turn prevents the release of essential nutrients by decomposers.

The organic soil building process
There is worldwide agreement within organic standards that organic farming systems should maintain or increase soil fertility on a long-term basis. This is achieved through management practices that create soils of enhanced biological activity, such that plants are fed through the soil ecosystem and not primarily through soluble fertilisers added to the soil. Organic farming systems rely to the maximum extent feasible upon crop rotations, crop residues, animal manures, legumes, green manures, mechanical cultivation, approved mineral-bearing rocks to maintain soil productivity and tilth and to supply plant nutrients.

Conversion from a conventional fertiliser regime to an organic soil building process firstly involves eliminating the use of artificial chemicals in the farming system. This means that fertilisers such as super phosphate and ammonium nitrate are excluded and are replaced by practices which foster the cyclic renewal of nutrients to maintain crop health.

Organic matter content, microbial activity and general soil health are taken as measures of soil fertility. An analysis of organic farming systems in Europe (Stolze et al., 2000) has found that organic farming increased microbial activity by 30-100% and microbial biomass by 20-30%.

A comparative study of organic, conventional and integrated apple production systems in Washington State from 1994 to 1999 found that organic and integrated systems had higher soil quality and potentially lower negative environmental impact than the conventional system. The data indicated that the organic system ranked first in environmental and economic sustainability, the integrated system second and the conventional system last (Reganold, et. al., 2001).

Research into the sustainability of organic farming systems in Australia has been limited. The research has tended to focus on comparative studies in extensive cropping and livestock systems. These systems are characterised by their low use of external inputs. Phosphate rock, lime, dolomite, legume rotations, incorporation of green manures and crop refuse, manure application during livestock grazing, and the application of microbial preparations, may be used for building soil fertility.

Studies by Penfold (1995), Derrick (1996), Derria et al. (1996) and Schwarz (1999), suggest a trend towards deficiencies in phosphorous, nitrogen and sometimes sulphur, under current organic management regimes in broad-acre (extensive) cropping and livestock systems.

Limited studies of intensive organic farming systems in Australia have generally shown an increase in soil health compared to conventional practice (Wells and Chan, 1996; Huxley and Littlejohn, 1997; Stevenson and Tabart, 1998). This could largely be a reflection of the cost effectiveness of applying larger applications of commercial organic fertilisers, compost and incorporation of green manures, to high value crops such as fruit, vegetables, and herbs.

Organic farmers have a range of options to sustain soil health. Applications of these methods are discussed below.

Increasing biological activity
Organic conversion begins with a process that encourages increased microbial and arthropod activity within the soil. The elemental composition, structure, and organic matter content of the soil need to be favourable if soil biological activity is to be enhanced.

Biological activity begins with the breakdown of soil organic matter. During the decomposition process, the organic molecules in organic matter are broken down into simpler organic molecules that require further decomposition or into mineralised nutrients. Organic farmers supply organic matter through incorporation of green manure crops and crop refuse and the addition of compost.

The use of bio-indicators is becoming an increasingly important way to assess soil health. Pankhurst et.al (1997) reviews how soil organisms and biotic processes can be used as indicators of soil health.

A range of techniques are available for assessing a soil's biological activity. These include measurements of soil microbial activity based on the soil's CO2 respiration; DNA testing to determine the diversity and abundance of microorganisms present; and an 'in-situ' technique to measure activity based on measuring the tensile strength of a cotton strip that has been buried in the soil. Commercial laboratories that offer services to assess soils for microbial status are now becoming more common in Australia.

Green manuring
Green manure crops are grown specifically to be cultivated back into the soil to build up soil organic matter and nutrients and to stimulate biological activity. The type of green manure crop and stage it is turned in determines the amount of organic matter or nutrients returned to the soil. A lush, actively growing legume sward (for example vetch, faba beans or lupins) contains large amounts (50-140 kg N-gain / Ha) of nitrogen that is released to the soil upon cultivation. The same crop when allowed to mature, contributes more organic matter but less available nitrogen. If a soil is low in organic matter, then a green manure crop that increases soil organic matter is desirable (for example oats).

Green manures may also act as break crops to reduce the carryover of pests and diseases in subsequent crops in the rotation. Green manure crops are an essential component in intensive organic annual cropping rotations.

Nitrate leaching following the incorporation of a green manure crop may occur when rainfall exceeds evaporation resulting in net drainage. There is some evidence to suggest that nitrate leaching may be less under organic than under conventional systems (Lampkin, 1990). Nitrate leached below the root zone is effectively lost from the system. Rotation design within the organic system needs to consider how large nitrogen losses following ploughing in of the green manure crop can be minimised. Early establishment of a cereal crop immediately following incorporation of the green manure has been shown to be the simplest and one of the most effective methods of reducing nitrate leaching.

Undersowing crops
Undersowing of crops (e.g. of barley with the grass/clover pasture that will follow in the rotation in the succeeding year, or of almost any crop with a leguminous green manure), is a key practice in organic systems. This practice has been shown to have beneficial effects on the diversity and abundance of insect species (Vickermann, 1978). Other benefits include the potential for higher protein content in cereals undersown with a legume due to a small net nitrogen gain, enhanced weed suppression and improved pest and disease control (Lampkin, 1990).

Permanent swards and pastures
In livestock and cropping enterprises, legume-based pastures provide the systems major nitrogen input and livestock largely recycle other nutrients. In orchards, permanent swards (sods) are sometimes planted between the rows, and are the preferred method of inter-row management because the soil ecosystem remains undisturbed. This favours the development of plant roots, soil microfauna and flora, worms, and mycorrhiza; and helps retain good soil structure.

A mixture of deep-rooted and shallow-rooted species increases the potential for accessing soil nutrients. For example, in organic pastures, herbs such as chicory, plantain, yarrow and caraway are often added. Ideally, an orchard sod consists of a range of perennial plant species. Grasses (for example ryegrass or fescue) are efficient in obtaining potassium from the soil and able to utilise excess organic nitrogen. Legumes (for example clover or lucerne) may contribute 40-140 kilograms per hectare per year of nitrogen to the soil reservoir. Herbs (for example comfrey and chicory) often have a higher mineral content and have deep roots capable of bringing up leached elements that would otherwise be unavailable to the crop.

A study by Evans et. al. (2000), of organic cropping systems in the Riverina and Central West of NSW, will attempt to identify best practice for management of the pasture phase to optimise soil microbial activity and increase soil concentrations of mineralised nutrients. The study aims to quantify soil fertility trends and will introduce a range of innovative pasture management practices to improve yield and cropping frequency.

Compost
Compost is a primary source of nutrients and organic matter in intensive organic farming systems and an invaluable food source for soil microorganisms. The use of compost in Australian broad-acre organic cropping systems is not widely practiced, as its application is not cost effective.

Animal manures and crop refuse form the major ingredients of compost. Organic standards require that manure intended for application is composted before use.

The major benefits of compost are that it is a more stable form of organic matter than raw waste, and weed seeds and diseases are destroyed during the composting process. When manure is composted, it is easier to spread, and losses to the environment are minimised. Rock dusts and clay, added to compost in small quantities, may help to reduce nitrogen losses from the heap by absorbing ammonia (Lampkin, 1990).

There are many recipes and techniques advocated for composting. The Australian Standard for Composts, Soil Conditioners and Mulches (AS 4454-1999) defines composting as "the process whereby organic materials are pasteurised and microbiologically transformed under aerobic and thermophilic conditions for a period of not less than 6 weeks." The pasteurisation process is described as having "the whole mass of constantly moist material subject to at least three consecutive days at a minimum temperature of 55 degrees C".

The major aim of composting is to produce a stable humic compound. This is achieved by mixing major ingredients together in quantities that achieve a suitable carbon:nitrogen ratio. The ideal C:N ratio lies between 25 and 35:1 (Lampkin, 1990). Moisture content is also important and ideally should be in the order of 55-70%. Compost heaps should be designed to allow for sufficient air access. Microbial activity quickly raises the temperature of the heap to above 55 degrees C, after which it is turned (ASA Standards specify a minimum of 3 turns) to allow for thorough mixing and a further heating of any undecomposed material.

Rock dusts and re-mineralisation
Many Australian soils are leached of elements essential for plant growth. Moreover, many years of farming with emphasis on supplying an N:P:K fertiliser regime at the expense of minor elements, may have resulted in further ëminingí of certain trace elements. This theory has some support, with evidence (McCance and Widdowson, 1940-2000) suggesting a gradual decline in the elemental composition of fresh fruit and vegetables since the 1940's.

Soils having higher biological activity play an important role in increasing the availability of micronutrients. Significant research has been undertaken in the symbiotic roles of arbuscular mycorrhiza fungi in increasing phosphorus availability in plants and rhizobium bacteria and its ability to fix atmospheric nitrogen for plant use. However, little research has been undertaken into the role of other soil microorganisms in improving micronutrient uptake by plants.

The re-mineralisation of Australian farming soils is a more recent strategy proposed by some soil health practitioners. Various techniques for re-mineralisation are having an increased following amongst farmers, largely based on balancing the CEC of soils and achieving a satisfactory calcium to magnesium ratio (Albrecht, 1975). The effectiveness of these techniques is yet to be scientifically evaluated under Australian conditions.

Re-mineralisation involves the addition of various fertilisers of mineral origin. These are rock-based materials and include rock phosphate, dolomite, limestone and rock dusts (from silicate rocks, including basalt and bentonite and some commercial organic blends.

Rock dusts may be added directly to the soil or added to compost heaps. Whichever method of application is favoured, release of nutrients from the rock dusts is accelerated by moist conditions, high temperatures and high biological activity (for example during a green manure stage or composting).

Soil structure improvements
Improvements in the biological activity and CEC of soils will generally lead to an improvement in soil structure. However, this needs to be supported by suitable cultural practices. Use of appropriate machinery at correct soil moisture, incorporation of soil organic matter, and improvement of soils utilising different crop root physiology are techniques used by organic farmers to develop soil structure.

Lampkin (1990) describes cultivation practices as having the most significant impact on the soil of any agricultural activity. He summarises the organic approach to soil cultivation as one that seeks to maintain soil structure and allow the soil to have vegetative cover for as long as possible within the rotation. Shallow cultivations, where only surface layers of the soil are mixed, are an important element of this approach. Deep cultivation of dry soil is practiced to loosen and aerate soil, avoiding inversion of the lower layers. Green manures or cereal crops are sown as soon as practicable following cultivation, their roots helping to stabilise loosened soil and minimise nitrate leaching.

Organic Soil Conversion
Organic conversion is not just about replacing a high-input chemical system with a no-input system. I propose that the organic soil building process goes through three critical stages. For the purpose of this paper I will refer to these as the adjustment phase, the comfort phase and the maintenance phase.

i. The adjustment phase
The first critical stage is during the organic conversion process. The adjustment phase involves developing a system that reduces the crop's reliance on artificial chemicals. This could be likened to overcoming ëcold turkeyí for those farming systems that are heavily dependent on chemical inputs. During this phase some farmers have observed that crop yields may decline as the system converts from a chemical to a biological one and is starved of its regular 'fix' of readily available, chemical fertilisers.

The length of this preliminary soil building process will largely depend on the soils' pre-existing condition. The adjustment phase involves increasing biological activity by providing optimal soil conditions. The challenge for the organic farmer is to implement a cost-effective strategy which encourages and builds biological processes within the soil whilst still maintaining optimal plant nutrition. In addition to standard organic practices such as the planting of legumes and green manures and applications of compost and rock dusts, commercial organic fertilisers, foliar applications of seaweed, fish emulsion, sugar solutions and microbial preparations, are applied to stimulate soil biological activity and supplement plant health.

ii. The comfort phase
The comfort phase coincides with an increase in biological activity and a corresponding release of previously 'locked-up' or unavailable nutrients. During this phase optimal crop yields are reached. Organic farmers need to be diligent that over fertilisation does not occur during the comfort phase. This is more likely to occur in intensive horticulture systems where applications of compost and green manuring are common practice. Evidence of over-fertilisation usually manifests itself through crop physiological problems and increased pest and disease incidence.

Organic farmers are encouraged to regularly monitor soil nutrient levels. Soil and plant tissue-testing enables nutrient requirements to be tracked thus avoiding 'overfeeding' the soil system.

iii. The maintenance phase
Research has indicated that some organic systems have, over a longer period of time, undergone a decline in soil nutrient reserves (Small et al, 1994; Penfold, et.al, 1995). This could be attributed to long term drawing down of nutrients during harvesting of crop or livestock products and through natural processes such as leaching.

In Australia, this has been particularly evident in broadacre cropping and livestock enterprises where phosphorous deficiency has been found. This has implications for cereal and legume crops. Phosphorous deficiency in legumes will impact on the plants ability to fix atmospheric nitrogen in root nodules. Nitrogen fixed by legumes forms is an essential nutrient in subsequent crops in the cropping rotation.

Nutrient budgeting by reconciling inputs and outputs to the soil system and correlating with regular soil tests and crop performance can help organic producers track the performance of the soil nutrient cycle.

Correcting deficiencies organically
Unseasonal weather conditions, such as a prolonged dry spell or excessive wet, or just a miscalculation of crop nutrient requirements may result in a deficiency within the crop. If this happens during a critical crop growth period, plant health may decline, predisposing crops to pest and disease attack and a permanent yield depression could result, so it becomes necessary to correct any deficiency quickly. Leaf analysis is the usual method to detect deficiencies during the crop growing period. Organic farmers make use of foliar sprays such as fish and seaweed extracts, molasses and trace elements to correct temporary deficiencies.

Case Study - Organic conversion at Yanco
At Yanco in the Murrumbidgee Irrigation Area in southern NSW, NSW Agriculture, in conjunction with the Natural Heritage Trust's National Landcare Program, has established a demonstration site to illustrate the organic conversion process to farmers.

Practices demonstrated include: increasing of soil fertility and biological activity by use of legumes, green manure and other crops in appropriate rotations, the application of composts, special preparations and other organic and mineral fertilisers, and various tillage techniques; non-chemical methods of pest, disease and weed control, and the evaluation of crop varieties for pest and disease susceptibility and their adaptability to diverse, low input cropping systems.

The 4 Ha site rotates cereals, oil seeds, legumes and vegetables over the area and aims to achieve sustainable production and maintenance of soil health. Initially the entire site was sown down to green manure crops. Following incorporation of the green manure crop, two organic soil treatments were applied. Both treatments comply with organic standards.

Soil treatment A is referred to as 'organic - biodynamic'. In this treatment, crops are fertilised pre-sowing with compost and rock phosphate and foliar applications are applied after 6 weeks at 8-10 day intervals, depending on leaf analysis results. The foliar applications consist of biodynamic preparation 500, biodynamic fish emulsion, brown sugar, worm liquid and seaweed liquid.

Soil treatment B follows a re-mineralisation strategy that aims to achieve a satisfactory calcium to magnesium ratio. In this treatment the basic organic-biodynamic fertilisation program is followed (as per treatment A), and in addition, small, pre-sowing applications of lime (up to 300 kg/Ha) and gypsum (up to 500 kg/Ha) are applied as per soil analysis results.

The project, now in its fourth year, has been evaluating soil and crop health in both treatments.

Results
The primary aim of the Yanco demonstration site has been to show farmers that organic practices can be sustainable and that some practices may offer opportunities for conventional farmers to reduce chemical inputs. This is being achieved, with some Riverina district farmer's now practising organic management of corn, soybean and vegetable crops. Alliances between local processors and producers are being forged, enabling producers to investigate opportunities in lucrative organic export markets.

A diverse range of crops have been grown at the site including wheat, oats, sunflowers, linseed, safflower, sweet corn, maize popcorn, soybeans, pumpkins, melons, tomatoes, lettuce, and green manures. Marketable yields have been achieved for most crops, although in most instances yields are below the district average. Soybeans have been the exception with crop failures due to significant green vegetable bug damage. Heliothus sp. has been the other major insect pest, and whilst some minor crop losses did occur, pest populations were manageable. Disease incidence has been negligible on all crops.

Standard soil analysis has been carried out each year, pre- and post- planting. Soil analysis trends show that organic matter has increased (from 1.5% to 3.0%), pH has generally remained unchanged (7.3), Ca:Mg ratio has increased (1.7 to 2.5), and CEC has decreased slightly, largely it is believed, due to an increase in potassium from compost applications.

The real benefit of organic management at Yanco will be to demonstrate the long-term impact of the intensive organic rotation. Soil health improvement, pest and disease incidence, sustainable crop yield and quality, will continue to be monitored over the coming years to assess these changes.

Conclusion
Maintaining soil health organically, relies on nurturing the soils biological and mineral processes. Incorporation of green manures and legumes in the cropping rotation, applications of compost, mineral rock dusts and organic fertilisers, livestock grazing, combined with appropriate tillage are some of the techniques used by organic farmers to meet this objective.

More research is required under Australian conditions to determine organic soil management strategies for optimum crop performance and to assess the effectiveness of current practice. Essential to this research is gaining a better understanding of the relationships between soil microorganisms and soil and plant health, including mineral uptake, pest and disease resilience.

References
Albrecht, W.A. (1975). The Albrecht Papers (C. Walters Jr ed.). ACRES U.S.A.; Raytown, Missouri.

Derrick,J.W. (1996). A comparison of agroecosystems: organic and conventional broadacre farming in south-east Australia. Ph.D. Thesis, Australian National University, 346 pages.

Deria, A., Bell, R.W. and O'Hara, G.W. (1996). Wheat production and soil chemical properties of organic and conventional paired sites in Western Australia. Ed. M. Ashgar. 8th Australian Agronomy Conference, Toowoomba. Publ. Australian Society of Agronomy, 200-202.

Farmer, F. (1977). Gene Poirot, farmer, philosopher, poet. ACRES U.S.A., 7 (6), 13-15.

Huxley, J. and Littlejohn, M. (1997). Jessica and Maria measure the difference between matched pairs of organic and conventional macadamia orchards. In Going Organic. Official Journal of TROPO. No.36 Oct. - Dec. 1998.

Lampkin, N. (1990). Organic Farming. Farming Press Books; Ipswich U.K.

Pankhurst, C., Doube, B.M. and Gupta, V.V.S.R. (1997). Biological Indicators of Soil Health. CAB International; Oxon, U.K., New York, U.S.A.

Penfold, C.M., Miyan, M.S., Reeves, T.S. and Grierson, I.T. (1995). Biological farming for sustainable agriculture production. Australian Journal of Experimental Agriculture, 35, 849-56.

Reganold, J.P., Glover, J.D., Andrews, P.K., & Hinman, H.R.. (2001). 'Sustainability of three apple production systems'. Nature 410, 926 - 930 Macmillan Publishers Ltd.

Schwarz, J., Graham, R., McDonald, G. and Shepherd, K. (1999). Organically vs Conventionally Grown Wheat: Grain Mineral Content. University of Adelaide.

Small, D., Wales, W. and McDonald, J. (1994). Soils, pasture and milk production on bio-dynamic farms in south-eastern Australia. 10th Organic Agriculture IFOAM Conference, Lincoln University, New Zealand. IFOAM.

Stevenson, G. and Tabart, T. (1998). Tasmanian Organic Farm Monitoring Project (1995-1998). NLCP. Tasmanian Organic-Dynamic Producers. Dec. 1998. ISBN 0-646-36407-3.

Stolze, M., Piorr, A., Haring, A., and Dabbert, S. (2000). The Environmental Impacts of Organic Farming in Europe. In, Organic Farming in Europe: Economics and Policy (Vol. 6). Ed., S. Dabbert, N. Lampkin, J. Michelsen, H. Nieberg, and R. Zanoli. Publ. University of Hohenheim, Stuttgar, Germany, 127 pages.

Vickermann, .P. (1978). The arthropod fauna of undersown grass and cereal fields. Scientific Proceedings of the Royal Dublin Society (A) 6: 273-283.

Wells, T. and Chan, K. (1996). Environmental impact of alternative horticultural production systems in the Hawkesbury-Nepean catchment. NSW Agriculture. Gosford.

Readers' Comments

From: "lk hazarika" <lkhazarika@aau.ac.in>
Date: 18 Oct 2003

The webpage subject matter is excellent and to much informative. The new thoughts on soil quality aspects under organic farming are realistic. I suggest you to include additional information on the microbial community shifts- conventional versus organic, if available.

Thanks
I will connect you later.

Dwipendra Thakuria
Assam Agricultural University

From: "Hilary Harding" hilm@hotkey.net.au
Date: Thu, 15 Jun 2006

I was disappointed to find no reference to commercial outlets to buy this kind of soil.

Cheerio for now.....................Hilary

If you have some relevant experience, please send us your comments to be added to this page.

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