Effects of Organic and Chemical Inputs on Soil Quality

Crop and Soil Environmental News, December 1997

Greg Evanylo,
Extension specialist
Waste and Nutrient Management

The following is a summary after the second year of a project designed to research the effects of conventional and alternative fertility practices on the biological, chemical, and physical attributes that constitute soil quality and influence crop yield on six limited resource vegetable farms in the mid-Atlantic region. This project is being funded by the USDA Sustainable Agriculture Research and Education Program.

Objectives

1. Assess the effects of organic and inorganic soil amendments on selected soil biological, chemical, and physical properties indicative of soil quality on limited resource vegetable farms in the mid-Atlantic region.
2. Teach extension agents to perform simple on-farm tests to determine the effects of production practices on soil quality.
3. Develop fact sheets for distribution to farmers and educators on:
   1. Effects of organic and inorganic fertility on soil quality, and
   2. Sampling and monitoring soils for indicators of quality.
4. Conduct field days for farmers, extension agents and educators, agricultural consultants, and researchers to share the results of the field studies and the methods that farmers can use to monitor soil quality.

Methodology

The six farms are located in the Coastal Plain, Piedmont, and Blue Ridge soil physiographic provinces. All farms produced tomato (Lycopersicon esculentum L.) during the second year of the study. Three of the producers are long term biological farmers and three have a history of chemical weed and insect control. Two fertility treatments, a conventional treatment using commercial fertilizer and an alternative treatment using compost, were employed. The conventional fertilizer consisted of ammonium nitrate, triple superphosphate, and muriate of potash designed to meet the recommendations of nitrogen (N), phosphorus (P), and potassium (K) for each farmer's crop and pre-existing soil test level. The alternative soil amendments, composted cotton gin trash or composted yard waste, were applied at rates to meet or supplement the nitrogen needs of each crop according to estimated N mineralization rates. Individual experimental plots measured 25 feet by 25 feet, and treatments were replicated three times in a randomized complete block. Fertility sources were applied onto plowed and disced or roto-tilled soils in the spring and immediately cultivated into the soil. Tomato seedlings ('Celebrity' or 'Mountain Spring') were transplanted within one week of soil amending.

Measurements

Soil was sampled twice from each plot during the season. Samples were collected to a depth of 12 inches at early flowering for extractable P, K, Ca, Mg, Mn, Zn, Cu, and B; pH; total Kjeldahl N; NH4-N; NO3-N; total C; organic matter; cation exchange capacity; total P; and microbiological indicators. Immediately following harvest, soil was sampled for extractable P, K, Ca, Mg, Mn, Zn, Cu, and B; pH, bulk density, water-holding capacity, aggregate stability, and microbiological indicators of soil quality. Fresh yield was determined by collecting and weighing ripened, marketable tomato fruit from a 12-foot section of the center row from each plot.

Results

Populations of beneficial microorganisms in soils with a history of organic production practices (especially populations of Trichoderma and Gliocladium spp. and total bacteria) were higher initially (at planting in 1996) than in soils under conventional production practices (inorganic fertilizer and synthetic pesticides). Furthermore, soils with a history of organic production practices have a greater capacity for population growth than the conventional farms. The addition of composts to soils with a history of conventional practices increased the baseline populations of the beneficial soil fungi Trichoderma and Gliocladium spp. and the thermophilic microorganisms. Soils with alternative amendments always had lower populations of plant pathogenic Phytophthora and Pythium spp than soils amended with fertilizers.

The composted waste improved soil quality more than commercial fertilizer by: increasing soil concentrations of the nutrients P, K, Ca, Mg, Mn, Zn, and B; increasing soil pH; increasing soil organic matter; and reducing bulk density after only the second year of treatments. A trend toward increasing soil total C, cation exchange capacity, and plant available water holding capacity was observed after one year, but the second season's analyses have not yet been completed; and aggregate stability has not yet been run.

The results after only two years of treatment is interesting because previous research has shown that three to five years are normally required to see any benefit from organic amendments, usually in the form of yield increases. Our research indicates that many soil chemical, physical, and biological attributes can be enhanced after only two years of organic amending. Composted waste applications increased organic matter and reduced bulk density, two properties especially difficult to improve during short time periods, within two years. Yields were not significantly different between fertility treatments, possibly due limitations of plant available N due to incorrectly estimating N mineralization from the organic sources. Populations of thermophilic microorganisms and Trichoderma and Gliocladium spp in historically inorganic fields can be brought up to levels comparable with organic fields in a short period of time. Increases in populations of thermophilic microorganisms and Trichoderma and Gliocladium spp and reductions in Phytophthora and Pythium spp can occur over the span of a season. If this corresponds to an increase in yield (or an increase in future yields) then information on the abundance of these microorganisms may be important as part of a soil quality indicator package.

Impacts of results

Sustainable agricultural systems require that non-renewable resources not be depleted and air, water and soil quality be maintained or improved. Recycling of organic "wastes" as soil amendments and fertility sources for agricultural soils can slow the depletion of oil and mineral reserves by substituting materials normally diverted to landfills for commercial fertilizers, whose mining is environmentally degrading and production is energy-intensive. Furthermore, recycled organic wastes provide organic matter, an important constituent of biologically active and productive soils, which is not found in commercial fertilizers. The results after two years indicate that the chemical and physical attributes of composted waste-amended soils can be enhanced above those of commercially-fertilized soils, and that yields with composts can be comparable to those of commercially-fertilized soils. In addition, organically-managed soils have higher populations of beneficial and lower populations of pathogenic microorganisms, which may predispose conventionally-fertilized soils to greater incidences of disease. Continuing study will determine whether long-term fertility, soil quality, and crop productivity can be enhanced with organic fertility sources.

Potential contributions to producers and consumers

The use of organic soil amendments may result in a soil that has greater capacity to resist the spread of plant pathogenic organisms; thereby, requiring reduced use of fungicides. This can lower farmers' costs and the risk of pesticide contamination of food. The improvement in overall soil quality may reduce the potential for nutrient contamination of ground and surface water and produce more vigorous-growing and high-yielding crops. Many potential sources of organic soil amendments are currently buried in costly landfills or underutilized in relation to commercial fertilizers. Increasing use of composted wastes will reduce the costs of landfilling wastes and the environmentally damaging (mining) and energy intensive (petroleum-based) fertilizer production practices.

For further information contact:
Gregory K. Evanylo
421 Smyth Hall, CSES Dept.
Virginia Tech, 0403
Blacksburg, VA 24061
540/231-9739; gevanylo@vt.edu

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