Effects of Organic and Chemical Inputs on Soil Quality
The following is a non-technical summary of the first 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.
The farms are located in the Coastal Plain and Piedmont soil provinces. Three farms produced sweet corn and three farms produced cucurbits (cassava melon and watermelon) during the first year of the study. Three of the producers were long term biological farmers and three had a history of chemical weed and insect control. Two fertility treatments, a conventional treatment using commercial fertilizer and an alternative treatment using an available organic fertility source, were employed. The conventional fertilizer was a mix of ammonium nitrate (33.5-0-0), triple superphosphate (0-46-0), and muriate of potash (0-0-60) mixed to meet the soil test recommendations of nitrogen (N), phosphorus (P), and potassium (K) for each farmer's specific crop and pre-existing soil test level. The alternative fertility sources, composted cotton gin trash compost (3 farms), composted yardwaste and manure (2 farms), or beef manure (1 farm), were applied at rates to meet the nitrogen needs of each crop according to estimated N availability. Experimental plots measured 25 feet by 25 feet, and each treatment was replicated three times.
Fertility sources were applied onto plowed and disced or roto-tilled soils in spring 1996 (except one late season corn site, where soil preparation, fertilization and planting occurred in August) and immediately cultivated into the soil. Corn seed was planted or melons were transplanted within one week of soil fertilization and amendment addition. Farmers implemented their normal practices, except no herbicides, insecticides, of fungicides were used.
Soil was sampled when the melon vines began to run and at corn silking for available nutrients, organic matter, and microbial indicators. Immediately following harvest, soil was sampled for bulk density, water-holding capacity, aggregate stability, and the microbiological indicators. Fresh yield was determined by collecting and weighing all corn ears and melons from 12-foot sections of each of two center rows from each plot. The corn growth at the late-season planted location was assessed by sampling total stalk dry weight because corn earworms and an early frost prevented ears from forming.
The alternative soil amendments increased soil concentrations of most nutrients above that of the commercial fertilizer because the alternative amendments were a rich source of most essential nutrients, but only N, P, K, and, sometimes, limestone was recommended for the conventional treatment. The organic amendments tended to increase soil pH, organic matter, and total C above that of the commercial fertilizer, but no first year effect on soil bulk density occurred.
Some interesting trends have emerged from analysis of the biological samples removed at planting and harvest at the six on-farm locations. The most striking trends are between the historically organic and conventional farms. Two historically organic farms had higher numbers of the beneficial soil fungi Trichoderma and Gliocladium species initially at planting than the other farms. Populations of Trichoderma and Gliocladium species increased over time in the three historically conventional farms and were greater in plots treated with organic than inorganic fertilizers at harvest. These microorganisms have been demonstrated to play a role in biocontrol of soilborne pathogens. Numbers of thermophilic microorganisms (mostly species of Actinomyces and Streptomyces) also increased over time at most sites and were also higher in plots treated with organic than inorganic soil amendments. Our data indicate that beneficial soil organisms can be increased via organic amendments. None of the grower fields had significant levels of recognizable soilborne diseases this season; plant pathogenic oomycete fungi (Pythium and Phytophthora species) increased with time and were greater in plots treated with inorganic fertilizers than organic amendments at harvest.
Despite the beneficial effects of the alternative fertility sources on soil chemical and microbiological properties, the organically-fertilized treatments did not yield higher than those commercially fertilized. Available N may have been lower from the alternative source than from the commercial fertilizer, but soil N analyses have not yet been completed for verification.
Impacts of results on sustainable agriculture
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 first year's results indicate that the nutrient value of waste-amended soils can be raised above that of commercially- fertilized soils, and that initial yields with wastes 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.
Plans for the remainder of the project
The remaining soil chemical, physical, and biological analyses from the first growing season will be conducted this winter. In addition, a greenhouse study will be conducted to determine better estimates of N availability from the cotton gin trash and yard waste composts to refine next season's application rates. Farmers have prepared their fields for next season's crop, which will be tomato at all locations. Research assistant and principal investigators will work with producers to provide training in the use of simple, in-field soil quality tests and data will be collected to determine the accuracy of the quick tests. The producers' use of the kit will permit greater understanding of the impact of various practices and empower producers to redesign their production systems based on their own observations and measurements. Principal investigators, graduate research assistants, and producers will plan and design field days, present study results at workshops presented at the Virginia Sustainable Agriculture Conference and the North Carolina Farm Stewardship Conference, and jointly author extension facts sheets, and research articles.
For further information contact:
Gregory K. Evanylo
421 Smyth Hall, CSES Dept.
Virginia Tech, Blacksburg, VA 24061-0403