Effects of Organic and Chemical Fertility Inputs on Soil Quality in Limited Resource Vegetable Farms

1995 Annual Report for LS95-070

Project Type: Research and Education
Funds awarded in 1995: $184,319.00
Projected End Date: 12/31/1997
Matching Non-Federal Funds: $79,351.00
Region: Southern
State: Virginia
Principal Investigator:
Greg Evanylo
Virginia Tech

Effects of Organic and Chemical Fertility Inputs on Soil Quality in Limited Resource Vegetable Farms


Biological vegetable farmers produce high value crops whose continued success is dependent on enhancing the quality of their soil. The objective of this project was to study the effects of conventional and alternative fertility practices on biological, chemical, and physical attributes of soil quality and crop yield on six limited resource vegetable farms in Virginia and Maryland and at two experiment stations in North Carolina.

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 vegetable farmers to perform simple on-farm tests to determine the effects of their production practices on soil quality.

3.) Develop fact sheets for distribution to farmers on:
a.) Effects of organic and inorganic fertility on soil quality, and
b.) 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.

5.) Present the results of the effects of organic and inorganic fertility on soil quality and a practical guide to monitoring soil quality at the Virginia Sustainable Agriculture and the Carolina Farm Stewardship Association conferences.

6.) Write research articles on the effects of organic and inorganic fertility on soil quality.

Farms produced sweet corn (Zea mays L.) or melon (cucurbitaceae) during 1996 and tomato (Lycopersicon esculentum L.) during the 1997. Three of the farmers were long term organic farmers and three employed conventional chemical practices. Two fertility treatments, a conventional treatment using commercial fertilizer and an alternative treatment using compost, were employed. The conventional fertilizer was a mix of ammonium nitrate, triple superphosphate, and muriate of potash designed to meet the soil test recommendations for N, P, and K for each farmer’s specific crop and pre-existing soil test level. The alternative soil amendments, composted cotton gin trash and composted hay and manure or yard waste and manure, 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. Melons (muskmelon or watermelon) and tomato (‘Celebrity’ or ‘Mountain Spring’) seedlings were transplanted and sweet corn (‘Silver Queen’) was drilled within three weeks of soil fertilization and amendment addition. Soil was sampled twice from each plot each year. Samples were collected at sweet corn height of 12 inches and at melon and tomato early fruit set to a depth of 6 inches for: Mehlich I-extractable P, K, Ca, Mg, Mn, and Zn; pH; total Kjeldahl N; NH4-N; NO3-N; total C; organic matter; cation exchange capacity; and total P; and to a depth of 12 inches for biological indicators. Immediately following harvest, soil was sampled in the same manner for Mehlich I-extractable nutrients, pH, bulk density, plant available water-holding capacity, and the biological indicators. Fresh yield was determined by collecting and weighing ripened, marketable sweet corn ears, melons and tomato fruit from a 12-foot section of the center row from each plot during three week periods.

This experiment station research was conducted at the Horticultural Crops Research Station in Clinton and the Center for Environmental Farming Systems (CEFS) in Goldsboro. The experimental design was a split plot with either tillage or surface mulch (wheat straw) as main plots, and soil amendments including either synthetic fertilizer, composted cotton gin trash, swine manure, or a rye/vetch cover crop as subplots. Each experimental unit consisted of six 25-ft rows. Soil were limed to a pH of 6.2 and a rye/vetch cover crop was planted in the fall of 1995 and 1996. In the spring of 1996 and 1997, soils were amended with synthetic fertilizer, compost, swine manure, or the rye/vetch cover crop. Tomatoes (‘Rio Colorado’) were planted 14 days after soil amending. Soil samples were collected two weeks after planting and at harvest in a uniform pattern, placed in coolers with ice and returned to cold storage on the same day. All soil dilutions and extractions were done within two weeks of soil sampling.

Soil samples were analyzed for microbial population densities using serial soil dilutions and eight different media. Total numbers of culturable bacteria, total fungi, thermophilic microorganisms, fluorescent pseudomonads, Trichoderma and Gliocladium species, Fusarium species, Phytophthora and Pythium species, and sclerotia of S. rolfsii were quantified. The incidence of southern blight was recorded in each experimental plot, beginning 40 days after transplanting until harvest. Tomatoes were harvested and yields were taken at 72 days after transplanting.

Many important conclusions were drawn from the combined on-farm and experiment station data. The alternative fertility treatment 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 improved several soil physical and chemical properties that normally require longer periods for positive change. Within only two years of the start of the study, the application of compost increased soil organic matter, total C, and cation exchange capacity above those of the fertilized soils. Bulk density was significantly reduced in the compost-amended plots compared to the fertilizer treatment. However, crop yields were not significantly increased by the alternative treatments on the farms or at the experiment station despite the improvement in many soil physical and chemical properties. Incorrectly estimating plant available N may have reduced the yields obtained with the alternative fertility treatments, and presents the major nutritional challenge to organic production.

The addition of compost to soils with a history of conventional production increased the baseline populations of several important groups of microorganisms, including Trichoderma and Gliocladium spp, and thermophilic organisms. Fields with a history of organic production have higher baseline populations of microorganisms and a greater capacity for population growth. Populations of microorganisms in fields with a history of organic production (especially populations of Trichoderma and Gliocladium spp and total bacterial populations) were initially higher than in fields under conventional production practices. Populations of thermophilic microorganisms and Trichoderma and Gliocladium spp in historically inorganic fields can be brought up to levels comparable to organically-managed fields within a short period of time. These results are significant because these fungal species are known antagonists of pathogenic soilborne fungi and are known biological control organisms.

Tillage and mulching were important factors affecting plant pathogens and yields. Yields were highest in surface mulched plots with synthetic fertilizers, cotton gin trash, or swine manure. Disease incidence at the experiment station site was 67% in tilled plots amended with synthetic fertilizers, whereas disease incidence was 3%, 12%, and 16% in surface mulched plots amended with cotton gin trash, swine manure, or rye-vetch, respectively. Organic soil amendments were suppressive to disease and enhanced beneficial soil organisms.

Research has shown that three to five years are normally required to observe any benefit from organic amendments, usually in the form of yield increases. Our research demonstrates increases in populations of thermophilic microorganisms and Trichoderma and Gliocladium spp over the span of a growing season and enhancement in several important soil chemical and physical properties within two seasons. If these changes correspond to an increase in future yields, then information on the abundance of the beneficial microorganisms and key soil chemical and physical attributes should be important as part of a soil quality indicator package. December 1998.