Soil Quality in Organic Agriculture

Kathleen DelateIowa State UniversityCynthia Cambardella & Douglas KarlenUSDA National Soil Tilth Laboratory

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Definition of Organic
According to the USDA National Organic Standards Board (NOSB), organic agriculture is defined as "an ecological production management system that promotes and enhances biodiversity, biological cycles, and soil biological activity. It is based on minimal use of off-farm inputs and on management practices that restore, maintain, or enhance ecological harmony. The primary goal of organic agriculture is to optimize the health and productivity of interdependent communities of soil life, plants, animals and people." (NOSB, 1997) The term "organic" is defined by law. In 1990, the U.S. Congress passed the Organic Food Production Act (OFPA). OFPA requires that anyone selling products as "organic" must follow a set of prescribed practices that include avoidance of synthetic chemicals in crop and livestock production, and in the manufacturing of processed products. To sell a product as "organic" the crop must have been raised on land to which no synthetic chemical (any fertilizers, herbicides, insecticides or fungicides) inputs were applied for three years prior to its sale. In addition, no GMO crops are allowed in organic production (e.g. Roundup-Ready® soybeans and Bt-corn®). "Split operations" or conventional and organic fields located on the same farm are allowed by Iowa law, but special care, including a border of 30'  between organic and conventional fields, is needed in mixed operations. Only naturally-occuring materials are allowed in production and processing operations and all treatments must be noted in farm records. Other practices specifically disallowed for organic production in Iowa include the use of "biosolids" or sewage sludge, due to concerns with bacterial and heavy metal contamination.

Soil Health:
The Basis for Organic Farming
The basis for all organic farming systems is the health of the soil. In addition to maintaining adequate fertility, organic farmers strive for biologically-active soil containing microbial populations required for nutrient cycling. Crop rotations, required for all organic operations, provide nutrients, such as nitrogen in the case of legume crops (alfalfa and red clover), and carbonaceous biomass upon which beneficial soil microorganisms depend for survival. Naturally-mined lime products are used to adjust the soil pH to 6-7 (depending on crop requirements). In addition to lime, manure and composted manure are the most common forms of soil amendments for organic operations. Iowa rules require that raw manure be applied three months prior to harvest for agronomic crops and four months for horticultural crops, in order to allow adequate decomposition, and avoid any problems of bacterial contamination of produce. Raw manure cannot be applied to frozen or snow-covered ground. Composting is the preferred method of stabilizing manure. Composting is a controlled process where nitrogen-containing materials (manure, yard/kitchen waste) are mixed with a carbon-containing source (corn stalks/cobs, straw, wood chips) to produce a substance preferably in a Carbon-to-Nitrogen ratio (C:N) of 30 to 1. The compost mixture must reach a temperature of 140° F for at least three days during the composting process. Adequate moisture and temperature are required for proper composting. Additional information on composting practices is listed in the references.
There are many soil amendments available for organic farming. The key, however, is that these materials are naturally-based, and that no synthetics are used in the processing or collection of the minerals. In addition to manure-based fertilizers, many organic farmers rely on fish emulsion and seaweed preparations to supply nitrogen and other elements. When phosphorus and potassium are limiting, rock phosphate and naturally-mined potassium chloride are allowed. It is imperative that you check with your certification agency before application of any materials. Should you apply a material found to be contaminated with toxic materials, your certification may be revoked for three years.

Crop Rotations: A Requirement and Wise Practice
In order to be certified, a crop rotation plan must be in place to protect against pest problems and maintain soil health. No more than four out of six years should be in row crops, and the same row crop cannot be grown in consecutive years on the same land. Legumes (alfalfa, red clover, berseem clover, hairy vetch) alone, or in combination with small grains (wheat, oats, barley), must be rotated with row crops (corn, soybeans, amaranth, vegetables) to ensure a healthy system. A typical six-year rotation in Iowa would be corn (with a cover of winter rye)-soybeans-oats (with an underseeding of alfalfa)-alfalfa-corn-soybeans.  Horticultural crops must be rotated with a leguminous cover crop at least once every five years. In research conducted at Iowa State University, after one growing season under organic management, Microbial Biomass Carbon (Mb-C) was 128% greater in the organic system; Maggroaggregate Stability (AggS) 15% greater; Organic Carbon (Org-C) 6% greater; Particulate Organic Matter Carbon (Pom-C) 8% greater; and N mineralization potential (PminN) 7% greater in the organic system. Nitrate N (NO3-N) was 44% greater in the conventional system, as reflected in the excess corn stalk nitrate. 

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