Management of crop nutrients and soil fertility

Organic soil fertility management is based on feeding the soil a rich, complex diet of plant residues, animal manures, and compost. In contrast, conventional agriculture simplifies crop nutrient and soil fertility management by feeding the plant soluble nutrients. Thus, crop nutrition in conventional agriculture can be managed with three pieces of information: 1) the amounts of nutrients used by the crop during the growing season, 2) the amount of plant-available nutrients in the soil, and 3) the amounts of fertilizer nutrients that need to be added to account for differences between crop nutrient needs and available soil nutrients.

Nutrient management in organic systems is more complex. Organic inputs cannot easily be added to the soil to provide the exact balance of nutrients needed by the plant for at least three reasons. First, many organic inputs (such as cover crops, crop residues, weeds, and compost) are added to the soil for reasons other than fertility management, yet they contribute to the pool of nutrients in the soil. Second, most organic materials, including compost and manure, have only a small component of soluble nutrients; most of their nutrients must be transformed through biological processes before they become available to plants. Thirdly, most manures and composts do not have a consistent nutrient content (such as the 10%N-10%P-10%K found on bags of synthetic fertilizers). They also contain a ratio of nutrients different from that needed for optimal plant growth. For example, dairy manure has a nitrogen to phosphorus ratio of approximately 2:1. However, hybrid corn requires eight times more nitrogen than phosphorus.(10)

You could try to combine plant and animal residues to get a nutrient ratio similar to that required by your crops. But these residues may not provide nutrients in the amounts and at the times needed for optimal plant performance. Nutrient availability in organic systems is primarily the result of biological processes. Soil organisms feed on some of the nutrients in the added organic materials, retain some in the soil as humus, and slowly mineralize and release others over time. In addition, environmental factors, such as soil moisture and temperature, affect organic matter decomposition and mineralization. Consequently, mineralization is slow during dry periods and in the spring and fall, when the soil is cool.

While crop and animal residues do not afford precise nutrient management, the integrated mineralization and humification process provides soils with a buffered storehouse of nutrients. The diverse populations of soil organisms involved in decomposition store residue nutrients in their bodies (biomass). Biomass nutrients are available for plant uptake, but they are protected from runoff, leaching, or volatilization.(11) Microbial processes also make humus from residues that are resistant to decomposition. Humus is responsible for improving soil tilth, water-holding capacity, and nutrient-holding capacity. Following mineralization, soil nutrients provided by organic materials are subject to the same chemical reactions as soluble nutrients from conventional fertilizers, such as sorption of phosphorus by iron oxide or precipitation of calcium with sulfate.

This provision of the standard does permit fertilizers with high levels of soluble nutrients, as long as they are derived from allowed crop and animal materials (such as fish and soybeans). These organic fertilizers provide nutrients in standardized amounts that are immediately available to plants. Ideally, these products are used as a complement to compost or other slow-release organic inputs, in order to provide a better nutrient balance or meet temporary high nutrient demands of the plants. Producers can use these soluble products to manage nutrient availability in a manner that mimics conventional fertility programs, while legally complying with the materials section of the National Organic Standard.(12) However, these producers do not comply with the

Farmers are encouraged to test their soils for nutrient needs, and to apply only what nutrients are needed. Photo by Lynn Betts, USDA NRCS.

intent of the standards, nor will they reap the multiple benefits of integrated, soil-based fertility management.

In summary, plant-available nutrients in organic systems include minerals, nutrients that have been mineralized from plant and animal residues, nutrients held in microbial biomass, and nutrients that are being mineralized from decomposing residues. As a good organic producer, you should account for each of these nutrient sources to determine current and future nutrient availability. You should also view nutrient management as but one component of an integrated crop and soil management plan. Cover cropping, for example, not only produces and conserves nutrients but is also used for weed and pest control and for soil conservation.

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