Northwest Soil Science: Nitrogen Mineralization
by: WSU Extension: Doug Collins Posted on: May 13, 2012
Photo: Simon Davis-Cohen
By Doug Collins, WSU Extension, Statewide Small Farms Educator
Editor’s Note: Get a Soil Science lesson from a man working to merge the interests of organic farmers, water quality, and soil.
Healthy soils sustain plant and animal production without damaging the environment. Increasing organic matter in soils is one way to build soil health. Cover cropping, compost, and manure additions increase soil organic matter and improve the soil’s ability to hold and supply nutrients, water, and air to plants and animals.
Soil organic matter is a bank of soil nutrients that can increase plant productivity. Formed from dead and decaying plants, the organic component of soil is generally only a very small portion of the total soil mass. The largest portion is composed of weathered rocks of varying sizes, known as sand, silt, and clay. Nutrients like nitrogen, phosphorus, potassium, calcium, magnesium, and iron are the building blocks of plants and are taken up by plant roots, generally with soil water. Nutrients that are in the organic matter “bank” are not able to be incorporated by plants. As soil bacteria and fungi forage on decaying organic matter they release nutrients in a plant-available form, converting organic nutrients to their mineral form. The process whereby nutrients are converted from organic matter to plant-available minerals is called mineralization. Through a research project supported by the Organic Farming Research Foundation, soil scientists at WSU Puyallup are collaborating with seven organic farms in western Washington State to better understand and predict how and when nutrients are made available from soil organic matter.
Understanding how and when nutrients are made available from soil organic matter can help farmers in choosing and timing their fertilizer applications. When studying soil nutrients it’s logical to start with nitrogen, the nutrient that most frequently impacts plant growth. Though nitrogen mineralization from organic matter has been widely studied, there is still uncertainty on how to incorporate this knowledge when making recommendations for nitrogen fertilizer. Organic farmers and researchers alike are confounded by how to effectively account for previous years’ soil amendments and cover cropping additions. This uncertainty is particularly acute for organic producers who do not have access to conventional fertilizers.
Soil testing labs often provide “book values” for estimated nitrogen release based on total organic matter, which can be easily tested for. However, soil organic matter exists in two different pools: active and stable. The stable organic matter is not a source for mineralizeable nutrients, but the active organic matter is. For example, recent studies with organic potato producers in Oregon and Washington have shown highly variable rates of nitrogen mineralization that did not correlate with total organic matter.
Plant productivity relies on the availability of nutrients in a timely fashion but an excess is both an environmental hazard and can be a waste of money. In the Maritime Northwest, winter precipitation flushes plant-available nitrate-nitrogen into groundwater, possibly contaminating adjacent bodies of water. A recent survey of organic farms in western Washington found that more than 40% of samples had high or excessive nitrate-nitrogen in fall soil samples, a symptom of over fertilization. Certified organic fertilizers cost growers between $4 and $60 per pound of nitrogen and are an expensive component of vegetable production. Under-fertilization is also costly to the grower that has invested in planting, weeding, and irrigation but does not optimize yield due to poor fertility.
Fertilization requirements vary greatly depending on historical farm management, such as cover cropping and soil amendments. It is difficult and even misleading to offer fertilizer recommendations without in depth knowledge of recent management practices. A site-specific test would be valuable for improving soil quality, profitability and protecting natural resources. Methods to predict nitrogen mineralization have evolved to become faster, simpler, and cheaper. The Solvita™ colorimetric test for soils is one such method. The efficacy of these tests for predicting nitrogen mineralization has not been widely tested in field conditions, especially in western Washington.
For our study we have taken soil from WSU Puyallup and 7 other farms (including Growing Washington’s Alm Hill Gardens pictured above) and will run 3 different tests designed to predict nitrogen mineralization for each. We will then use two different approaches to carefully track how much nitrogen is actually released from the soil under field conditions. The first approach involves growing broccoli plants in what is called a “zero nitrogen” plot. Each farm will plant a bed of broccoli without any additional fertilizer. They will periodically send broccoli plants in for tissue analysis so we can track the broccoli’s nitrogen uptake. The second approach is to directly measure mineral nitrogen in soil at 10 different time periods on each farm from March to October.
Through this project we will be able to quantify nitrogen mineralization on the farms. We will see how well the Solvita and other prediction tests hit the mark and we will also see if historical management at the 8 sites had a significant bearing on available nitrogen. It is our hope that farmers can use knowledge of available nitrogen release to adjust their fertilizer applications and maximize productivity while protecting water quality.
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