Nitrogen Dynamics in Organic Cropping Systems

CN-MR-1-Nitrogen dynamics 1by Sanne Kure-Jensen
John Spargo, UMass Extension soil scientist and director of the UMass Soil and Plant Tissue Testing Laboratory discussed Nitrogen (N) dynamics in organic cropping systems at the Agriculture & Food Conference of Southeastern Massachusetts. Spargo explained how to efficiently and effectively manage N to optimize crop quality and yield as well as to minimize losses to the environment.
Spargo reminded growers that one of the fundamental principles of organic management is feeding soils so that soils can feed crops. Both nutrient deficits and excesses can be detrimental to crop health and yields.
According to Spargo, the most sustainable sources of N are legume cover crops. Nitrogen-fixing bacteria living within legume root nodules fix atmospheric N into a plant available form. When legume cover crops are killed, they can provide a substantial amount of available N to subsequent cash crops. Cover crops also help conserve essential nutrients, control erosion and provide water-holding organic matter for future crops.
Timing
Traditionally, many growers planted cover crops after late summer or fall harvesting to minimize erosion and soil loss between cash crops. When farm plans and space allow, greater benefits are reaped when winter cover crops are planted at ideal times and allowed to grow well into the spring. Once cover crops are turned in, they will be broken down by microbes, making N and other nutrients available for plant uptake.
After harvest time, there is often excess N available in soils that can leach out or dissipate during the off-season. Cover crops absorb, create additional N, prevent winter leaching and protect water quality. When long cover crop cycles are not possible, Spargo recommended including winter cereal crops to keep N in farm systems.
Nitrogen Cycle
Our atmosphere is composed of approximately 78 percent N gas. Atmospheric N cannot be absorbed or fixed by non-leguminous plants. Lightening can also blast N from the atmosphere into water and soil. Plants are eaten and excreted by animals. Plants, manure or dead animals return N to soils during their decay. Bacteria and algae break them down into Ammonium and then Nitrates. Organic matter locks up or immobilizes N in this form. Nitrates are ideal for plant uptake but are very prone to leaching in heavy rains. Nitrates are prone to denitrification in saturated soils. Ammonia volatilizes easily.
Soil moisture, precipitation and temperature influence the nitrogen cycle. Heavy spring and fall rains during cooler weather can cause significant Nitrate leaching. Sustainable farming practices encourage high levels of Soil Organic Matter (SOM). Promptly tilling in animal manures or cover crops encourages N to ‘stick’ organic matter in soils, retaining N rather than letting ammonia volatilize.
Mineralization is the biological decomposition of organic matter including oxidation or decomposition of organic matter compounds to plant available forms. Rocks and minerals undergo chemical dissolution and physical or disintegration weathering caused by moisture, oxygen and repeated freezing and thawing.
Managing Nitrogen Levels
Spargo recommends having soils tested to determine soil acidity, P, K, Ca, and Mg (and micronutrient) levels before adding any amendments. In humid climates, the most useful test of N is the Pre-Sidedress Nitrate Test (PSNT) typically done in mid to late June just before sidedressing.
According to Spargo, average farm soils have 4-5 percent Soil Organic Matter (SOM). Between two and four percent breaks down or mineralizes each year and offers as much as 2,000 pounds of N per acre. Soil N levels vary during the growing season reaching their highest level in mid-summer because summer heat has maximized soil activity by late July.
Mineralization, immobilization and transformation levels of Soil Organic Matter (SOM) can be difficult to predict.
Maximum Performance from Cover Crops
To achieve the best returns from cover crops, plant fresh, quality seed and ensure good soil contact. Before planting, adjust the soil pH to the ideal range of 6.5 to 7.2. Be sure to plant at the optimal time for germination. Keep cover crops in place until they reach their maximum biomass or dry matter content. This is typically reached when 50 percent of the flower buds are open.
Livestock operations or large-scale, diversified farmers can afford to keep land in forage legumes or sod crops for two to three years for maximum nutrient levels. Red clover and alfalfa offer great livestock forage. It can take two to three years for them to they reach their peak biomass. Integrated farms often include perennial forages in the rotation. Perennial forages (both legumes and non-legumes) build soil.
Nitrogen Sources
Not all legumes offer the same results. Crop rotation constraints and site adaptations can limit results. Vetch offers a quick fix with a boost in N levels, but its half-life is just seven days after tilling. Vetch has a 15:1 C:N ratio. Clover is slower to develop. If allowed to reach maturity, clover seed and woody matter will break down slowly in soils. Clover also has a 15:1C:N ratio but produces less biomass. Therefore, it provides less N to subsequent crops as slow-release N.
Rye is not a legume and has a much higher C:N ratio than vetch and clover. Rye is very affective at scavenging residual soil nitrate, preventing it from leaching over the winter and early spring. Growers may manage rye with a roller-crimper to lay it on the soil surface rather than tilling it in. Decomposition will progress more slowly with less soil contact. The vegetative mat will suppress weeds as it lays on the surface. Some stems may regrow.
Cottonseed meal and blood meal are easily mineralizable and retained in soils. Urea is quite volatile and must be incorporated promptly. Compost has a low mineralization factor and offers slow-release N. Its biggest benefit is its organic matter content.
With the highest level of mineralization, feather meal, or a blend of feather meal and pelletized poultry litter, remains available after two to three years.
Nutrient amendment levels are hard to calculate with manure and compost. Without incorporation, much of the N can volatilize. Nutrient ratios do not always match crop needs. The phosphorous levels are often higher than needed, especially in manure-based composts. Where phosphorous is over-applied, it is vulnerable to runoff losses and can cause significant water quality problems. A secondary concern is that high Phosphorous levels can also lead to micronutrient deficiencies. Once soil phosphorus levels are high, it takes a very long time to reduce levels.
Long-term cover crop or pasture management increases organic matter while deep roots bring up minerals providing micronutrients when they decay.
Spargo strongly encouraged all growers to include cover crops in their crop rotation. He discouraged growers from relying on composts and manures, especially without soil tests. High phosphorous levels are too risky and hard to get rid of.
Email questions to John Spargo at spargo@umass.edu or call 413-545-1002.

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