Reducing nitrogen runoff in fields
After a dry summer, reduced corn yields generally results in excess soil nitrogen and excess nitrogen leaching in the fall, winter and spring. Most farmers fertilize for maximum yields and hope that the weather cooperates. When corn yields are reduced due to drought, the excess nitrogen that was applied is often lost through leaching when the rains return or it may be lost to the atmosphere as ammonia gas under saturated soil conditions through denitrification. Soil nitrogen runoff is lost at 100 times higher rates than soil phosphorus and nitrogen is a major pollutant in Lake Erie and the Mississippi River.
Keeping nitrogen in the soil should be a major goal of every farmer, so keeping nitrogen in a form that is both plant available but not soluble would be a great accomplishment. Microbially-bound nitrogen is both plant available but also not soluble. When soils get dry, nitrogen is often not plant available but when these soils finally get wet, the soluble nitrogen can be lost by leaching or denitrification. The best way to reduce these problems is to convert soluble nitrogen to amino acids to keep the soil nitrogen from being lost. So healthy soils keep our nutrients recycling, plant available and result in less nitrogen pollutions — less nitrogen runoff or nitrogen lost as atmospheric gases.
Soil microbes need three things to tie up nitrogen as amino acids: carbon, sulfur and molybdenum. A typical carbon to nitrogen (C:N) ratio of 30:1 is needed to stabilize soil nitrogen. Carbon is the skeleton or backbone to tying up nitrogen in the form of amino acids and proteins. The best way to make sure adequate carbon is available especially on sandy soils is to grow a cover crop; especially grass cover crops like cereal rye, oats, annual ryegrass, triticale etc. Grass roots produce sugars and carbohydrates which supply the carbon needed to stabilized soluble nitrogen. When nitrogen fertilizer is applied in the spring, adding stable humic products at a 3% rate ties up soluble nitrogen. Soil organic matter or humic products have the ability to tie up negative charged anions like nitrates and keeps soil nitrogen from being lost as pollutants.
Sulfur (S) is another element used by soil microbes to make essential amino acids like cysteine and methionine. The soil ratio is 10 parts nitrogen to one-part sulfur (10:1 N:S ratio). A simple remedy for keeping nitrogen tied up is to apply sulfur at the same time that nitrogen fertilizer is applied. For a simple example, if your soil nitrogen levels are 60 pounds available nitrogen per acre and you apply 60 pounds nitrogen fertilizer, you have a total of 120 pounds of nitrogen per acre. If a soil test indicates only 8 pounds of available sulfur, farmers need to apply 4 pounds of sulfur to get a 10:1 N:S ratio because 120 pounds of nitrogen needs 12 pounds of sulfur to stabilize the nitrogen. Farmers often add ammonium thiosulfate (12% nitrogen and 26% sulfur) at planting in their starter fertilizer to stabilize the nitrogen. Another source of sulfur is gypsum (calcium sulfate) which is 23% calcium and 18.5% sulfur. Due to the Clean Air Act, farmers are not getting sulfur for free like they did 20 years ago, so sulfur is often soil limiting.
Molybdenum (Mo) is a micronutrient needed to keep nitrogen from being lost. The nitrogen reductase enzyme needs molybdenum to convert nitrates into amino acids. Sandy soils and soils that are acid (low ph) generally have shortages of available molybdenum, so liming improves molybdenum availability. Adding 1 pint per acre of molybdenum can greatly increase nitrogen microbial uptake and amino acid formation, stabilizing soil nitrogen.
Another strategy uses microbial bio stimulates to promote beneficial bacteria which tie up soluble nitrogen fertilizer. When concentrated nitrogen fertilizer is applied to the soil, the beneficial bacteria often cannot survive in this environment. Soil organic matter, humic acids and microbial bio stimulates not only buffer the soil, but also allow these beneficial bacteria to thrive, so that the nitrogen is used more efficiently. Microbial bio stimulates also contain complex carbohydrates like molasses and other simple sugars which may provide another microbial carbon source. An added benefit is that these microbial bio-stimulates help break down crop residues and improve soil structure and drainage. Nitrogen inhibitors may also be used to inhibit nitrogen transformations but are generally slightly more expensive. Utilizing humic acids, sulfur, molybdenum and microbial bio stimulates along with cover crops utilize natural processes which keep soil nitrogen recycling effectively.