Sulfur and nitrogen are an important component of crop production. They often come from multiple sources and can be lost to the environment or immobilized during decomposition. Accounting for all sources of these nutrients can improve farm profitability by reducing application needs or accounting for shortfalls with additional commercial fertilizer. Although the release of some sources of these nutrients are harder to predict than others. Currently, the corn nitrogen rate calculator has the most profitable nitrogen rate based on a nitrogen price of $0.70 per pound and corn price of $5.50 per bushel ranging from 156 to 182 pounds of nitrogen per acre.
Nitrogen availability from manure
Manure is an excellent source of nitrogen but the way it is applied greatly affects how much of the manure test nitrogen will be plant available. When liquid or solid manure is incorporated at application or shortly after for a pre-plant or sidedress application 95% of the Ammonium-N will be available for this year’s crop. The incorporation of manure also reduces the odor as part of manure’s odor in ammonium nitrogen escaping. One day after application, the available Ammonium-N decreases to 50% for solid manure and 70% for liquid manure. Two days later solid manure falls to 25% and liquid 45%, by a week after application without incorporation the amount of Ammonium-N for this year’s crop is 0% for both solid manure and liquid manure if temperatures are above 50 degrees F. Organic nitrogen is not affected by incorporation but is slowly released as soil bacteria break down the organic matter. For most manures, 40% of the organic nitrogen is plant available with 20% released the next year, 10% the year after, and 5% on the 4 years. If your fields are receiving manure nitrogen every year or even every other year this slowly release organic nitrogen can be a major source of nitrogen. For example, if 100 pounds of organic nitrogen from dairy manure is applied each year 40 pounds of nitrogen would be supplied from this year’s application, and an additional 35 pounds from the previous application would become plant available for this crop. Glen Arnold’s research has shown that corn sidedressed in liquid swine manure can yield as well or better than commercial fertilizer.
Nitrogen and cover crops
Cover crops can be both a source and a sink for nitrogen. Legume cover crops are a source of nitrogen, the longer the crop grows the more nitrogen it produces. A two- to three-ton red clover crop frost seeded into wheat produces 70 to 150 pounds of nitrogen for the following crop. But legume crops that grow for less time will produce less nitrogen. Other cover crops that are nitrogen sinks such as grasses and brassicas scavenge nitrogen that is left over from the previous crop. As these cover crops decompose, they may release nitrogen for the next crop if the carbon-to-nitrogen ratio is less than 20 to 1. Cover crop mixes can either be a source or sink of nitrogen depending on what the carbon-to-nitrogen ratio of the cover crop is at termination. As the carbon-to-nitrogen ratio increases, research has shown a benefit to an additional nitrogen application to overcome the decomposition penalty as soil microbes utilize other nitrogen sources to be able to decompose the cover crop. Purdue researchers studying corn planted into headed cereal rye needed an additional 50 to 70 pounds of nitrogen over corn without that cover crop. The utilization of precision ag technology such as NDVI sensing for crop nitrogen status can be a good tool to determine at tassel if more nitrogen is needed for your crop.
Sulfur comes from many sources manure, atmospheric sulfur, elemental sulfur, organic sulfur, gypsum, and sulfate sulfur. The best way to know how much sulfur is coming from manure is through a manure test but liquid dairy manure has approximately 4.2 pounds per 1,000 gallons and liquid swine manure contains about 7.6 pounds per 1,000 gallons. Solid beef manure contains 1.7 pounds per ton and poultry 3.2 pounds per ton. Just like nitrogen, not all the sulfur in manure is available in the first year. Approximately 55% of the sulfur in manure will be available in the first year. The sulfur that is bound in the manure organic matter goes through bacterial oxidation transforming it to sulfate-sulfur, which is plant available.
Atmospheric sulfur levels have been decreasing across Ohio. There are still 10 to 20 pounds of sulfate-sulfur deposited each year. Other forms of synthetic sulfate sulfur that may be applied are ammonium sulfate, ammonium thiosulfate, magnesium sulfate, and potassium sulfate which will all be available for the crop year of application. If elemental sulfur is used, it will require soil bacteria to transform it to sulfate-sulfur that is plant available. The rate of this transfer depends on soil temperature, sulfur particle size, and the amount of incorporation into the soil. Elemental sulfur is a good sulfur source as long as a delayed release is acceptable. Response to sulfur fertilization in Ohio has been most common on sandy soil with only minimal responses on clay loam soil. A 60-bushel soybean crop or 200-bushel corn crop each removes about 10 pounds of sulfur in the grain while additional sulfur is left in the fodder to be returned to the field.