Nitrogen in soil is always changing. Frequently, this dynamic, ever-changing, system is described by what is known as the “nitrogen cycle.” Many educational programs that focus on nitrogen refer to this “cycle” as being very complex. Potential sources of nitrogen for crop growth include animal manures, nitrogen from a previous legume crop in the rotation, crop residues, nitrogen from the atmosphere, soil organic matter, and of course commercial fertilizers. Over time, each source of nitrogen input has been researched at several Land Grant universities. As a result of this research, there is a relatively good understanding of the various transformations affecting nitrogen in soils. However, it is more difficult to quantify the various inputs.
In Minnesota, soils formed from prairie vegetation are characterized by a high organic matter content. This organic matter, present in all soils, can be a significant source of nitrogen for crop production. This organic matter is broken down (mineralized) by bacteria to form usable nitrate-nitrogen (NO3-N). However, the amount of nitrate-nitrogen derived from organic matter each year can vary over a wide range. The rate of nitrogen mineralization from organic matter is affected mostly by soil moisture and temperature. But, just how important is this source of nitrogen in crop production and how long can it be effective?
A recent special project of Discovery Farms in Kandiyohi County Minnesota provided an opportunity to get some measure of the amount of nitrogen supplied from the organic matter of prairie soils each year. For this project, a small field on a cooperating farm (3.27 acres) was planted to soybeans in 2007 and then to continuous corn thereafter. Since 2007, there has been no substantial application of manure or commercial fertilizers. Only a pop-up fertilizer was applied at 3 gallons per acre at planting each year (about 3.5pounds of N/a ). Other management practices conducive for optimum corn yields were used each year. The field is tile drained and water in the lines was collected and analyzed for nitrate-nitrogen each year. Using the same practices and procedures, this field has been in continuous corn since 2007. The soil is calcareous (pH 7.6 to 8.0) and tests high in P and K. Soil organic matter content ranges from 4.0% to 5.0% and soil texture varies from clay loam to silty clay loam.
For the 2015 growing season, corn yield was 112 bu/acre which was higher than anticipated. Other fields on the same farm with similar soil texture produced yields in the range of 205 bu/acre, where adequate nitrogen was used. Except for the popup fertilizer at planting, and in the absence of nitrogen from manure or fertilizer, the mineralization (breakdown) of organic matter supplied all of the nitrogen needed for this yield. Considering nitrogen uptake of approximately 1.5 lb./bushel of corn, a total of about 160 lb. of nitrogen per acre would have been supplied by the soil organic matter. The yield achieved in the absence of no other nitrogen inputs highlights the importance of soil organic matter for supplying nitrogen for crop needs.
Each year water flowing through the tile lines was analyzed for nitrate nitrogen. In 2015, a total of 1.62 pounds of nitrate-nitrogen per acre moved in the tile water. This occurred primarily in May (1.08 lb./acre) and June (0.54 lb./acre) These measurements correspond with 5.0 and 2.7 inches of rainfall in May and June respectively.
Soil organic matter is a mixture of organic compounds. Some are simple; some are complex. This mixture includes, but is not limited to proteins, amino acids, and other related compounds containing nitrogen. Some of these nitrogen containing compounds in organic matter mineralize (breakdown) over several years and can be considered to be slow release sources of nitrogen and other compounds mineralize more quickly in months. The 2015 measurements provide evidence that some N-containing compounds do persist in soils and supply nitrogen for at least nine years and counting.
It would be nice if we could use soil testing methodologies to predict the amount of nitrogen released from the soil organic matter. Research is ongoing in this area. There are routine analytical procedures for measuring soil organic matter and nitrate-nitrogen. However, as stated earlier, the nitrate-nitrogen in the soil can originate from several sources—not just soil organic matter.
In conversations about this project, some people expected that the tile water should have “zero” nitrate-nitrogen when no nitrogen fertilizer is applied each year. This is not a reasonable expectation and is not what this study has shown so far. Even though no manure or commercial fertilizer was added to this soil for nine years, the organic matter in this prairie soil was able to supply some nitrogen for a low yielding corn crop. In spite of the high crop demand for nitrogen from the soil, some nitrate-nitrogen was not taken up by the corn crop and was found in the tile water. This project proves that whenever there is mineralization of nitrogen from soil organic matter, there will always be some nitrate-nitrogen present in tile water. The crop is not able to take up 100% of the N mineralized from the organic matter.
This special project of Discovery Farms-Minnesota is not being conducted for the purpose of predicting the amount of nitrate-nitrogen released from soil organic matter. Instead, this project shows that in a prairie soil, soil organic matter is an important source of nitrogen. Many thanks to the excellent farmers and crop consultants who are cooperating in the Discovery Farms Initiative in Minnesota. With their help, this project will continue and provide more insight in the future. Stay tuned.