Plants need nitrogen to grow and produce high-quality crops. How much will be required is difficult to predict with absolute certainty. What we do know is that the soil in crop fields can be a very important source of nitrogen for crop growth.
Farmers and crop advisors will need to better understand a soil’s ability to supply nitrogen to ensure that added nitrogen from fertilizer and manure is enough for optimum yields. Estimating the nitrogen contribution from soil is challenging as weather conditions and management can greatly influence the supply. However, some estimation is needed as mistakes in nitrogen fertilizer management can be costly.
Let's do a calculation
If a farm field with a silt loam soil has 3.5 percent organic matter by weight and we assume there are about two million pounds of soil per acre in the top six inches of soil, the total amount of organic matter in that “acre-furrow slice” is about 70,000 pounds. If organic matter has an average of about 10 percent nitrogen, there are approximately 7,000 pounds of nitrogen per acre in the top six inches of soil. Only a small portion of this nitrogen will be available to a growing crop each season, but a small portion of 7,000 pounds can be significant.
A common and possibly conservative rough estimate is that one percent of the total nitrogen will be available each year to a crop. In this example, 70 pounds of nitrogen is expected to be available for crop uptake due to breakdown of soil organic matter alone. Fields with higher overall organic matter, a history of manure addition, or large amounts of active organic matter are likely to provide even more nitrogen to a crop each year.
All nutrient management plans should account for soil’s natural ability to supply nitrogen. Granted, in most situations, nitrogen made available through organic matter mineralization alone is not enough to support optimum crop yields for corn or grass, but ignoring this contribution can result in excess nitrogen application with fertilizer and manure.
We know crops like corn and grass hay that take up large amounts of nitrogen, and are unable to fix nitrogen from the air, will suffer significantly when nitrogen is in short supply. We also know that due to unavoidable losses and biological inefficiencies, more nitrogen needs to be supplied than will be taken up by the crop.
To the farmer, the yield and quality penalties for not supplying enough nitrogen are substantial. However, excess nitrogen has environmental implications. In wet conditions, nitrogen fertilizer remaining after harvest is not likely to be carried over to the next growing season and may be lost to the environment. Increasingly, in irrigated regions, we are hearing about groundwater concerns from the nitrate form of nitrogen leaching into groundwater.
In addition, the retail chain is increasingly interested in production practices on dairy farms, and processors are being asked by many organizations to fill out forms about practices used on the dairy farms that ship milk. Some retailers want to know how much nitrogen is used to produce a ton of corn silage. This suggests that it will also be important to continue to refine dairy farm nitrogen use to maintain a market for milk. Routine annual evaluations of nitrogen management are an essential step toward this goal. Such assessments leverage data from current practices to provide a basis to fine-tune estimations of nitrogen supply from organic matter, crop residue, and manure applications under various weather conditions and management practices.
This article appeared in PRO-DAIRY’s The Manager in March 2021. To learn more about Cornell CALS PRO-DAIRY program, visit PRO-DAIRY Cornell CALS.
