Nitrogen is one of the nutrients most needed by plants, and it is also one of the most difficult to track. The Nitrogen Cycle is complex, influenced by many factors.
Cover crops can impact N cycling on the farm by:
- “Fixing” N (legume cover crops) from the atmosphere, providing “free” N to the system.
- “Catching” N (especially deep-rooted cover crops), keeping it from leaving the system.
- Providing fresh organic matter to the system, the composition of which, along with the soil characteristics, determines the rate of decomposition and subsequent N availability.
Generally speaking, cover crops with lower C/N ratios such as legumes and brassicas will release N more rapidly after they have been killed/incorporated than cover crops with high C/N ratios such as grasses.
To optimize nitrogen use efficiency:
- Know your crop’s demands- understand the N uptake patterns of crops. Cash crops generally have a period of rapid N uptake only in the middle of their growth period. Supplying N too soon or too late can result in N losses. At a certain point in most crops’ growth, there is very little or no N uptake occurring, but the soil microbes may still be mineralizing N from organic matter, especially if the weather is warm. Planting a cover crop at this time would not interfere with cash crop maturation and can expand the growing period for cover crops.
- Supply N when and where it is needed- target your fertilizer and compost applications, and grow cover crops that will make N available when the cash crop needs it–not too soon, or too late.
- “Mop” up after cash crops with a cover crop to capture mineral N that wasn’t used- even good management practices can leave mineral N in the field. Make sure there is a plant there to keep it from leaching. Choose deep-rooted cover crops to follow shallow-rooted cash crops. If you can’t “afford” to plant a cover crop, follow shallow-rooted cash crops with deeper-rooted cash crops to keep N from leaving the system.
For more on nitrogen cycling on (organic) farms see:
Soil Microbial Nitrogen Cycling on Organic Farms by Louise Jackson, UC-Davis
Soil Fertility in Organic Farming Systems: Much More than Plant Nutrition by Michelle Wander, University of Illinois
Also check out Joseph Heckman’s fact sheet on the pre-sidedress nitrate test for vegetables from Rutgers.