Do Cover Crops Produce Nitrogen for Your Next Crop?

Cover crops provide many benefits to agricultural soil, increased organic matter, erosion reduction, weed suppression, moisture retention, improved soil structure, disrupting pest cycles, and the list goes on. However, a common question is, do they provide nitrogen to the next crop? The straightforward answer is yes, maybe, sometimes, and it depends…

The first thing to consider is the type of cover crop. Legumes such as clover and peas have the potential to provide nitrogen. Grass species such as rye and oats are not likely to provide any nitrogen to the next crop. It all comes down to the carbon to nitrogen ratio. The majority of nitrogen in plants is tied up in proteins. Legumes have more. The problem is that protein is not plant available and must be decomposed by soil microbes to be released as ammonium or nitrate for the plants to utilize. In order for the microbes to do their job, they need a C:N ratio of about 25:1. Grass species generally have a C:N around 30:1 to 50:1. This means that the microbes need to find another source of nitrogen to breakdown these cover crops leaving none to be released. On the other hand, legumes generally have C:N around 15:1 to 25:1. When there is more nitrogen in the cover crop than the microbes need, it can be released to the soil solution for the crop to utilize.

As with anything in agriculture, weather has the greatest impact on the potential for nitrogen release from cover crops. The microbes need warm soil temperatures and adequate moisture. Fortunately, when conditions are favorable for microbial activity, it is also favorable for plant growth which means there is a greater chance of utilizing the nitrogen.

So, how much nitrogen can you realistically expect from a legume cover crop? Some research has claimed that a clover cover crop can provide 70 to 90 pounds of nitrogen per acre. This is not entirely true. When testing the biomass of the cover crop, it is definitely possible for there to be that much nitrogen, but it does not, the crop will have access to all of it. Another misconception in some of the research is that all the yield gained following a legume cover crop is a result of a nitrogen contribution. As mentioned above, cover crops can provide many benefits that can improve yield. So, if you reduce a nitrogen application by all the nitrogen in the cover crop biomass, you may be under applying leaving missing out on potentially higher yields. A more realistic expectation following a well established legume cover crop is about 30 pounds of nitrogen per acre. Another thing to consider is tillage. A cover crop terminated through conventional tillage is more likely to decompose in a timely manner than one that is terminated with herbicide in a no-till situation due to the increased soil contact with the plant tissue.