How to manage nitrogen applications in cover cropping systems

Whether you are a seasoned cover cropper or at the beginning of your process, you may be wondering, “How can I maximize the nitrogen fixing or scavenging ability of my covers this season?” or “How should I change my nitrogen management with the use of cover crops?”  Rightly so, as these questions have quickly become hot topics given the high input costs.  As an agronomist and self-labeled soil health nerd, I’ve learned some critical management tips that can help you maximize your nitrogen cycling for reducing input costs.

Manage your cover crop like it is a cash crop to take full advantage of the potential nitrogen savings and maximize soil health benefits. Like a cash crop, we want to ensure our cover crop has the best start. This may require finding unique ways of planting earlier or simply letting the cover crop grow longer to maximize benefits associated with producing a greater amount of biomass. Not only would planting earlier to establish legumes or waiting to terminate longer be important for nitrogen cycling, but you would also pick up the advantage of sequestering more carbon. Click here to learn tips and tricks for planting cover crops. 

This is critical for cycling cash crop and cover crop residue. Why do many tilled and no-till fields have residue dating back to two or three seasons ago? Do we have a fully functioning soil food web with appropriate food sources to sustain all the critters necessary for the residue decomposition process? There are several ways to enhance residue and cycling, and they include variations of the following materials: a light rate of manure, sugar, molasses, amino acids, enzymes, soluble calcium, a little sulfur, a little nitrogen, or other bio-stimulants. These products (when combined appropriately) help provide the necessary foods for our soil life to thrive and prevent residue from fermenting.

However, it may not be the most economical long-term strategy. The best long-term solution should be creating a robust and balanced soil ecosystem that is teeming with life and powered by superior photosynthesis (aka healthy soil that leads to healthy plants that lead back to healthy soil).

Adjust nitrogen timing to account for what the soil microbes and plants need at the different points in the growth cycle of your cash crop. Children provide an apt metaphor for crops.  Imagine trying to feed a newborn a steak. It just doesn’t work because the baby is not capable of chewing, swallowing solids, or producing the necessary enzymes for the digestion of complex foods. In a similar vein, why would a young seedling need a large shot of nitrogen before it has reached the rapid growth phase in corn, for example?

Research from the University of Illinois shows approximately 10-15% of the total nitrogen needs for corn occur before V6. The remaining 85% is taken up from V6 to the end of the season. A cover crop can temporarily cause minor nitrogen tie-up up and applying 25 - 40 units of nitrogen with 4-8 units of sulfur on the planter is helpful for preventing a carbon penalty. Plus, improved nitrogen timing is a qualifying carbon farming practice.

Personally, I like to pair my nitrogen applications with a natural carbon source such as Humic acid, Sugar, or Molasses. These products help buffer the harmful salt effects on soil microbes, create exchange sites for nitrogen to attach to, slowing conversion and reducing leaching (Dong, L. et al. 2012), and they stimulate soil biology.

Sulfur, specifically sulfate, can have a net stabilizing effect on ammonium. The negative charge of sulfate is attracted to the positive charge of ammonium, helping to stabilize both for a short period until soil microbes work on these compounds. In addition, sulfur is a vital part of protein in the plant an,d as such, is a critical building block of multiple amino acids. In many places around the world, we see nitrogen fertilizers as ineffective without the addition of sulfur (Wang et al, 1976; Marschner, 2012)

Molybdenum, not manganese (and yes, it is hard to pronounce) is often overlooked in terms of its importance in nitrogen use efficiency. This nutrient is often needed in such limited quantities that we don’t afford it the attention so deserves. When looked at under a more critical lens, we see that muh-lib-duh-nuhm is essential for two specific enzymes: nitrogenase and nitrate reductase. Nitrogenase is fundamental for converting atmospheric nitrogen into ammonia (rhizobium in legume nodules and free-living nitrogen bacteria like Azotobacter require this) and nitrate reductase is required in the conversion process of taking nitrate and making an eventual protein in the plant.