Should You Fertilize?
Goals and growing conditions will determine fertility requirements. If a crop is poor due to drought, flooding hail, etc. cover crops will scavenge and utilize residual nutrients in the soil. If covers are following a high-yield crop where few residual nutrients can be found, some extra fertility may be needed. This is especially important if you are early in a system of no-till and cover crops, as your soils are still trying to reach a state of balance and equilibrium. Covers that are designed to be grazed may also require additional fertility for maximum production. We recommend soil testing and supplementing nitrogen to get to 40-50 units of total nitrogen in a grazing mix with a low percentage of legumes.
NItrogen-Fixation from Legumes
Fixing atmospheric nitrogen by utilizing rhizobium colonized on legume roots is a common goal for many producers. We inoculate all of our mixtures that include legumes to give the best chance at good nodulation. Many factors can affect nodulation, including soil pH, residual soil N, and length of time inoculant is on seed. The longer legumes grow, the more N fixation potential you will realize. Please note that if your field has more than 30 units of N in the soil, this could hamper N fixation because the plants will be lazy, utilizing the N that is available instead of fixing their own. Organic N should be looked at like manure, where on average you can expect a nitrogen credit of 50% to be rendered available in the first year, 30% the second, and 10% during the third year.
Balancing the carbon to nitrogen ratio of a cover crop will determine nutrient cycling and residue maintenance. High-carbon crops like wheat and corn leave long-lasting residue which can be offset by utilizing lower-carbon crops from the brassica and legume families. Conversely, lowcarbon crops like soybeans leave little residue to protect the soil’s surface. It is vital in these systems to have a high-carbon living cover, such as cereal rye, to protect and build the soil.
Cover crop mixes should be developed to produce the desired carbon to nitrogen ratio to meet the goals that you have. Mixes that produce biomass with a C:N ratio of 20 or less will decompose and cycle nutrients quickly but leave the soil without significant residue cover. Cover crop mixes that produce biomass with a C:N ration of 30 or more will break down more slowly and leave more residue cover but can also tie up nitrogen and other nutrients for a longer period of time. C:N ratio mixes between 20 and 30 will generally be a good compromise between too fast of a breakdown and too much nutrient tie up. Green Cover Seed’s SmartMix® calculator is the only tool available that will give an indication of C:N ratios of cover crop mixes as they are being developed. Refer to pages 28-29 for a complete description of the SmartMix® Calculator.