It’s often said that no-tillers choosing cover crops for their farm operations should have goals in mind. One could be scavenging nitrogen in the soil and holding it for the next cash crop, rather than letting this expensive nutrient run off fields into local watersheds.

So how much nitrogen will be released by a cover crop next year, and when? The age and stage of the cover crop when it dies, and how fast it decomposes, are important factors that no-tillers can’t entirely control, says Purdue University soil physicist Eileen Kladivko.

But no-tillers can still optimize this nutrient availability by knowing the carbon-nitrogen ratio of the residue in many popular cover crops, Kladivko told attendees at the National No-Tillage Conference earlier this year.

A carbon-nitrogen ratio greater than 25:1 may cause nitrogen immobilization, with smaller ratios releasing nitrogen more quickly, Kladivko says. Cereal rye, for example, has a higher-carbon residue of 26:1 to 37:1. Corn stover stands at 57:1, and wheat straw at 80:1.

But legume cover crops have lower carbon levels, she says, and will release it quickly as they decompose. Hairy vetch has a ratio of 11:1. “Legumes have excess nitrogen in their tissues, and the microbes release that nitrogen so you get mineralization,” Kladivko says.

You can learn more about Kladivko’s lessons about cover crops — and some interesting facts she shared about winter drain flow and runoff — in our main feature story in this edition of No-Till Farmer E-Tip.