Dec 8, 2016

Growing cover crops for nitrogen on vegetable farms

Many different cover crops can recycle or add nitrogen to the soil. Legume cover crops are most often associated with adding nitrogen because they have developed relationships with symbiotic soil bacteria which can convert nitrogen gas in the air, which plants cannot uptake, to ammonia, which plants can use. Non-legumes, such as grasses and brassicas, take up nitrogen from the soil and prevent it from leaching away into groundwater. When cover crops are killed and begin to decompose, nitrogen can be released from the residues back into the soil where it can feed the next crop. Recent research has looked as using single species and mixtures of cover crop species for adding and recycling nitrogen.

Figure 1. Graphic by Charlie White, Plant Science Department, Penn State

Charlie White, a research associate at Penn State who studies cover crops, provided Figure 1 here showing how different cover crops and mixtures of cover crops affect nitrogen availability or tie-up. The figure has a two-way arrow with carbon-to-nitrogen ratios of cover crop residues starting at 5:1 and going to 40:1. Carbon-to-nitrogen ratios are important in determining nitrogen availability or tie-up by affecting mineralization when cover crop residues decompose. Mineralization is the process where organic nitrogen, which is largely not available to plants, is converted by soil microorganisms into inorganic (or ‘mineral’) nitrogen that is readily plant available.

When carbon-to-nitrogen ratios of plant material are below about 20:1 these microorganisms release excess nitrogen into the soil which plants can then use. When ratios are above about 20:1 microorganisms tie-up nitrogen from the soil which can result in plants being nitrogen deficient. Looking at the figure, clovers, peas, and radish have low carbon-to-nitrogen ratios while oats and sorghum sudangrass have high ratios. Ratios of canola, cereal rye, triticale, and annual ryegrass are highly variable.

This has to do with when the cover crop is terminated. If it is terminated when it is still young and lush, before it has produced flowers and seed, carbon-to-nitrogen ratios are lower than if terminating when the cover crop is mature. Cover crop mixtures from grower’s farms and those tested at the Penn State research farm also had variable carbon-to-nitrogen ratios. When the mixture had more rye in it the carbon-to-nitrogen ratio was on the higher end of the scale, meaning it would tie-up nitrogen upon decomposition.

When using rye as a cover crop, growing it for maximum above-ground growth is a good strategy when your goal is to harvest it or leave it on the soil surface as a mulch. When rye is tilled into the soil it can inhibit the growth of vegetables by tying-up nitrogen and allelopathy (producing harmful substances to vegetable plants). To add nitrogen to the soil, till in rye while it is still lush and green. Also consider planting it with hairy vetch to add more nitrogen to the mix. Research at Penn State and elsewhere suggests that a seeding rate for non-legumes in a mixture that is 20% to 30% of the typical monoculture seeding rate provides a good balance between soil nitrogen scavenging by the non-legume and atmospheric nitrogen fixation by the legume, with carbon-to-nitrogen ratios generally staying below the critical 20:1 threshold.

A seeding rate of the non-legume species greater than 30% is likely to smother the legume companion and increase the carbon-to-nitrogen ratio. So for rye, which has a typical monoculture seeding rate of 120 lbs/acre, use a seeding rate between 24 and 36 lbs/acre with a 70% to 80% of monoculture seeding rate for the legume companion.

More information on managing cover crop mixtures, including calculating seeding rates.

– Elsa Sanche, Penn State University