Cover crops offer many advantages for agriculture. They contribute organic matter, curb soil erosion, promote soil health, suppress weeds, help control pests, attract pollinators, provide forage, improve air quality, conserve moisture and boost fertility.
Several plant families comprise cover crops. Legumes, with their nitrogen-fixing roots and taproots, include the clovers, peas and hairy vetch. Grasses, such as barley, oats, ryes, sorghum-sudangrass and wheats, typically have fibrous root systems. The brassicas are nonmycorrhizal, have a taproot and include mustards, radishes and rapeseed. Buckwheat and sunflowers also can be used as cover crops.
Leguminous cover crops fix atmospheric nitrogen. However, the seed must be inoculated with the proper Rhizobium strain. The amount of the nitrogen fixed relates to the volume of biomass produced by the leguminous cover crop. This is due to its nitrogen content and carbon to nitrogen ratio (C:N) staying relatively constant.
In annual legumes, the nitrogen fixed peaks just at their blossom time. Early recommendations for cover cropping have noted that fixed nitrogen does not become available to the next crop until the legume decomposes and is released when plowed into the soil. But recent research with hairy vetch reported in Pennsylvania’s “The Agronomy Guide, 2015-2016,” has shown that the cover crop need not be incorporated to benefit the following crop. In their spring or summer cover crop trials, such as red clover, white clover, field peas and sweetclover legumes, incorporation did indicate more rapid decomposition and greater amounts of nitrogen released earlier in the season.
Weather and soil moisture play a role in the nitrogen release. Since growing requirements and performance of cover crops vary widely among regions, growers should consult their local extension office to determine the best cover crops and appropriate planting times for their area. Also, more research is needed on this subject.
Cover crop protection from erosion is widely recognized. More importantly, cover crops take up excess nutrients that might leach out of the soil profile. With bare soil, if the crop has not used all the nitrogen at season’s end, nitrates will leach in rain events. Leached nutrients not only pollute streams and groundwater, but also deplete the soil. Nitrogen in the nitrate form is the most water-soluble and therefore highly vulnerable to leaching. Nonlegume cover crops with deep, extensive root systems work well at conserving nitrates. Cold-tolerant rye is often recommended. Cover crops also help retain phosphorus in the soil by reducing erosion.
In Pennsylvania, winter-hardy cover crops can capture the nitrogen released in the fall and winter and make it available to the following crop when the cover crop decomposes. A crop such as oats, which dies in the winter, does not provide the nitrate leach protection. Plus, the more growth the cover crop produces, the higher the nitrate taken up. Rye, wheat, barley and ryegrass are recommended.
Figuring out fertilizer needs
In any crop, determining fertilizer needs is essential for management. Dr. Heather Darby, University of Vermont Extension agronomic specialist, advised, “Don’t guess, soil test.”
In her guide “Feeling the Squeeze: Manage Nutrients Efficiently to Offset High Fertilizer Prices,” Darby said soil should be sampled every three years and when crops are rotated. She also noted that individual fields vary greatly in their capacity to supply essential nutrients, including phosphorus and potassium. Organic matter and pH affects management, too.
Darby noted that nitrogen recommendations for corn are based on an estimate made from expected yield, nitrogen credits from previous crops, and manure and the soil drainage class. She said to evaluate the potential of a legume or grass hay crop to provide nitrogen and noted the role of the pre-sidedress nitrate test (PSNT) to indicate nitrate release. Instructions for measuring manure and its availability are included.
The land grant universities can perform the test and provide information on the PSNT, plus the early-season chlorophyll meter test for corn and the late-season cornstalk nitrate test.
Sjoerd Duiker, Penn State University professor of soil management and applied soil physics, said, “Cover crops are becoming an integral part of crop production in the Northeast. This is due in large part to the increasing adoption of no-tillage systems, because cover crops can be managed more easily than with tillage, while cover crop residues in no-till systems lead to multiple benefits.”
In the “Managing Cover Crops Profitably, Third Edition,” published by the Sustainable Agriculture Network, Dr. Duiker noted the diversity of cover crop options in the Northeast. He reported, “Rye, wheat, oats and ryegrass are the most common grass cover crops; hairy vetch, crimson clover and Austrian winter pea are important legumes; buckwheat finds a place in many vegetable systems and brassica crops such as forage radish are increasingly being tested and used in the region.”
In “Nutrient Management with Cover Crops,” Ohio State University professor and Extension educator Alan Sundermeier explained research with oilseed radish. In the study of capturing and recycling of excess soil nutrients in plant biomass, the results revealed that compared with no cover crop after manure application, oilseed radish reduced soil nitrate levels by 70 percent. In addition, 9.8 tons per acre of dry matter biomass absorbed 603 pounds per acre of nitrogen, 126 pounds per acre of phosphorus and 1,060 pounds per acre of potassium. Due to its fleshy composition and low C:N, the oilseed radish plant material easily decomposes and the nutrients become available for the following cash crop.
“Managing Cover Crops Profitably” lists top species by purpose by region. For the Northeast, red clover, hairy vetch, berseem and sweetclover are listed as nitrogen sources. Except for the omission of sweet clover and the addition of crimson clover, those are also listed for the Mid-Atlantic.
In the performance and roles chart in that publication, the legumes berseem clover, hairy vetch and sweetclover have an excellent rating; crimson clover and red clover rated very good as nitrogen sources.
In the same chart as nitrogen scavenger ratings, the nonlegumes radish and rye are rated excellent; annual rye and berseem clover are very good; and crimson clover and red clover rated good. For brassicas, radish is rated excellent and mustard is good.
Among the legumes, berseem clover’s rating is very good; crimson and red clover was good; and hairy vetch and sweetclover rated fair.
The publication also includes details for many cover crops and designates whether the crop is for summer or winter by region; its role such as soil builder, fertility source, weed suppressor and erosion preventer; and other cover crops in which mixing is desirable.
More diversity of cover crops is possible with mixtures. Also, cover crop mixtures can be more effective than a single species, especially with respect to the growing season and maturity dates and growing habit. For example, the vines of hairy vetch can obtain more light by climbing a grass, and thus, fix more nitrogen.
Research on the vast benefits of cover crops is ongoing, including the effects of differing mixtures, crop rotation, plus the role of cover crops in nutrient management and assessments of new and less-used cover crops.
Read more: Nutrient Management: Plan to Make a Plan