Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
Cover crop acreage has been growing in the region, largely due to nutrient management efforts and cost share programs. In the last year, there has been an emphasis on growing cover crops for soil health benefits and programs are underway from NRCS and Conservation Districts to increase cover crop plantings for soil improvement.
Nutrient management goals and soil health goals are not necessarily the same. You can think about this with the question are you growing cover or crops?
In nutrient management based cover crop programs, the goals are to have crops that can take up residual nitrogen and also provide cover to reduce erosion losses. Non-legumes predominate, with most of the acres planted in small grains such as rye with some recent use of radishes (Maryland programs are non-legume based while Delaware conservation district programs allow for the use of legumes). No fertilizer can be used with these cover crops. In this case the answer to the question above is that a cover is being grown. While there will be soil health benefits, they are not maximized.
In contrast, when soil improvement is the primary goal, the cover crops are grown as crops. You are growing plants to maximize the benefits they provide. To increase organic matter and improve soil health the main goal is to produce maximum biomass above ground and below ground. A secondary goal would be to provide different types of organic matter (such as with cover crop mixtures) to support a diverse soil microbial environment.
In other cases the goals will be different. With leguminous cover crops a goal may be to maximize the amount of nitrogen fixed. With soil compaction reducing crops such as radishes, the goal is to maximize the amount of “biodrilling” – the amount of tap roots being produced. With biofumigant crops, the goal is to maximize the production of fumigant-like chemicals the crops produce. With mulch based systems, the goal is to maximize above ground biomass.
What these soil improvement and specific use goals have in common is the need to treat the cover crop as a crop to optimize plant growth. This would include seeding at the proper rate to achieve optimal stands, planting at the right time, using seeding methods to get maximum seed germination and plant survival, having sufficient fertility to support good plant growth, providing water during dry periods, managing pests (insects, diseases, weeds), and inoculating legumes. If cover crop mixtures are being used, the ratios of seeds being planted must be considered to have the best balance of plants in the final stand.
The best cover crop stands are obtained with a drill or seeder that places the seed at the proper depth, at the proper seeding rate, with good soil to seed contact. Fertilization and liming programs should be used to support season-long growth – fertilizers and other soil amendments will be necessary in most cases. Nitrogen will need to be added for non-legumes.
When the crop is terminated is also key. The cover crops should be allowed to grow to the stage that maximizes the benefits they have to offer before killing the crops. Allowing a winter cover to grow for an extra week in the spring can make a large difference in the amount of biomass produced.
Again, consider the question are you growing a cover or a crop? The answer is important to achieve your cover crop goals.