Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
For most of Delaware, the optimal window for cover crop planting is in the month of September and we are nearing the end of the planting window where soil health benefits can be maximized. As we move into October, cover crop selection becomes limited due to reduced daylength and lower temperatures thus limiting potential soil health benefits.
Vegetable growers understand the benefit of growing cover crops to maintain soil heath. Most vegetable production systems are tillage intensive and organic matter is oxidized by soil microorganisms at a high rate. Cover crops are an important means to add organic matter back into vegetable production systems.
Cover crop acreage has been growing in the region, largely due to nutrient management efforts and an emphasis on growing cover crops for soil health benefits. Successful programs have been implemented by the USDA-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 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. No fertilizer or limited 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 with cover crop mixtures to support a diverse soil microbial environment.
In other situations, 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 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.
Above-ground biomass for a mulch-based vegetable production system after spring burn-down. Note the differences between the cover crop strips.