Cover Crops and Green Manure Crops

A farmers’s goal is have productive fields every year. Applying synthetic fertilizers during the growing season is not sufficient for maintaining a sustainable soil.  After harvesting crops, a good management practice is to buildup and maintain the soil during the off season so that it will be more fertile and productive for the next growing season. Growing cover or green manure crops is a key for this desired goal since they help maintain soil fertility, soil health and productivity instead for harvesting. The terms cover crops and green manure crops are sometimes used interchangeably based on the growers perspective. A cover crop is usually a specific annual, biennial, or perennial grasses or legumes or a combination of two or more grown between regular growing seasons for the main purpose of preventing soil erosion by protecting and improving the soil. When cover crops are tilled into the soil while they are still growing is referred to as green manure crop. A green manure crop is usually grown to help maintain soil organic matter and nitrogen availability.

Why grow cover/green manure crops?
Cover crops can protect soil from wind and water erosion, suppress
weeds, fix atmospheric nitrogen, scavenge soil nitrogen, build soil structure, reduce surface crusting, improve water infiltration, break hardpan, improve soil/water quality and reduce insect pests. Benefits from these crops depend on the amount of growth of the crop, know as biomass, before the soil is prepared for the next crop. When cover crops are buried and tilled into the soil, the green manure that is added enhances soil fertility and structure by feeding soil microbial populations which also glue together soil particles to form soil aggregates. When plant material is decomposed by soil microbes, they break down and release nitrogen and other nutrients to the soil. Nitrogen accumulation and release is greater with legumes, which have nitrogen fixing bacteria in their roots.

crimson clover on left
hairy vetch on right

Cover Crop Nitrogen Accumulation*

• Hairy vetch has 3.2 lbs/1000 sq. ft of nitrogen accumulation
• Crimson clover has 2.6 lbs/1000 sq ft of nitrogen accumulation
• Austrian winter pea has 3.3 lbs/1000 sq ft of nitrogen accumulation
• Winter (annual) rye has 2.0 lbs/1000 sq ft of nitrogen accumulation

*Nitrogen accumulated in growing crop prior to tilling under

Source: ATRA: Overview of Cover Crops and Green Manures

Selection of Cover Crops
Success in the growth of cover crops requires proper selection of the cover crop, correct timing of seeding and management practices. Species selection depends on targeted planting date and the purpose for growing it. Legume cover crops have a symbiotic relationship with bacterial that fix atmospheric nitrogen into a form plants can use. Non legumes species scavage existing soil nitrogen and other nutrients and reduce leaching losses of nutrients your cash crops needs for the next year. There are many traditional cover crops to select from, including annual rye grass, cereal rye, winter wheat, oats, white clover, sweet clover, crimson clover hairy vetch and buck wheat. Grasses are easier to establish than legumes such as clover as they germinate quickly and do not require inoculation.

Early vegetable harvest begins in mid to late summer. Rather than leaving the ground open to weeds, the land can be improved by planting over crops. For planting in July/August the main choices are buckwheat, clovers and Sudan grass. These cover crops are best when sown during July through early August. If crop space becomes available after harvest in late August and September, barley, annual rye grass, oats and clover can be successfully established. The last date by which cover crops can be planted in Missouri will be the end of October to early November.  Winter annual grasses such as cereal rye and wheat can be planted by the beginning to mid October.
Given the growing conditions in Missouri, annual rye grass can be considered first for a vegetable cover crop. Winter rye is another good choice that is best for late planting.


Establishment of cover crops are similar to planting any garden seed including raking the crop area and removing the residues. Next broadcast the cover crop seed of your choice and lightly rake the soil to  incorporate the seeds with the surface soil and water the soil surface lightly to provide the required moisture for germination.

When to kill cover crops in spring?
Early to mid April is the best time to kill over wintering grass cover crops whereas legumes should be allowed to grow longer into the spring. They can be killed with an herbicide or plants can be killed by plowing them in to the soil.. To get the most nitrogen out of grains such as rye, the best time to kill is when they have greened up after winter and are about 6 inches tall. When rye is larger than 6” nitrogen can get tied up in soil by a process referred to as nitrogen immobilization which can prevent it from being available when your plants needs. To get the full nitrogen benefit from legumes they must be allowed to grow until they begin to bloom. Afterward they can be killed by shallow tillage.  Do not let them go to seed.

A good reference on cover crops and green manure crops is Managing Cover Crops Profitably which explores how and why cover crops work and provides all the information needed to build cover crops into any farming operation.

(by Manjula Nathan, Director, MU Soil and Plant Diagnostic Service Laboratories and Tim Reinbott, Superintendent, Bradford Research and Extension Center)