Cover crops are an important part of an organic crop rotation and are vital for improving and maintaining good soil health. Research has shown that cover crops can:
- Reduce erosion and water runoff from agricultural fields by intercepting raindrops and increasing infiltration
- Promote microbial biomass and activities involved in decomposition, leading to increased carbon retention in soil
- Increase soil aggregate stability, total porosity, and plant available water
- Increase hydraulic conductivity and soil water holding capacity
- Reduce bulk density
- Control weeds
- Take up nutrients that might otherwise be lost through leaching or denitrification
- Provide nutrients for a succeeding crop
Cover Crop Mixes
Mixes can increase the amount of above ground biomass and nitrogen, increase winter survival rates of some plants, increase weed suppression, decrease nutrient immobilization by providing optimum carbon to nitrogen ratios, and conserve nutrients for subsequent vegetable crops that might otherwise be lost to the environment.Cover crops can be grown individually or in mixes. Cover crop mixes take better advantage of above and below ground niches of nutrient, light and water demand and are more adaptable to natural soil variability than monocultures.
Mixing legumes and grasses can also increase the amount of atmospheric nitrogen fixed by legumes because faster rooting grasses will deplete soil nitrogen levels, causing the legume component to fix more nitrogen.
We are conducting research on seeding rates for cover crops grown in mixes.
Winter Cover Crops
Winter cover crops can be planted from September till November, depending on the weather and the species being planted. Overwintering species must be destroyed in the spring through mowing, tillage, or rolling/crimping. At Bradford we plant cereal rye and hairy vetch by themselves or as components of a mix. These two cover crop species have proven that they grow well in Missouri conditions and produce lots of biomass. Hairy vetch is a legume and will fix its own nitrogen. It will put on most of its growth in the spring.
Other winter cover crops include crimson clover (left), Austrian winter pea, oats, and wheat. Crimson clover finishes its life cycle earlier in the spring than hairy vetch and may be a good cover crop before corn. However, it does not fix as much nitrogen as vetch. Austrian winter peas should only be used in a cover crop mix because they may winter-kill in very harsh winters. Wheat, cereal rye and oats are fairly cheap cover crops but will attain better growth when mixed with legumes such as vetch, crimson clover or Austrian winter pea.
Summer Cover Crops
Cover crops can also be grown in the summer after a wheat crop or between spring and fall vegetable crops. The photo below shows a plot of organic cowpea on the left and a plot that had no cover crop planted on the right. The plot with no cover crop is full of weeds while the cowpea plot is nearly weed free, illustrating that summer cover crops are much more desirable than fallow fields, especially in organic production. Summer cover crops can also provide nitrogen for subsequent crops if legumes such as cowpea, sesbania or sunn hemp are planted.
In addition to those three legumes, we also grow sorghum-sudangrass and buckwheat. Buckwheat does well in Missouri if the weather is not too dry. Sorghum-sudangrass is very drought tolerant, has thick regrowth when mowed and is excellent for weed control. It is known for its very strong allelopathy, so should probably not be followed by winter wheat. Cowpea, sesbania and sunn hemp also proved excellent at weed control and all grew well in the drought. Summer cover crops can be left to winter kill, providing residue to reduce runoff during winter and early spring precipitation events.
Tillage radish (left) and forage turnip can be planted in late summer. These should be planted no later than mid-August for maximum growth. In 2012, Missouri experienced an increase in tillage radish planting because so much corn was removed from the fields early and chopped as silage, leaving time for a late summer cover crop planting. Tillage radish is believed to scavenge nutrients from the subsoil and to help break up soil compaction. When the tubers die in the winter, they rot and leave large holes that improve water infiltration. Tillage radish and forage turnip provided some weed control, but did not have as much leaf area as the other summer cover crops, thus did not keep weeds back as well as the others previously listed.
Cover Crops and Reduced Water Runoff
In 2012, a runoff demonstration was conducted at the Bradford Research Center comparing corn in conventional tillage, corn in no-till without a cover crop and corn in a no-till plot with a rolled hairy vetch cover crop. In that demonstration, 1.6 inches of water was applied to the tilled corn plot with 50% of it running off within 24 minutes. The no-till corn plot produced no run-off after 3.55 inches of water was applied, but water was about to begin running into the catchment basin at the 24 minute mark. The no-till plot with the hairy vetch cover crop also experienced no run-off within 24 minutes after 3.55 inches of water was applied. Unlike the no-till plot without the cover crop, this plot did not approach the point of run-off.