Wednesday, March 12, 2014

Cover Crops to Build Soil, Crowd Out Unwanted Plants, and Create Biomass

It is very important the the earth be able to grow and breathe at all times of the year. Our gardens and fields are a living being and will always try to fill an open space with growth. You can decide what grows there, you can choose the plants that dominate an area. Let me explain. I weed an area by throwing the proper seeds on top to compete. The more aggressive the plant the taller the cover crop I plant. This “canopy closure” puts tiny, emerging weeds in the shade and hinders their growth. I have eliminated crabgrass with shade out, by planting tall fava bean cover crops and tall densely planted garden varieties and by using all matter produced on that spot as a very deep mulch between cycles.  I pull very few plants, most I chop and drop in place, keeping nutrients where they were for the most part. 
 Soil must always be naturally covered and shaded by plants or organic matter. I do not recommend any printed cardboard (which is less offensive) to plastic for weed suppression, plastic is toxic and should not be used at all. Plastics degrade in sunlight, but more worrying is when in direct contact with the ground will begin to percolate plastic laden water from distillation off the inside surface of the plastic. Would you drink water made in a plastic distiller?  

Buckwheat is a lovely choice for creating green matter, will re-seed if allowed. It is a short season annual which makes it very useful for weed suppression, and to create shade between rows and after harvesting early vegetables. It draws, concentrates, and makes available phosphorus & calcium very quickly. It is a easy first plant in a new plot as it develops aggregate stability and is easy to turn in even by hand.

buckwheat between rows of broccoli 

Clover for nitrogen and soil building in low fertility soils as well. Dutch White clover is a slow growing nitrogen fixing perennial. Once established it provides long term cover, either alone or with other plants. Red clover is shorter lived and should be mowed down when flowers arrive. Both are excellent in high traffic areas. Clovers are beneficial bug attractants as well as medicinal for teas.

Red and rose colored clover gone wild

Fava Beans are also used as a cover crop to protect delicate soil, because they grow quickly and produce a great deal of lush foliage. In addition, like most legumes, fava beans are nitrogen fixers, and they replenish the soil with this vital nutrient. Many farmers plant fava beans and plow them back into the field after the growth has peaked or chop them down for mulch.

fava beans

Daikon Radish, a relatively new idea for a cover crop, special oil producing strains produce a very large and long root, which can penetrate into the soil 8 inches or more to breakup hardpans. It is an excellent accumulator of excess nitrogen and phosphorous, left in the soil from previous crops or manure applications. Also, being in the Brassica family, the vegetative and root growth are low in lignin, which allows the plant material to break down rapidly, releasing nutrients for following crops. Large roots rot in place allowing water deeper into hard soils even without tilling.

Imagine all those roots punching down into hard soil.

Last seasons dead radish root left in place creates an easy path
 to deep cool soil and moisture for a young lettuce.

Oats, Spring planted oats are used for a green manure while a fall planted oat cover crop is intended to die off and create a protective cover for soil all winter. Easy to kill off and creates good cover for other plantings of cover crops.
Sow your oats!

Rye is a fantastic grain and soil saver it has deep roots that reduce the leaching of nitrates. It can be difficult to control but can be broadcast directly on surface of soil eliminating the need for soil prep. Most useful for ravaged land and erosion control. Hairy Vetch is often added to rye oat or wheat cover crops as a support plant.

mature winter rye and hairy vetch

 Cow Peas, fodder beans, black eye'd peas are an annual legume and nitrogen fixer and source. Fast growing they quickly crowd out weeds. They are and excellent soil builder on transitional land or long fallow land. They decompose quickly and offer high nutrition to grazing animals. An excellent choice for areas to be cleared by animals before planting out vegetable crops. 

regular cow pea

a purple cow pea variety
Hairy Vetch another legume and nitrogen fixer, showy with showy purple flowers good for stabilizing steep banks road sides and as a decent graze fodder in the field . Many folks use it as a nurse crop between rows of trees or grapes to shade soil and reduce watering. It is often mixed with oat or rye cover crops as well.

close-up of vetch flowers and leaves
habit is trailing and climbing

Inoculation of legumes; Legumes  live in a close relationship with rhizobia bacteria that invade and establish themselves in the roots of the plant as it grows. These bacteria take nitrogen from the atmosphere (which is about 80 % N2) and make it available to the plant in a usable form. In return for the nitrogen, the legume gives the bacteria carbohydrates. Seeds are treated with an inexpensive powder before planting out by wetting and coating seed with powdered bacteria sold by your seed supplier. 

:: additional study sources, citations and photo's ::