Why use Cover Crops?
Cover crops have been grown in agricultural situations in California since the early part of the last century. They have been used in perennial crops and in a variety of field and row crops, either as an integral part of the annual cropping system or as a rotational crop. Cover crops are associated with soil benefits such as improved tilth and fertility, reduced erosion and crusting, and increased water-holding capacity. Field margins, roadsides, banks, levees and slopes can also be planted with cover crops for weed suppression. Cover crops also provide valuable cover, nesting and foraging habitat for a variety of wildlife and can support beneficial insects, which aid in pest control.
A variety of perennial and annual grasses and forbs (broadleafed plants) can be used for cover crops as either single-species or multi-species mixes depending on the farmer’s needs. Fast-growing grass species provide high biomass for boosting organic matter in the soil, while some species of legumes can provide high volumes of nitrogen for the following cash crop. Mixed cover crops can be used to provide a combination of biomass and crop nutrient production. “Green manure” cover crops are typically incorporated into the soil before a cash crop is planted. In an orchard or vineyard setting, many annual cover crops can be managed to self-seed, minimizing needs for replanting and soil disturbance. Perennial grass cover crops can provide basic soil cover, and are typically selected to minimize water and sunlight competition with adjacent trees and vines.
Conditions Where Cover Crops Apply
Cover cropping is useful in a variety of agronomic situations where either rainfall or adequate irrigation are available. In an annual cropping system, the cover crop can be planted after harvest of one crop and before planting of the next to provide soil management and fertility benefits. In a perennial cropping system, a cover crop can be managed between plant rows for similar benefits.
The basic ingredients for a cover crop are outlined below:
- Seed & inoculum (for legumes)
- Standard farming equipment for bed preparation, planting, incorporation and smoothing could include disk, harrow, planer or roller, seed drill or broadcaster, power mulcher, and herbicide. Choice of tools is dependent upon the farmer’s choice and available equipment.
- Mower (for cover crops in orchard and vineyard settings or for cover crop “knock down”)
- Standard irrigation equipment and/or rainfall
Before a cover crop is planted, a suitable seedbed should be prepared. This is usually started after the post-harvest irrigation for perennial crops or after seedbed preparation for annual crops. On farmland, light disking or some other form of tillage is usually sufficient for most cover crops. Disking should be followed by some smoothing operation such as floating or planning so that larger clods are broken and the seedbed is smooth. This is particularly important for smaller seeded cover crops such as clovers. In non-tillage orchards, or with shallow-rooted citrus trees, care should be taken not to till too deeply where a large percentage of tree roots may have grown near the surface.
Unless non-leguminous or grass-only cover crops are used, additional fertilizer is not usually required for cover crops. Otherwise, follow your seed company representative’s recommendation for fertilizer type and rate. Excess nitrogen fertilizer may actually reduce overall nitrogen fixation and give weedy species a competitive edge.
Prior to planting, mixes including large-seeded legumes should be inoculated with the appropriate rhizobial bacteria. Host-specific bacteria work in combination with special root structures to bind or ‘fix’ nitrogen into plant tissues. Some seed is sold pre-inoculated, but large-seeded legumes such as vetch, peas and beans should be inoculated immediately before planting at a rate of about 8 oz. of inoculum per 100 lb. of seed and layering it into the planter hopper. If the seed is broadcast rather than drilled, it should be wet-inoculated to provide better adhesion of the inoculum to the seed.
For planting, the cover crop seed can be broadcast or drilled in. Drilling may require less ground preparation, and is the method of choice for first-time plantings. For single species or larger seeded types, an alfalfa drill can be used. Broadcasting seed is faster and less expensive, but will require a light harrowing to incorporate the seed followed by a final floating or rolling to finish the seedbed. In established perennial cover crops, supplemental seeding may be needed every 2-5 years.
If fall rains are not expected immediately, a light irrigation will settle soil around the seed and hasten germination. Summer annual cover crops will require regular irrigations just as any other warm season crop.
In orchards or vineyards, two to six mowings, beginning in February or March, may be needed from cover crop planting until the cash crop is harvested. If self-seeding is desired, mowing should be delayed until the cover crop has matured seed. When mowing a cover crop mix that includes legumes, care should be taken to not cut below the growing point, or re-growth will be hindered. Mowing, spot-spraying or hand-hoeing may be needed to keep sprinkler or drip emitters clear, but using low-growing cover crops or extending sprinkler risers could reduce the need for such maintenance.
Incorporation of the cover crop (if necessary) should be timed to allow at least two weeks of decomposition in the soil before planting. Timing of incorporation should also be made in consideration of adequate soil moisture for decomposition, otherwise additional irrigation may be necessary to adequately break down the organic matter for proper seedbed preparation for the following crop. In spring, care must also be taken not to enter a field with excessive soil moisture, which would obviously hinder equipment access and also damage the soil with excessive compaction and clodding. The simplest scenario for cover crop incorporation involves “knocking down” the cover crop with either mowing or herbicide, followed by disking the plant material into the soil. After a period of decomposition, the soil surface would then be reshaped and smoothed, as needed. Power incorporators such as the Wilcox Performer have also been used in Yolo County without any other equipment to chop and incorporate a cover crop and prepare the bed for the following cash crop in as few as three passes with ideal soil conditions.