Posts Tagged ‘cover crops’

Soil Health and Vegetable Production

Friday, April 3rd, 2009

Gordon Johnson, Extension Ag Agent, Kent Co.;

Experienced growers and crop advisors know that one of the keys to vegetable productivity is a healthy soil. According to the Cornell Soil Health Group, “Soil health describes the capacity of a soil to be used productively without adversely affecting its future productivity, the ecosystem or the environment.” “Soil health emphasizes the integration of biological with chemical and physical measures of soil quality that affect farmers’ profits and the environment.”

From a biological standpoint, soil health relates directly to the root environment and organisms that inhabit the soil. A healthy soil for vegetables will be one that has few limits to root growth; supports high numbers of beneficial soil organisms, such as earthworms; supports a diverse microbial community with high levels of beneficial bacteria, fungi, Actinomycetes, protozoa, and nematodes and low levels of plant pathogens (such as root rot fungi, bacterial and fungal wilt organisms, soft rot bacteria, and plant parasitic nematodes). In a healthy soil, vegetable crop root systems explore a large portion of the soil volume, crops are under reduced stress, and pest problems are minimal. A healthy soil will also support mineralization of organic matter by soil microorganisms at levels appropriate to the climate.

From a chemical standpoint, healthy vegetable soils will be at a proper pH (6.0-6.8 in most soils); have a high cation exchange capacity; have optimal levels of calcium, magnesium, and potassium held on exchange sites; contain optimal but not excessive levels of other mineral nutrients needed by crops, have high levels of organic matter in various levels of decomposition and high levels of stable humus; support aerobic mineralization processes; and be free of toxic minerals from natural sources (such as high free aluminum levels) or from toxic chemical contaminants.

From a physical standpoint, healthy soils will have high levels of stable aggregates in the topsoil (creating a stable granular structure); an optimal mix of pore sizes (macropores and micropores) so that it is well aerated in the root zone, well drained, but also has a high available water holding capacity; and a low bulk density relative to the soil texture. They will be free of compaction, which limits root growth. Healthy soils are highly permeable to water and not prone to crusting.

From a management standpoint, vegetable growers have several tools at their disposal to maintain and improve soil quality including:

Crop Rotations
It is critical to choose crop rotations that minimize soil born diseases and at the same time can help to improve or maintain good soil physical and chemical characteristics. Mixing in deep rooted crops, crops with extensive root systems, and crops with high residue in the rotation will add organic matter, leave root channels which benefit future crops, break up compaction, and recycle nutrients from deeper in the soil. Crops that have similar pest profiles should not being planted consecutively, especially those vegetable and field crops that are susceptible to the same soil born diseases. Crop diversity in rotations is a key to maintaining or improving soil quality health.

Cover Crops and Green Manures
These are crops that are specifically used to recycle nutrients and to add organic matter to the soil. They occupy land and time periods in the rotation when grain and feed crops are not being grown. It is important to always have something growing on the land, even when not in production, to maintain soil health. Including cover crops and green manures in rotations increases crop diversity and provides the benefits associated with that diversity. For example, certain cover crops and green manure crops have been found to have benefits in reducing soil born diseases.

Reduced Tillage
It is important to reduce the levels of tillage in soils to maintain soil health. The more that soils are tilled the more soil aggregates are broken down and the more quickly soil organic matter is oxidized (decomposed). Soils that are excessively tilled generally have lower organic matter levels and often have poor physical characteristics. While some vegetables and vegetable cropping systems are not well adapted to no-till planting, there have been some great successes with vegetable no-till, such as pumpkins. Reduced tillage tools may be appropriate for other vegetable cropping systems. Zone tillage, vertical tillage (such as turbo till), and soil aeration are all examples of approaches that may be used successfully in vegetables. Other field crops in the rotation should be planted using no-till or reduced tillage tools as much as possible and attempts should be made to conserve crop residue (as long as it does not interfere with the vegetable portion of the rotation).

Compost, Manures, and Other Organic Matter Additions
Compost, manures, and other organic matter sources can be added to vegetable soils to improve soil quality. This approach is most appropriate where heavy tillage must be used, such as in plasticulture. By adding these organic matter sources you can counteract the effect of the heavy tillage and maintain soil health. These materials offer all of the benefits associated with increased organic matter in the soil: increased microbial diversity, reduced disease pressure, increased nutrient holding capacity, slow release of mineral nutrients, increased water holding capacity, improved aeration, and reduced bulk density.

Traffic Management
Managing traffic in vegetable crops is another soil health key. By reducing trips across a field with heavy equipment and trucks, soil compaction is reduced and soil health is maintained. Limiting traffic to designated areas, driveways, drive lines, or tram lines is another way to achieve this because areas in between are conserved and remain uncompacted. These heavy traffic areas can then be targeted with a subsoiler or other tillage equipment to break up compaction. While it is not always possible, reducing trips across vegetable fields when wet is also important. One pass by heavy equipment over wet soils can destroy the productivity of that area for a long period of time.

In 2009, the University of Delaware Cooperative Extension is launching a soil health education initiative specifically aimed at vegetable growers. In this initiative we will provide vegetable growers with information on soil health and vegetable production, soil health testing methods, how to evaluate soil health on farms, and how soil health testing can fit into an integrated pest management plan. We will be working with growers on how to create healthy rotations for vegetable crops specific to their farms. We will be doing demonstrations and field trainings on the use of different cover crops and green manure crops in vegetable rotations and demonstrations on the use of different types of composted materials and their the effects on soil health and subsequent vegetable production. For more information on this initiative, contact Gordon Johnson or Joanne Whalen.

Cover Crops for Vegetable Rotations

Friday, August 8th, 2008

Gordon Johnson, Extension Ag Agent, Kent Co.;

August is here and it is time to consider late summer and fall cover crop options for vegetable rotations. Reasons to use cover crops:

● Return organic matter to the soil. Vegetable rotations are tillage intensive and organic matter is oxidized at a high rate. Cover crops help to maintain organic matter levels in the soil, a critical component of soil productivity.

● Provide winter cover. By having a crop (including roots) growing on a field in the winter you recycle plant nutrients (especially nitrogen), reduce leaching losses of nitrogen, reduce erosion by wind and water, and reduce surface compaction and the effects of heavy rainfall on bare soils. Cover crops also compete with winter annual weeds and can help reduce weed pressure in the spring.

● Reduce certain diseases and other pests. Cover crops help to maintain soil organic matter. Residue from cover crops can help increase the diversity of soil organisms and reduce soil borne disease pressure. Some cover crops may also help to suppress certain soil borne pests, such as nematodes, by releasing compounds that affect these pests upon decomposition.

● Provide nitrogen for the following crop. Leguminous cover crops, such as hairy vetch or crimson clover, can provide significant amounts of nitrogen, especially for late spring planted vegetables.

● Improve soil physical properties. Cover crops help to maintain or improve soil physical properties and reduce compaction. Roots of cover crops and incorporated cover crop residue will help improve drainage, water holding capacity, aeration, and tilth.


There are many cover crop options for late summer or fall planting including:

Small Grains
Rye is often used as a winter cover as it is very cold hardy and deep rooted. It has the added advantage of being tall and strips can be left the following spring to provide windbreaks in crops such as watermelons. Rye makes a very good surface mulch for roll-kill or plant through no-till systems for crops such as pumpkins. It also can be planted later (up to early November) and still provide adequate winter cover. Wheat, barley, and triticale are also planted as winter cover crops by vegetable producers. Spring oats may also be used as a cover crop and can produce significant growth if planted in late August or early September. It has the advantage of winter killing in most years, thus making it easier to manage for early spring crops such as peas or cabbage. All the small grain cover crops will make more cover with some nitrogen application or the use of manure.

Both perennial and annual ryegrasses also make good winter cover crops. They are quick growing in the fall and can be planted from late August through October. If allowed to grow in the spring, ryegrasses can add significant organic matter to the soil when turned under, but avoid letting them go to seed.

Winter Annual Legumes
Hairy vetch, crimson clover, field peas, subterranean clover, and other clovers are excellent cover crops and can provide significant nitrogen for vegetable crops that follow. Hairy vetch works very well in no-till vegetable systems where it is allowed to go up to flowering and then is killed by herbicides or with a roller-crimper. It is a common system for planting pumpkins in the region but also works well for late plantings of other vine crops, tomatoes and peppers. Hairy vetch, crimson clover and subterranean clover can provide from 80 to well over 100 pounds of nitrogen equivalent. Remember to inoculate the seeds of these crops with the proper Rhizobial inoculants for that particular legume. All of these legume species should be planted as early as possible – from the last week in August through the end of September to get adequate fall growth.

Brassica Species
There has been an increase in interest in the use of certain Brassica species as cover crops for vegetable rotations. Rapeseed has been used as a winter cover and has shown some promise in reducing certain nematode levels in the soil. To take advantage of the biofumigation properties of rapeseed you plant the crop in late summer, allow the plant to develop until early next spring and then till it under before it goes to seed. It is the leaves that break down to release the fumigant-like chemical. Mow rapeseed using a flail mower and plow down the residue immediately. Never mow down more area than can be plowed under within two hours. Note: Mowing injures the plants and initiates a process releasing nematicidal chemicals into the soil. Failure to incorporate mowed plant material into the soil quickly, allows much of these available toxicants to escape by volatilization. More recent research in the region has been with forage radish. It produces a giant tap root that acts like a bio-drill, opening up channels in the soil and reducing compaction. When planted in late summer, it will produce a large amount of growth and will smother any winter annual weeds. It will then winter kill leaving a very mellow, weed-free seedbed. It is an ideal cover crop for systems with early spring planted vegetables such as peas. Brassicas must be planted early – mid-August through mid-September.

Mixtures of rye with winter legume cover crops (such as hairy vetch) have been successful and offer the advantage, in no-till systems, of having a more rapidly decomposing material with the longer residual rye as a mulch.

There are many other lesser known cover crops that can also be used. For more information on cover crops go to: