Soil Management for Vegetable Crops
Cover Crops in Vegetable Production
An intense vegetable cropping system removes large amounts of nutrients from the soil. Unfortunately, some of the practices used in high intensity vegetable crop production can be detrimental to soil health if proper attention is not paid to soil management. Problems with loss of organic matter, soil erosion, and nitrate contamination of ground water can arise in vegetable production systems if issues associated with tillage, soil organic matter levels, and off-season leaching of nutrients are not addressed. Long-term fallow periods can be very beneficial for improving soil quality, but are not always practical or cost effective.
Benefits of Cover Crops
Cover crops with shallow fibrous root systems, such as many grasses, build soil aggregation and alleviate compaction in the surface layer. Cover crops with deep tap roots can help break-up compacted layers, bring up nutrients from the subsoil to make them available for the following crop, and provide access to the subsoil for the following crop via root channels left behind. Cover crops can thus recycle nutrients that would otherwise be lost through leaching during off-season periods.
Cover Crop Selection for Vegetable Growers
Matching suitable cover crop species to objectives and regional conditions is critical. The first step in the selection of a suitable cover crop is identifying its hardiness zone of the location and crop tolerance to cold temperatures (Section 16.8). Once producers select a main objective (e.g., nitrogen fixation, C:N ratio, biomass production, etc.) the next step is to choose a cover crop species that will fit into the vegetable production system. In selecting a cover crop, producers should consider the objective the cover crop is intended to accomplish, the seasonal adaptation, and the following cash crop. Generally, one of three groups of plants—or a combination of these—can be used as a cover crop. One group is the grasses or grains. Examples include wheat, rye, or oats. Legumes are another group that would include plants such as clover, vetch, sunn hemp, and Austrian winter pea. Finally, several of the brassicas also are used as cover crops.
Single Species vs. Mixture Cover Crops
Mixtures of cover crop species can be planted to optimize the benefits associated with cover crop use. Mixtures which include species that establish quickly can reduce soil erosion. Above-ground biomass, and consequently nitrogen in the above-ground biomass, can be increased by a mixture that can utilize more below-ground and above-ground niches for nutrients, water, and light. For example, a deep-rooted cover crop can be combined with a shallow rooted cover crop to utilize water and resources in more of the soil profile. Competition for soil nitrogen in mixed stands results in increased biological nitrogen fixation by the legume. Cereal crops usually germinate and establish effective root systems more rapidly than legumes and effectively lower soil nitrogen concentration.
Crop Rotation with Cover Crops
Integrating cover crops into a vegetable production system is somewhat different than with field crops. Vegetable production is seasonally more diverse than most field crop production systems, with both short and long season crops and more intense management throughout the season. Cover crops can be chosen to fit within existing vegetable rotation sequences, or new rotations can be designed with cover crops in mind to revamp production systems. In either case, cover crop types and practices should be carefully selected to match the desired outcomes for the crop as well as the integration with the timing of the cash crop management practices.
Cover Crop-Vegetable Rotation Practices
The sample rotation sequences vary from farm to farm. The following provide some of the crop rotation practices employed vegetable farmers:
Strip-Cropping with Cover Crops
Strip-cropping involves planting strips of legumes or legume/grass/grain mixes and strips of vegetables across fields. Cover crop strips are left for 1 to 3 years then rotated back to vegetables. Strip widths can be adjusted as well as to accommodate changes in slope or other field characteristics. Strip-cropping is best suited to fairly rectangular fields larger than ½ acre. The legume strips are mowed as needed. This method has advantages over whole field growing of both vegetables and legumes. Traffic and compaction are limited to those strips best suited to handle it—legume sod strips.
Interplanting with Cover Crops
In this method, the cover crop legume is planted in early spring. Several weeks after germination, a strip is tilled out; vegetables are planted in this tilled strip. The cover crop between the strips is managed just as it would be if the whole strip was in legumes. This method is especially adapted to transplanted crops such as brassicas (e.g., cabbage, broccoli, cauliflower, Brussels sprouts, collards, kales, and kohlrabi) or tomatoes. Selecting the right cover crop is important. Considerations are fast growing versus slow growing legumes. Fast growing legumes will provide a lot of mulch but may interfere with the cash crop. The distance between vegetable strips is based primarily on the mowing width of the grower’s mower unless the crop will be harvested before the cover crop starts rapid growth.
Seasonal Cover Crops for Vegetables
Cover crops can be added to a vegetable rotation at several points in the year. They can be grown in the winter when sown in early fall, in the summer when sown in late May or June, as a spring cover sown as soon as the ground can be worked, as an intercrop between rows, beds, or blocks of vegetables, or as a long-term fallow in a field taken out of vegetable production for a season or more. Some of the common seasonal cover cropping niches include: winter fallow, spring fallow, summer fallow, or full-year fallow.
Winter-Fallow Cover Crops
Winter cover crops are generally planted in late summer to fall before a hard fall frost, typically following harvest of a cash crop, left over the winter, and then incorporated in the spring before planting the vegetable crop. Although in northern climates the choices are limited by the short growing season, planting a winter cover crop can provide protection from soil erosion, suppression of weeds and root pathogens, contribution of nitrogen to the next crop, and increased soil organic matter and aggregation. An overwintered cover crop is more likely to supply nitrogen to the following crop if it is destroyed earlier in the year, when the crop is still relatively green and “fresh.” Crops that are destroyed later with more stem and a deeper, denser root system are more likely to increase soil organic matter.
Spring-Fallow Cover Crops
For vegetable crops planted later in the season, an early spring cover crop can provide organic matter, scavenge or provide nutrients, and keep the soil free of weeds until planting. Legumes can be mixed with oats or barley, which serve as nurse crops to outcompete weeds as legumes get established. Yellow mustard can be used as a good source of organic matter, with potential for soil-borne disease suppression. It can also suppress weeds, as can annual ryegrass. These crops are sown as soon as the ground can be worked in early spring.
Summer-Fallow Cover Crops
Summer fallow cover crops are more common in vegetable than field crop rotations. A fast-growing cover crop can be planted between vegetable crops. Many vegetable crops are harvested at or before midsummer (broccoli, snap beans, peas, early carrots, etc.). While growers can plant a second crop at this point in the season, some may opt for a cover crop instead to build soil organic matter and provide other ecosystem services. There are two windows of opportunity to plant a cover crop in the summer: cover crops can be planted in late spring or early summer, terminated at midsummer, and followed with a late season vegetable crop.
Full-Year Fallow Cover Crops
If the soil is very low in organic matter and/or fertility, a full-year of cover cropping may be an excellent option for building the soil and controlling difficult weeds. Perennial cover crops (e.g., alfalfa, clovers), biennials, or mixtures of different crops can provide numerous benefits. One option is to use sequential cover cropping. An example rotation could be to plant a summer annual (sorghum-sudangrass) and mow and incorporate it in the fall, then plant hairy vetch which is terminated in spring before growing a vegetable crop.
Management of Cover Crops
How a cover crop is managed greatly influences the benefits derived from planting one. Careful management will increase the chance of success and maximize benefits both for the cover crop and for the vegetables grown later. Time planting the cover crop to match when it will establish well and to allow adequate time for sufficient growth. For example, cool-season legume cover crops like peas do not germinate well and grow poorly if planted too late in the spring. Thus, the benefits of cover cropping can be maximized by using optimum planting dates, field preparation, seeding rates and methods, and termination.
Optimum Planting Dates
Seeding cover crops within the listed date range is important to reduce the risk of planting failure. Research has found that planting cover crops outside recommended planting dates can have a dramatic effect on final plant stand and overall biomass of the cover crop. Some of these effects may be overcome by increasing the seeding rate if the actual planting date is later in the planting date window.
Field Preparation
To establish a cover crop successfully, field preparation is the first critical component to address. Two of the primary issues to consider are weed management and the development of a smooth seed bed for good seed-to-soil contact. Ideally field preparation will mirror that for annual cash crops, but can be less intensive. A seed drill usually results in the best stands. Broadcasting seed followed by harrowing to cover the seed or applying a layer of compost can also be effective. Consult seed bag labels for recommended seeding rates and depths. Control weeds prior to planting to minimize competition during stand establishment.
Cover Crop Termination
The five common methods of terminating cover crops are: winter-kill, tillage, mowing, roller-crimping, and applying herbicides as discussed in Chapter 16. Each method has its disadvantages and limits. For example, winter-kill (the cover crop is terminated by a hard freeze) is only applicable to certain crops and climate regions; mowing is limited to certain cover crops and crop growth stages. Cover crops are most often incorporated by tilling into the soil as a green manure prior to planting the next vegetable crop. Mow heavy cover crops a few days before tilling.
Termination Timing
Termination timing is important to meet your goals and achieve desired cover crop benefits. Once a crop flowers, biomass accumulation and nutrient uptake dramatically decreases. Since biomass accumulation is one of the main goals of planting a cover crop, termination just prior to flowering ensures maximum nutrient cycling and rapid tissue breakdown when incorporated. In addition, cover crops allowed to form seeds can become a weed problem.
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