Conservation Tillage Methods

Selecting Crops

Successful vegetable production with conservation tillage depends on careful crop selection. Crops that germinate quickly and grow rapidly in the first few weeks after planting do well because they are more competitive with weeds than crops that initially grow slowly. Examples of rapidly-growing plants are transplants and seedlings of large-seeded vegetables such as sweet corn and beans. Examples of slow growing plants are seedlings of small-seeded vegetables such as lettuce and carrots. Cool-season vegetables also do better in spring no-till plantings than warm-season crops. Low spring temperatures in untilled fields will delay growth of squash, cucumbers and other warm season crops more than that of cole crops, potatoes, leafy greens and other cool season vegetables.

Equipment

The lack of appropriate planting equipment has limited no-till practices in vegetable production. Some growers and researchers have planted large-seeded vegetables such as sweet corn, snap beans and squash with no-till planters designed for field corn or soybeans, selecting appropriately sized seed plates or otherwise adjusting plateless no-till planters for the proper seed size. The important characteristic of these planters is a coulter to cut through crop residues and double disk openers or shoes to ensure proper placement and coverage of the seed. Conventional transplanters can also be modified for no-till by fitting a shank and a cutting coulter in front of the transplanter to open a slit in the soil for the transplant. A heavier-than-normal pack wheel then presses the soil around the roots.

Yields of tomatoes, cole crops, peppers, and tobacco as much as 15 percent higher have been reported when planted using no-till techniques with a subsurface tiller transplanter (SST-T) developed specifically for conservation tillage systems. This transplanter has two main components, a subsurface tiller to loosen a narrow strip of soil and a conventional transplanter to set plants in the tilled strip. The SST-T's smooth coulter cuts heavy residues rather than just pressing them uncut into the soil. This assembly loosens a soil strip 2-to-4-inches wide and 8-inches deep so that the double-disc shoe of the transplanter can move more easily through the soil in heavy residues or hard, compacted soil. In earlier no-till planter designs without this soil loosening, the shoe was expected to both till and plant, resulting in damage to the shoe in compacted or rocky soils and in inadequate contact between the soil and the roots of the transplant. Finally, two rubber press wheels bui lt to work in extremely heavy sod and rocky soil close the soil around the transplant.Because of the high-clearance design of the transplanter, the shoe does not clog up in fields with heavy residues.

Cover Crops, Weed Management, and Fertilizers in Conservation Tillage

In a true no-till system, the soil would never be tilled, and fertilizer and lime would be added to the soil surface, while cover crops would be established with special no-till planters. In practice, however, even 'no-till' fields are usually cultivated every 3 or 4 years to incorporate lime and fertilizers and to control perennial weeds. Some row crop growers in North Carolina who have extended the period between tillage operations to 5 years see decreases in soil pH, but are still satisfied with crop performance.

Cover Crops. In vegetable crops, researchers and growers are experimenting with a number of management options. One option which has worked well in a number of research and on-farm settings is to cultivate in the fall to establish a cover crop. The cover crop grows over the winter, protecting the soil. In the spring or summer, a vegetable crop is then planted into the cover crop with a minimum of tillage. This modified no-till system allows the incorporation of non-mobile fertilizers such as lime and phosphate. The incorporation of residues and better stand establishment of the cover crop also help reduce insect and disease carry-over from the previous crop. Potential disadvantages of this option compared to no tillage at all are loss of beneficials and less surface residue and organic matter.

The type of cover crop chosen depends on the crop which is to follow (see Chapter 2 for more specific criteria). In general, however, if the spring crops are cool-season vegetables, such as cabbage, broccoli or potatoes, which must be planted early, small grain cover crops are used because they grow rapidly enough over the winter to protect the soil and provide adequate organic residues early.

Legumes take longer to establish themselves and grow, so they are more useful for the warm-season vegetables such as tomatoes, sweet corn, bean and squash which are planted later in the spring. For these later-planted crops the nitrogen contribution fro m the legumes can be significant. Legume-grass mixtures are also popular for late-spring plantings, as they have some of the advantages of both types of covers.

In the spring, the cover crop must be killed before the vegetable crop can be planted. If no-till vegetable planters are available, then the crop can be desiccated with herbicides just before planting. This maximizes growth of the cover, increasing biomass, N fixation (in the case of legumes) and weed control. If, however, the crop is to be established in tilled strips (strip-tillage) then the cover crop should be killed 1 to 2 weeks before planting to allow residues to begin to break down. This will reduce the tendency for clods to form during tillage.

Instead of killing the cover with herbicides, some growers have successfully used flail mowers or bush hogs to kill the cover crops. The most important consideration in mowing is timing the operation so that significant biomass is formed, but the crop does not set seeds or regrow. The ideal time to mow is during flowering. If mowed during the early phases of growth, some legumes regrow, competing with the spring planted crop. If mowed after seed formation, the cover may reseed itself, which is not always desirable.

Good results have been reported by Virginia researchers who "mow-killed" hairy vetch and rye with a flail mower at flowering. The covers did not reseed and broccoli was successfully planted into the 2-to-3-inch residue mat and grown to harvest without herbicides.

Weed Management. The difficulty of weed control without cultivation is one of the most important limits on the use of conservation tillage practices for vegetable production. The general practice in reduced tillage systems is to substitute herbicides, mowing, or flaming for cultivation to kill the existing vegetation before the vegetable crop is planted, but few options exist for weeds that germinate after the crop is planted. Effective materials for pre-emergence weed control are currently available for beans, cabbage, potatoes, sweet corn, and tomatoes. Consult your state agricultural chemicals manual for specific recommendations.

Since the herbicide cannot be incorporated into the soil except in strip-tillage systems, the herbicide must be applied over the mulch or stubble and moved into the soil by rainfall or irrigation. More water is necessary to activate the herbicide in no-till systems because the residues intercept some of the herbicide before it reaches the soil. For the same reason, higher rates may be necessary.

In strip cultivation, it is usually possible to cultivate the tilled strip for the first 2 to 3 weeks after crop emergence or transplanting, but in other forms of reduced tillage, options are limited once the crop emerges, especially for crops where no post-emergent herbicides are registered.

Fertilizers. The following suggestions relate specifically to the fall tillage/cover crop system described above. Lime and phosphorus in amounts required by soil tests are incorporated in the fall while preparing the soil for the cover crop. If well mixed in the soil, the non-mobile lime and phosphorus will be more available to the plant and unlikely to be lost in significant amounts over the winter. Nitrogen and potassium are generally considered mobile in the soil and so are usually applied to the soil surface just before planting the vegetable crop in the spring.

Adding nitrogen fertilizer in the fall to increase the growth of the cover crop is also an option. Especially in the case of a grain cover, the grain should act as a 'catch' crop, taking up the nitrogen so it will not be lost over the winter. The extra nitrogen promotes growth of the grain, and is available for spring vegetable crop when the cover crop is killed. In a fertilization system in which nitrogen has been added from slowly-releasing organic sources, such supplemental nitrogen should not be necessary to ensure good growth of the cover. It may, however, be more convenient to incorporate composts or other organic matter in the fall rather than trying to apply them to the surface in the spring.

In the spring, the remainder of the nutrients needed by the crop is applied over the top of the cover crop residues, preferably by banding rather than by broadcasting. Nitrogen application rates depend on the type and amount of growth of the cover crop, but may need to be increased in no-till to get good early growth.

Grass or grain cover crops tend to decompose slowly, tying up soil nitrogen, while nitrogen applied to the surface takes some time to reach plant roots. For the same reason, starter solutions or fertilizers are often helpful. On the other hand, if the legume cover crop grew well, less nitrogen may be needed.

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