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INTRODUCTION
In conservation tillage, crops are grown with minimal cultivation of the soil. When the amount of tillage is reduced, the stubble or plant residues are not completely incorporated, and most or all remain on top of the soil rather than being plowed or disk ed into the soil. The new crop is planted into this stubble or small strips of tilled soil. Weeds are controlled with cover crops or herbicides rather than by cultivation. Fertilizer and lime are either incorporated earlier in the production cycle or plac ed on top of the soil at planting. Because of this increased dependence on herbicides for weed control and to kill the previous crop, the inclusion of conservation tillage as a "sustainable" practice could be questioned. It is included in this book for t wo reasons. First, on highly erodible soils, protecting the soil may be an overriding consideration. Second, growers and researchers are working on less herbicide-dependent modifications of conservation tillage practices, some of which are deSRCibed here.
Methods deSRCibed as no-till, minimum till, incomplete tillage, reduced tillage, or conservation tillage differ from each other mainly in the degree to which the soil is disturbed prior to planting. Even in 'no-tillā systems, the soil is opened by coulte rs, row cleaners, disc openers, in-row chisels or rototillers prior to planting the seed. By definition, conservation tillage leaves at least 30 percent of the soil covered by crop residues.
In another variation of reduced tillage, narrow strips are tilled and then planted with standard equipment. Where soils are compacted but subject to erosion, strip tillage is a good compromise because crops can be planted efficiently and grow well in the loosened soil of the tilled strips while the untilled portions of the field conserve soil and water and control weeds.
Advantages and Disadvantages
Reduced tillage practices in agronomic crops such as corn, soybeans, cotton, sorghum and cereal grains were introduced over 50 years ago to conserve soil and water. Crops grown without tillage use water more efficiently, the water-holding capacity of the soil increases, and water losses from runoff and evaporation are reduced. For crops grown without irrigation in drought-prone soils, this more efficient water use can translate into higher yields.
In addition, soil organic matter and populations of beneficial insects are maintained, soil and nutrients are less likely to be lost from the field and less time and labor is required to prepare the field for planting. In general, the greatest advantages of reduced tillage are realized on soils prone to erosion and drought, but significant advantages were seen in a 12-year study of Wisconsin silt-loams which were excellent agricultural soil. This study found improvements of many soil quality factors compa red to chisel or plow treatments. These included greater water-stability of surface soil aggregates, higher microbial activity and earthworm populations and higher total carbon. Soil loss was less from sprinkler irrigation than in the plow treatment.
There are also disadvantages of conservation tillage. Potential problems are compaction, flooding or poor drainage, delays in planting because fields are too wet or too cold, and carryover of diseases or pests in crop residue. Additional problems from r esidues may be caused by allelopathy and high C:N ratios. Allelopathic effects are most often seen when small-seeded vegetables, such as lettuce, are planted directly into rye residues. When the residues are incorporated, as in strip tillage, allelopathic substances break down relatively quickly and are usually not a problem. (See Weed Management for a discussion of allelopathic effects on weed seed germination.)
In vegetable crops, the difficulty of controlling weeds and the need for custom-built equipment have slowed the acceptance of reduced tillage practices. Commercial seeders which plant well into stubble have been developed for most agronomic crops, but are only now becoming available for vegetable crops. A subsurface tiller transplanter has recently been developed at Virginia Polytechnic Institute and State University that should, when it becomes commercially available, greatly increase the ability of vegetable growers to transplant their crops into stubble.
Other relative disadvantages of reduced tillage in vegetables relate to the intensive nature of vegetable production. Since inputs are high in terms of seeds or transplants, fertilizers, pesticides and harvest expenses compared to agronomic crops such as corn and soybeans, the economic return must also be high.
For example, one no-till tomato grower in Pennsylvania estimated he saved $70/acre by skipping moldboard plowing, three diskings, and two cultivations. For most growers, this represents a small percentage of total costs.
In general, vegetable growers want to harvest as soon as possible in the spring in order to get a high price and recover production costs. Without spring tillage, some no-till fields are too compacted and poorly drained for the crop to get a good start. Soil temperatures under the stubble are cooler in the spring, potentially delaying maturity of warm-season vegetables such as sweet corn, snap beans and squash. In addition, if the transplanter does not work well in stubble, the crop may be delayed and mature less uniformly.
Variable maturity is a costly problem in commercially grown vegetables especially those like cabbage where each plant is harvested once. It may not be cost-effective to bring crews in to harvest more than once or twice so late or early-maturing plants may not be harvested at all.
Another consideration in no-till production is an increased possibility of soil compaction in no-till compared to conventionally tilled soil. During one year of a four-year study, severe compaction and crusting prevented the transplanter shoe from penetrating the soil, resulting in cabbage yields 65 percent lower than conventional tillage. Planting also had to be delayed a month because the site was too wet to plant.
A further consideration is that as no-till is generally practiced in agronomic crops, the field is prepared for planting by killing the previous crop with herbicidal desiccants such as glyphosate (e.g. Roundup) or gramoxylin (e.g. Paraquat). The no-till seeders available for agronomic crops were designed to plant into these dried residues. Recently, agronomists have been developing no-till systems where cover crops are planted for weed control then killed with flail or other types of mechanical cutters instead of herbicides. No-till seeders must be modified to work on these tougher residues, but residues control weeds legumes contribute extra nitrogen. (See Cover Crops for information on cover crop cutting schedules.) Similar systems are under development for vegetable growers who want to reduce tillage operations without using herbicides.
With experience, and with the increasing sophistication and availability of no-till equipment for planting vegetables, no-till growers should be able to reach yields at least as high as with conventional tillage practices. If soil water-holding capacity improves, no-till systems may even produce higher yields. Assuming weeds can be controlled and appropriate planters found, most vegetable crops could probably be grown with reduced tillage. Asparagus, snap beans, lima beans, beets, cabbage, carrots, dry bulb onions, peas, potatoes, spinach, popcorn, sweet corn, sweet-potatoes, and tomatoes have been successfully produced in conservation tillage systems. The feasibility of using these systems without herbicides has also been demonstrated, but, as with any new technology, growers will need to experiment to develop a cover crop/vegetable crop system that works well for them.
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Last Modified: Thursday, October 4, 2001