NC State Extension Publications

Introduction

Tillage includes any physical manipulation of the soil, usually done in preparation for some aspect of crop production and management. The main contributions of tillage are improved capture and crop uptake of water and improved corn stand establishment. Since corn yield is very sensitive to the drought stress common in late May, through June and July, potential benefits in water use efficiency should be considered carefully. The corn crop also is population sensitive, so it is important to achieve the desired plant stand, with reasonably good uniformity of plant spacing.

Depending upon capabilities of the corn planter being used, preplant tillage can be very important in achieving a good plant population. In the case of conventional tillage, the ability of the planter to handle some soil clodiness and unlevelness may require more or less secondary tillage. However, unnecessary secondary tillage is costly, time consuming and frequently it is a major culprit in causing soil compaction and surface soil crusting. This undesirable effect of tillage contributes to erosion, water pollution and subsequent drought stress for the crop. In the case of conservation tillage, the planter may require some in-row preparation such as fluted or bubble coulters, row cleaners or in-row subsoiling - again, depending upon soil properties, residue conditions, and planter capabilities.

Tillage varies in:

  • width (seed zone only; strip tillage; complete width)
  • depth (seed zone only; shallow surface; "plow layer", subsoiling, and other deep tillage
  • fineness (or intensity)
  • frequency (several times a season; only certain times in a rotation; long-term no-till)
  • purpose or objective
    planter performance only; loosening compacted or rutted soils; water movement; water conservation; disease, insect or weed management)

Technology offers farmers many options. Mainly this is due to a wide array of effective chemical weed control options and to greatly improved planting equipment for hard soil and high residue conditions. Tillage practices can affect soil moisture and drought stress on the crop, water capture during rainfall, soil temperature, soil drying, crop rooting pattern, timeliness of all farm operations, potential yields and profits, and environmental protection or degradation.

The Conservation-Till/Clean-Till Decision

Conservation tillage refers to soil preparation that leaves a percentage of the soil surface covered by some form of plant residue after a crop is established. The minimum portion of surface coverage to qualify as conservation tillage is 30 percent. This residue coverage on the soil surface benefits the environment by reducing the risk of erosion and protects water quality from the degradation caused by runoff carrying sediment and possibly traces of fertilizer, pesticides, or both. Residue coverage, and the lack of stirring the soil through tillage, may increase water intake, reduce water loss by evaporation and runoff, and reduce certain weed pressures. Over several seasons conservation tillage may improve soil structure and organic matter, especially in the upper 2 inches of soil.

There are practical agronomic benefits of conservation tillage, too. What farmer wouldn't like to save the time and labor of one or several steps in the busy time of spring land preparation? There is also generally less "wear and tear" on tractors, less fuel usage and, over time, less equipment maintenance and storage. The practical trade-offs for these savings are in being sure that you have equipment which actually functions well under these field conditions and in choosing weed management methods which match the weed pressures and field conditions. Careful management is the key.

How Conservation Tillage Can Affect Water Intake

Where conservation tillage is used and residue covers much of the soil surface during all or most of the crop season, soil water intake rate is usually increased. The first effect of the residue cover is to reduce "soil crusting" by breaking up the impact of raindrops at the soil surface. The crusting tendency of soils varies, and generally, soils having textures which include a large portion of fine sand and silt with some clay are severely crust prone.

A second water intake benefit accumulates over time. There is gradual decomposition of the unstirred surface residue by tiny insects and microbes, and this contributes to stable soil structure and soil porosity. Soil-swelling mites and springtails ("microarthropods"), as well as beneficial nematodes are found at high numbers with conservation tillage. In some cases worm populations also develop because of increased food supply and the lack of physical disruption. This cumulative effect is promoted by a cropping system which leaves surface residue, especially the long-lasting residue of near-mature crops over most of the row area in most years of the crop rotation.

A long-term study at the Upper Piedmont Research Station in Reidsville has shown that soils under conventional tillage (chisel plowing followed by disking) absorb less than 0.25 inch of the 1 inch of water applied during a 30-minute period. No-till plots in that study absorbed more than double that amount, and the intake rate in the final 20 minutes was nearly triple that of the chisel/disked plots. When the crusted surface soil of conventional tillage plots was tilled by cultivation the water intake was greatly increased. After a couple rain events, however, the crust would be likely to redevelop.

In this study, the average corn yield over 12 years of continuous no-till was 93 bushels per acre, compared to 56 bushels per acre for the spring chisel plow/disk and 51 bushels per acre for spring moldboard plow/disk treatments. A shallow in-row subsoil treatment in the same years averaged 98 bushel per acre. The better yields with conservation tillage in this study were due to residue cover that increased water intake in summer rain events. The higher yields which occurred nearly every year in conservation-tilled plots increased soil productivity over time because these higher yields also returned greater amounts of crop residue to the soil. Conventional tillage chops and buries residue which then accelerates residue breakdown, losing the residue benefits and leaving soil productivity to fall behind that of areas where conservation tillage is practiced.

Water intake comparisons were also conducted in corn studies in some crust-prone soils of the coastal plain. In these soils no-till treatment absorbed about one-third more water than the disked soils.

Decisions about Ripping and Deep Tillage

Dense, hard soil zones below the topsoil layer seriously reduce the depth of crop root systems, especially when corn establishes most of its root system. Such dense zones are called "pan layers" or "tillage pans." This problem is very common in the light colored upland soils of the upper, middle, and lower coastal plain. In these areas pan layers readily and quickly develop in fields which predominantly have soils classified as well and moderately well drained.

Corn responds well to deep tillage when grown on soils which have compacted "pan layers." Deep tillage allows water extraction from deeper in the soil during periods of drought stress. There also is evidence that in light-colored soils with sandy properties to greater depth (about 16 inches or more), loosening existing pan layers with deep tillage may offer some added nitrogen and sulfur uptake as a result of deeper root exploitation. Much greater explanation of the cause and appearance of pan layers, the resulting crop rooting patterns and the crop response to subsoiling in various soils are given in Subsurface Compaction and Subsoiling in North Carolina, publication AG-353, available from the Cooperative Extension Service.

Pan layers may also develop in soils which are somewhat darker in surface color, but this occurs mainly when the soils also have some degree of sandy properties to at least a 10 inch-depth. However, there are various indications that pan layers are of less consequence to rooting and water uptake by annual crops in these soils with more organic matter and more influence of silt and clay. Corn is less responsive to deep tillage in fields with predominantly darker soils, because in these soils the limiting effects of pan layers are less likely to occur. In the lower coastal plain and tidewater regions, darker soils are dominant with much of the land of the tidewater classified as very poorly drained or organic soils. Compaction can still occur in these dark soils, but such compacted zones are usually shallower and of less drastic root-limiting effect than in the light-colored, more sandy land.

In the piedmont today most upland soils are at least somewhat eroded and soil textures have less effects from sand and more influence from clay and silt. Soil compaction certainly occurs but it is usually shallower and it should be addressed primarily through residue protection, utilizing the benefits of conservation tillage outlined earlier.

Corn and cotton are generally the most sensitive to the presence of compacted pan layers in the soil and are more strongly and consistently responsive to deep tillage. Soybeans responded well only under the more severe conditions. Grain sorghum and peanuts generally showed the least yield response to subsoiling when pan layers were present.

In general, the light colored, more droughtly soils of the coastal plain have lower yield levels but higher yield response to subsoiling practices. For the darker soils of the coastal plain average yields were higher and, in general, yield responses to the best of the deep tillage treatments was about half that of the lighter-colored, better-drained soils. This is because drought stress tends to be less severe even when compacted pan layers are present. Sometimes in the darker, more productive soils these pan layers are not as thick, and these soils often have finer texture with higher organic matter, resulting in greater available moisture storage capacity by the soil itself.

In sloping, crust-prone soils in the piedmont, conservation tillage, because of its benefits in moisture capture and conservation, shows more promise for increasing yields than deep tillage. At the same time, residue management and conservation tillage controls erosion and provides important environmental benefits. In general, our studies suggest that farmers should not use in-row depth tillage for either corn or soybeans in the piedmont. However, there are rare cases of soils in the piedmont with more sandy properties to at least a 12-inch depth, where severe pan layers have been noted, and where shallow subsoiling to the depth of clayey layer may be beneficial.

The recommended depth of subsoiling is based on running rippers deep enough to fracture all or most of the sandy pan layer zone, but only to the depth of the more clayey B horizon in the soil. (For more details refer to Cooperative Extension publication AG-353, cited earlier.) In one study in droughtly soils with a thick pan layer, running the subsoiler to about the same depth as the chisel plow resulted in a lower yield than the shallow tillage of disking alone. Running the same ripper tool to a 16-inch depth increased corn yield by 35 bushels per acre!

Because of variability of depth to clay layers in most fields and because current equipment generally requires manual adjustments, achieving the ideal ripper depth throughout the field is difficult to achieve. However, some adjustment can be made to hit "a happy medium" operating depth. To some degree this can be determined by the soil map and soil descriptions in all Natural Resource Conservation Service (NRCS) Soil Survey Reports. The better method to determine this requires more detailed examination by soil probe or shovel, including a good practical understanding of soil properties and soil classification. Perhaps in the future technological developments in on-the-go machinery adjustments, coupled with geographically referenced soil information (GIS database systems) may offer more effective and efficient use of soil property information in deep tillage applications.

Studies have been conducted under conservation tillage conditions to measure the need for subsoiling each year versus every second or third year. In a severely pan-layer prone soil the response to subsoiling is similar under clean tillage or conservation tillage. There was about a 60 percent carryover value of subsoiling to the following year but only about 20 percent in the second non-subsoiled crop.

Drought severity strongly controls the amount yield response from deep tillage. In fact, drought can be so severe as to limit the top yield and thus restrict the apparent response to subsoiling treatments compared to other methods. No amount of deep tillage can overcome severe drought stress. However, where rooting depth is severely limited - as can be the case with strongly-developed pan layers - deeper rooting can take advantage of water already present in the soil profile. And, as emphasized earlier, capturing more of the rainfall through good use of conservation tillage can often mean significantly better corn yields and opportunities for profitable production. More information about drought is found in the publication The Effect of Drought Stress on Crop Productivity, publication AG-519-14, available from Cooperative Extension.

Author

Publication date: Jan. 1, 2003
AG-590

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