Managing Soil Physical Characteristics
Soils are generally classified into four groups: sands, loams, clays, and mucks. Sandy soils dry out and warm up rapidly. Loamy soils contain sand, clay, and silt in nearly equal amounts. While loams are ideal soils for most crops, they are the least common soil in the southern United States.
The more common clay soils crust easily. They hold water well and may dry slowly. Clay soils also hold certain nutrients well so that losses to leaching are likely to be lower. Muck soils are not widespread in the southern United States, but where present, they are well-suited for production of vegetables such as celery and carrots. Like clay soils, muck soils hold water very well. Unlike other southern soils, however, their organic matter content is high, usually greater than 18 percent. Because of their high water-holding capacity and their heat-conducting properties, muck soils warm slowly in the spring.
Soil Compaction
Compaction is a frequent problem in agricultural soils. Crusting, cracks, erosion, subsurface compaction zones (tillage pans), and standing water, especially in wheel tracks, are visible in the field. Plants emerge poorly, grow slowly, vary in size, and have shallow or deformed roots. While compaction of any soil is undesirable, it can be particularly damaging to crop performance in sandy soils.
Surface crusts. A surface crust is a layer of soil more dense than the soil beneath it. Thickness of the crust may vary from 0.5 inch to less than 1/25 of an inch. A dense, impenetrable crust may trap germinating seedlings below the soil surface.
On sandy soils with very weak soil structure, crusts usually form as a result of raindrop impact during severe storms, although certain types of overhead irrigation can also contribute to crusting. When the soil dries, dense crusts form because the soil aggregates have broken down.
On soils with a tendency to crust, management options include:
Management practices for soils subject to pressure pan formation include reducing or eliminating disk harrowing, using a chisel plow or subsoiler to break tillage pans that have already formed, and limiting vehicle traffic to the smallest possible area of the field.
Surface compaction. Unless soil is very dry or already compacted, both wheel and foot traffic on the soil surface cause some compaction. Even driving once across a field causes compaction. In fact, 80 percent of the compaction resulting from four tractor passes in the same track will be caused by the first pass. The degree of compaction depends on the weight of the load, size of the tire or track, soil moisture content, and soil structure. Surface compaction can be reduced by making fewer trips across the field, pulling a chisel shank behind the wheel on every trip across the field, and restricting all vehicle traffic to the same wheel tracks.
Providing adequate drainage, staying out of wet fields, and increasing soil organic matter content will also help reduce surface compaction. Using flotation tires and keeping tractor wheels out of the furrow may also reduce compaction. A slight underinflation of tires on tractors or implements will also help. Pressures of 6 to 8 psi may be sufficient for radial tractor tires, for example.
In choosing tractors and other farm implements for use where compaction is likely to be a problem, try to minimize the weight pressure applied to any one area and the area to which pressure is applied. Sometimes tradeoffs between these are complex. Dual tires, for example, reduce pressure, but compact twice as much area. On implements such as large sprayers, using tandem axles following one behind the other instead of dual tires results in a smaller compaction area because the rear tires only add a little compaction. Similarly, a single tire four-wheel drive tractor may cause less compaction than dual tires on a two-wheel drive tractor.
Role of Organic Matter
Adding organic matter to the soil will improve its physical characteristics and increase fertility and biological activity. In general, adding organic matter improves soil structure which enhances soil aeration and promotes penetration of water and roots into the soil. Organic matter bonds sand, silt, and clay particles into soil aggregates, leading to what is called good 'tilth' or the ability of the soil to be worked. Soil water-holding capacity also increases when organic materials are incorporated into the soil.
It is very difficult to increase the organic matter of tilled soils, unless legumes or hay crops are included in a rotation or organic material from another area is added to the soil. However, even if soil organic matter levels are not significantly increased, cycling organic matter still increases biological activity which improves soil chemical and physical properties. For example, the increased microbial activity in worm castings increases soil stability by forming water stable aggregates. Aeration is also increased by worm activity and drainage can be increased 4 to 10 fold.
Water infiltration and root growth were promoted by lower bulk density of the soil after organic matter additions in a California study comparing the effects of poultry manure, sewage sludge, barley straw, and alfalfa on bulk density, aggregate stability, and water infiltration. Aggregate stability and water infiltration rates increased with organic matter additions. Bulk density tended to increase with intensive cropping, but decreased with organic matter additions.
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