Soil
Management
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INTRODUCTION |
Soil Management, the basis of all scientific
agriculture, which involves six essential practices: proper tillage;
maintenance of a proper supply of organic matter in the soil; maintenance of a
proper nutrient supply; control of soil pollution; maintenance of the correct
soil acidity; and control of erosion.
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TILLAGE |
The purpose of tillage is to prepare the soil for
growing crops. This preparation is traditionally accomplished by using a plow
that cuts into the ground and turns over the soil. This removes or kills any
weeds growing in the area, loosens and breaks up the surface layers of the
soil, and provides a bed of soil that holds sufficient moisture to permit the
planted seeds to germinate. Traditional tillage may harm the soil if used
continuously over many years, especially if the fertile topsoil layer is thin.
Today, many farmers use a program of minimum or reduced tillage to conserve the
soil. In this form of tillage, dead plant material that remains on the ground
after the crop is harvested is left on or near the surface of the soil, rather
than being plowed deeply into the ground as in traditional tillage. The dead
plant material at the surface of the soil helps to keep moisture within the
ground, and protects the soil from erosion.
Plows, which are chief mechanical implements used for
tillage in all parts of the world, may be designed for a number of purposes
ranging from the simple cutting of a furrow through the ground to the complete
reversal, or turning over, of soil, usually to a depth of 15 to 20 cm (6 to 8
in). In certain areas and for certain purposes the plow is replaced as a
primary tillage instrument by various types of harrows, tools that scrape or
scarify the surface of the ground without digging deeply into it. In most
areas, such implements are employed merely to break up and pulverize the soil after
plowing. Harrows or tools of the same general design are almost universally
used for the cultivation of ground between rows of growing crops.
Deep plowing and subsequent harrowing are necessary in
areas where the soil is compact and impermeable to water and plant roots.
Excessive tillage may result in the deterioration of soil structure,
particularly if performed in wet soil. This problem is more acute in finely
textured soils than in sand and loamy soils, which normally require less
tillage. Climate also plays its part in determining not only the amount but
also the time of tillage. In humid areas, tillage should be limited to times
when no great amount of rainfall is expected, for newly-tilled fields are
susceptible to water erosion. In arid or subhumid areas, on the contrary, land
should be tilled before periods of anticipated rainfall so that the ground can
absorb the maximum amount of water.
Among the secondary but important benefits of tillage
is the aeration resulting from pulverization. This aeration not only provides a
freer circulation of oxygen and water but also results in increased biological
activity in the soil, including that of organisms that fix atmospheric
nitrogen. Tillage contributes to the health of plants by inhibiting plant
diseases and by discouraging the development of various types of insects that
harm plants.
The type of tillage affects the loss of soil
through erosion by wind and water. When furrows are plowed in an
uphill-and-downhill direction, water tends to flow down the furrows, carrying
away small particles of the top layers of soil as it flows. By plowing across
the slope, the water stays in the furrows and sinks into the soil rather than
running off. Tillage of this type is commonly known as contour plowing because
the furrows follow the natural contours of the land.
The kind and amount of cultivation between the
rows of growing crops is determined essentially by the character of the soil.
Heavy, waterlogged soil benefits by the stirring up and aeration that
cultivation provides. On the other hand, hard, caked soils may require
cultivation to permit them to absorb the moisture that the crops need. For
soils that are in good physical condition, however, the primary purpose of
cultivation of row crops is to permit weeding (see Weed Control).
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MAINTENANCE OF ORGANIC MATTER |
Organic matter is important in maintaining good
physical conditions in the soil. It contains the entire soil reserve of
nitrogen and significant amounts of other nutrients, such as phosphorus and
sulfur. Soil productivity thus is affected markedly by the organic-matter
balance maintained in the soil. Because most of the cultivated vegetation is
harvested instead of being left to decay, organic materials that would
ordinarily enter the soil upon plant decomposition are lost. To compensate for
this loss, various standardized methods are employed. The two most important of
these methods are crop rotation and artificial fertilization.
Crop rotation consists of growing different crops in
succession on the same land, rather than utilizing a one-crop system or a
haphazard change of crops. In the rotation system, crops are alternated on the
basis of the amounts and types of organic matter that each crop returns to the
soil. Because frequent tillage hastens the oxidative loss of organic matter,
rotations usually include one or more sod crops that require little or no
tillage. Deep-root penetration on the part of certain leguminous crops, such as
alfalfa, provides better drainage as a result of the channels left after the
roots decay (see Legume).
The rotation system employs special types of crops
such as cover crops and green-manure crops. Cover crops are those crops planted
to protect the soil in the winter and, if a leguminous crop is used, to fix
nitrogen in the soil (see Nitrogen Fixation). Green-manure crops are
grown solely to be plowed under and serve to increase the organic-matter
content of the soil. Although no yield is expected of a green-manure crop, it
is supposed to increase the yield of subsequent crops planted in the same
fields.
The older method of increasing the organic
content of the soil is the use of such fertilizers as manure and compost. The
manuring of soil with animal wastes has been practiced for many thousands of
years and serves as a source of various complex organic compounds that are
important in the growth of plants. Compost, which usually consists of mixtures
of dead vegetable and animal matter, has a purpose similar to that of manure
and is often treated with chemical fertilizers to increase its effectiveness.
For a discussion of the various types of fertilizing materials, see Fertilizer.
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NUTRIENT SUPPLY |
Among soil deficiencies that affect productivity,
deficiency of nutrients is especially important. The nutrients most necessary
for proper plant growth are nitrogen, potassium, phosphorus, iron, calcium,
sulfur, and magnesium, all of which usually exist in most soils in varying
quantities. In addition, most plants require minute amounts of substances known
as trace elements, which are present in the soil in very small quantities and
include manganese, zinc, copper, and boron. Nutrients often occur in the soil
in compounds that cannot be readily utilized by plants. For example, phosphorus
combined with calcium or magnesium can be used by plants, but phosphorus
combined with iron or aluminum usually cannot. The supply of usable minerals in
soil is often increased by enrichment with artificial fertilizers and by
treatments hastening the breakdown of complex compounds. The supply of
available phosphorus, for example, is often increased by the addition of
superphosphate fertilizers. Adding calcium to soils also decreases soil acidity
and makes phosphorus more readily available to vegetation. The existence of
phosphorus in several unavailable forms is sometimes advantageous in that it
helps to conserve the phosphorus supply in the soil and makes the effects of
superphosphate applications last for several years. Copper and sulfur are often
added to the soil through the use of spray solutions. Other elements are added
by direct application or by the use of specific artificial fertilizers.
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SOIL POLLUTION |
The increasing amounts of fertilizers and other
agricultural chemicals applied to soils since World War II ended in 1945, plus
industrial and domestic waste-disposal practices, led by the mid-1960s to
increasing concern over soil pollution. Soil pollution is the buildup in soils
of persistent toxic compounds, chemicals, salts, radioactive materials, or
disease-causing agents, which have adverse effects on plant growth and animal
health. As of now, soil pollution is not widespread. Although the application of
fertilizers containing the primary nutrients, nitrogen, phosphorus, and
potassium, has not led to soil pollution, the application of trace elements
has. The irrigation of arid lands often leads to pollution with salts. Sulfur
from industrial wastes has polluted soils in the past, as has the accumulation
of arsenic compounds in soils following years of spraying crops with lead
arsenate. The application of pesticides has also led to short-term soil
pollution. See Environment.
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PESTICIDE RESIDUES |
The effectiveness of a pesticide as well as the
hazards of harmful residues depend largely on how long
the pesticide remains in the soil. For example, DDT, a chlorinated hydrocarbon,
has a half-life of three years in cultivated soils, while organophosphorus
insecticides persist for only days or months. Chlorinated hydrocarbons persist
longer in soils having a large amount of organic matter, although more of the
chemical must be applied to these soils to kill pests. Insecticides persist
longer if worked into the soil than if left on the surface. Herbicides applied
to soils may not persist at all or may persist up to two years or longer,
depending on the compound. Simiazine is one of the most persistent herbicides.
Eventually, all pesticides disappear because of evaporation and vaporization,
leaching, plant uptake, chemical and microbial decomposition, and
photodecomposition.
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SOIL ACIDITY ADJUSTMENT |
Maintenance of specific soil acidities is important in soil
management because it controls the adaptation of various crops and native
vegetation to different soils. For example, cranberries can be successfully
grown only in moderately to strongly acid soils, whereas alfalfa and other
legumes are successful only in weakly acid or slightly alkaline soils. The
ordinary procedure for correcting excess soil acidity is the application of
lime in the form of limestone, dolomitic limestone, or burnt lime. About 18.14
million metric tons of limestone are used annually on
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VIII |
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MECHANICAL CONTROL OF EROSION |
The mechanical loss of fertile topsoil is one of the
gravest problems of agriculture. Such loss is almost always caused by erosion
resulting from the action of water or wind. According to the U.S. Department of
Agriculture, more than half of all fertile