Soil Management

I

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.

II

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.

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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).

III

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.

IV

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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