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 plough that cuts into the ground and turns over the soil, removing or killing any weeds growing in the area, loosening and breaking up the surface layers of the soil, and providing a bed of soil that holds sufficient moisture to permit the planted seeds to germinate. Traditional tillage may harm the soil if practiced continuously over many years, especially if the fertile topsoil layer is thin. Today, many farmers use a programme of minimum or reduced tillage to conserve the soil. In this form of tillage, any 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 ploughed deeply into the ground as in traditional tillage; this helps to keep moisture within the ground, and protects the soil from erosion.
Ploughs, which are the 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 plough 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 ploughing. Harrows or tools of the same general design are almost universally used for the cultivation of the ground between rows of growing crops.
Deep ploughing and subsequent harrowing are necessary in areas where the soil is compact and impermeable to water and plant roots. Excessive tillage may, however, 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 loam, which normally require less tillage. Climate also plays its part in determining not only the amount but also the time of tillage. In areas, with high humidity, tillage should be limited to times when no great amount of rainfall is expected, for newly-tilled fields are susceptible to water erosion. On the contrary, in arid or sub humid areas, 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, exposure to the air, 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 ploughed 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 ploughing across the slope instead, the water stays in the furrows and sinks into the soil rather than running off. Tillage of this type is commonly known as contour ploughing because the furrows follow the natural contours of the land.
The kind and amount of cultivation between the rows of growing crops is essentially determined by the character of the soil. Heavy, waterlogged soil benefits by the stirring up and aeration that cultivation provides, while hard, caked soils may also 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 weed control.

III.MAINTENANCE OF ORGANIC MATTER
Organic matter is an important element 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 sulphur. Soil productivity is thus markedly affected by the organic-matter balance maintained in the soil. Because most 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 returns to the soil. Because frequent tillage hastens the oxidative loss of organic matter, rotations usually include one or more sod crops (crops that grow on the surface of the ground) 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 during winter and, if a leguminous crop is used, to promote nitrogen fixation. Green-manure crops are grown solely to be ploughed into the ground and serve to increase the organic-matter content of the soil. Although no yield is expected of a green-manure crop, it should 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 waste 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 a mixture 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.

IV.NUTRIENT SUPPLY
Among soil deficiencies that affect productivity, deficiency of nutrients is especially problematic. The nutrients most necessary for proper plant growth are nitrogen, potassium, phosphorus, iron, calcium, sulphur, 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. Enrichment with artificial fertilizers and by treatments hastening the breakdown of complex compounds often increases the supply of usable minerals in soil. The supply of available phosphorus, for example, is often increased by the addition of super phosphate 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, however, as it helps to conserve the phosphorus supply in the soil and makes the effects of super phosphate applications last for several years. Copper and sulphur 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.

V.SOIL POLLUTION
Soil pollution is defined as the build-up in soils of persistent toxic compounds, chemicals, salts, radioactive materials, or disease-causing agents, which have adverse effects on plant growth and animal health. The increasing amounts of fertilizers and other agricultural chemicals that were applied to soils following the end of World War II, plus industrial and domestic waste-disposal practices, led to increasing concern over soil pollution by the mid-1960s. 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. Sulphur from industrial waste 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.

VI.PESTICIDE RESIDUES
The effectiveness of a pesticide as well as the hazards of harmful residues depends 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 organ phosphorus insecticides persist for only days or months. Chlorinated hydrocarbons persist longer in soils that have a large amount of organic matter, although more of the chemical must be applied to such 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, although all pesticides disappear eventually because of evaporation and vaporization, leaching, plant uptake, chemical and microbial decomposition, and photodecomposition.

VII.SOIL ACIDITY ADJUSTMENT
Maintenance of specific soil acidities is important in soil management in order to control 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. When lime is added, the hydrogen of the complex soil colloid is exchanged for the calcium of lime. Acid soils are found predominantly in regions of high rainfall; in arid regions, the soil is usually alkaline.

VIII.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. For a discussion of the problem and the various methods for combating erosion.