Most soils cannot provide one or more plant essential nutrients in sufficient available form for modern crop production. Soil samples are frequently tested to determine the quantities of nutrients and other amendments which should be applied. Soil testing normally involves extraction or reaction of a sample with a specific chemical solution(s) which removes essential elements in amounts related to those required for plant growth. Soil testing is recommended to prevent both under and over fertilization of crops, thereby providing economic crop production in an environmentally effective manner.

Lime

The main purpose of liming is to raise soil pH and supply calcium and sometimes magnesium for plant growth. Other benefits from liming acid soils include increased biotic activity, enhanced mineralization of nutrients from soil organic matter, improved soil structure, decreased potential for aluminum toxicity, and increased availability of other nutrients, especially phosphorus.

Fertilization

A good fertilization program is based on soil testing, especially over several years, so that changes in nutrient availability and other chemical properties can be determined over time. Nutrients can be added in either organic or inorganic forms. Remember that plants utilize only specific ionic forms of the essential nutrients. These specific ionic forms can be provided directly through inorganic fertilizers or indirectly through the decomposition of manures, composts, and other organic amendments. Thus, plant uptake of nutrients should be similar whether initially added through inorganic or organic sources. Additional benefits which may accrue from application of organic materials include improved soil structure and increased water holding and cation exchange capacities. The degree to which these additional benefits occur will be determined by the quantity and quality of organic material applied and its decomposition rate.

Municipal sewage sludge, food processing wastes, and other similar organic wastes are commonly added to soils. Besides the obvious need for disposal, these materials can provide nutrients for plant growth and improve soil properties. Sewage sludges can contain heavy metals, however, especially in industrialized areas. Land application of sewage sludge is usually regulated by state and federal guidelines. When properly utilized, the above materials can be suitable sources of plant nutrients.

Irrigation

Irrigation is practiced to at least a limited extent on each of the earth's continents. Irrigation is a method of at least partially overcoming problems in natural precipitation patterns. These problems may result from an overall lack of precipitation or poor seasonal distribution. Irrigation is normally practiced not only to increase yield, but to provide yield stability. Yields may be increased many fold by irrigation depending on climate and the crop produced. Large scale irrigation projects may also be one facet of an overall program to provide hydroelectric power, flood control, municipal water supplies, and recreation.

Water sources for irrigation include lakes, streams, and groundwater. Numerous water delivery methods are used including furrow, flood, sprinkler, and drip irrigation. Each system is different in efficiency and cost. Both these factors are normally considered before a producer decides on the type of system to be used. Water districts and governmental agencies may also mandate the type of system used based on water quantity, quality, and environmental concerns.

Although development of irrigation capabilities almost immediately increases crop production, long-term effects must also be addressed. Irrigation water must be of reasonable quality (sufficiently low in dissolved salts and of proper ionic composition) to be used for an extended period of years. Many irrigation systems from ancient times through today have failed because of increasing soil salinity over time. Water applied through irrigation is partially transpired by plants or evaporated from the soil. Salts added in the water remain in the soil. Over time, salts can accumulate so as to decrease yields or prevent further crop production.

Providing adequate drainage is also imperative for irrigated lands. Proper drainage not only decreases salt accumulation, but prevents shallow water tables which may restrict plant rooting depth and contribute to upward salt migration during periods of high evaporative demand.

Irrigation has been extremely important throughout the world in providing a stable and abundant food supply, but proper planning and expertise are necessary to sustain economic and environmentally sound irrigated crop production.

Tillage Systems and Conservation Practices

Tillage systems vary widely from conventional, or "clean", tillage where all crop residues are incorporated into the soil with little residue remaining on the soil surface to no-tillage where crop residues are not disturbed and remain on the soil surface from one crop to the next. Conservation tillage denotes any tillage system in which at least 30% of the soil surface is covered by residue after planting. Tillage is used to destroy weeds, incorporate residues from a previous crop, open the soil surface for increased infiltration and aeration, shatter compacted zones which might restrict root growth, and prepare seedbeds for optimal seed germination.

The principal advantage of conservation tillage is protection of surface soil from water and wind erosion and is frequently used to decrease soil loss on erodible lands. Secondary advantages include increased water storage and organic matter contents. Erosion not only decreases the productivity of soils, but additional economic loss is associated with siltation of reservoirs, dredging of waterways, and removal of particulates to meet drinking water standards. As plant residue cover of the soil surface nears 100%, soil erosion decreases to near zero. Erosional loss can be reduced by more than 80% with a residue cover of 50%. Even a modest residue cover of 10% can reduce erosion by about 30%.

Other alternatives are available for erosion control. Construction of terraces, or berms, which slow and alter the movement of water across slopes has been used for centuries to decrease water erosion. Strip cropping, the planting of alternating strips of crops of different heights or seasonal maturities across a landscape, and contour farming, the production of crops across instead of with slopes, have also historically been used to reduce the erosional forces of wind and water.

Rotation, or planting different crop species, is normally beneficial compared to monoculture, or continual production of the same crop species. Rotation often decreases problems from weeds, disease and insects and may improve soil chemical, physical, and microbiological properties, and crop yield.

Farmers must make many decisions each year. These decisions must result in both economic and environmentally prudent crop production if food production is to be sustainable over the long term.

Lesson 10. Soils and Societal Issues

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