Plants require sunshine, water either from rainfall or irrigation, carbon dioxide from the atmosphere, and thirteen mineral nutrients from the soil. Of these, calcium, magnesium, nitrogen, phosphorus, and potassium are required in the greatest amounts. Calcium and magnesium are plentiful in soils located in dry climates, but in wetter climates these nutrients are often leached through the soil. In these regions, calcium and magnesium are returned to the soil in the form of lime, which is also sometimes added to soil to raise its pH (increase its alkalinity). Nitrogen, phosphorus, and potassium are the nutrients which are most often depleted from agricultural soils, and these nutrients are often referred to as the fertilizer elements. Because these nutrients stimulate plant growth and can greatly increase crop yields, it is necessary to apply them to the soil regularly in order to maintain fertility.
The amount of fertilizer applied to the soil increased more than 450 percent in the second half of the twentieth century while this increase in the use of fertilizers has more than doubled worldwide crop production, it has also caused some problems. The increased production of fertilizers has required the use of energy and mineral resources that could have been used elsewhere. In many cases, farmers tend to over fertilize. Over fertilization not only wastes money but also contributes to environmental degradation. Fertilizer elements, particularly nitrogen and phosphorus, are carried away by water runoff and are eventually deposited in the rivers and lakes, where they contribute to pollution of aquatic ecosystems. In addition, nitrates can accumulate in underground water supplies. These nitrates can be harmful if ingested by newborns.
See also: Water and Irrigation, Impact on Soil Resources
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