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Soil testing determines both the amount of soil nutrients essential for plant growth and the existence of any harmful ingredients in the soil. It can be used to make a recommendation for the amount and type of fertilizer, the timing and the exact location of fertilizer application for optimum plant growth. Furthermore, it can help achieve proper and effective nutrient management of crops. Improper application of fertilizer would be problematic because of the uniqueness of soils and crops. Understanding soil chemistry by measuring the exact content of chemicals in soil is the first step towards maintaining and managing environmentally sound soil. Soil testing reveals the chemical properties of soil include the following: pH, soil organic matter, available phosphoric acid, available silicic acid, substitution cations, and cation exchange capacity. The following maps show the acidity and the organic matter content of rice paddies and farmland in Korea. The survey was conducted from 1995 to 2006.
Soil Acidity(pH) of Paddy Rice Soils   Soil Acidity(pH) of Farmland Soils
The pH of soil can be an important parameter for crop growth. When a soil's pH is about neutral (6.5 ~ 7.0), availability of soil nutrients is at a maximum. As a soil becomes more acidic (pH <5.5), significant dissolution of manganese and iron from the soil not only impede the crop growth, but also hinder the propagation and function of useful soil microbes. The solubility of cations such as iron and zinc becomes low in the alkaline soils with a pH of greater than 8.0. As a result, crops may suffer a deficiency of these cations. The map indicates that the range of a pH of rice paddy and farmland ranges from less than a pH of 4.5 to above a pH of 6.5. According to Korean Rural Development Administration, National Institute of Agricultural Science and Technology, optimal ranges of pH for rice paddy and farmland are 5.5~6.5 and 6.0~6.5, respectively.