Soil Water

All soils contain water under natural conditions. The amount of water can be very low in air-dried soils. The optimum water content for microbial processes is 40-60% of the maximum water-holding capacity, or corresponds to the water content that is held in soil at suction pressures of -0.01 to -0.031 MPa. The spaces between soil particles are known as the soil pores; these are filled either with air or water (resulting in a soil solution) depending on the pore size and the water saturation of the soil. Depending on their equivalent diameters, soil pores can be divided into wide coarse (<50 |im), tight coarse (10-50 |im), medium (0.2-10 |im), and fine (<0.2 |im) pores. Pore sizes are assigned in accordance with adaptation to the water content at characteristic metric pressures. Equivalent diameters of 50 and 10 m correspond to water contents of the soil at field capacity 96 and 30 kPa, respectively, while an equivalent diameter of 0.2 m corresponds to the water content at the permanent wilting point (1,500 kPa). The amount of water available to plants and microorganisms lies between the field capacity and the permanent wilting point. Water stored at metric pressures of >1,500 kPa is accessible to neither fine plant roots nor microorganisms. Before undertaking an irrigation project, a soil survey is carried out. The history of irrigation shows that many soils have been damaged or ruined due to a rise in the water table and salinity or alkalinity. The main purpose of a soil survey, however, is to provide an inventory of soil resources. The scope of a soil survey is determined by the purpose in mind (Wilke 2005).

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