SOIL-WATER-PLANT RELATIONSHIPS SOIL TYPES Soil is formed by the interaction of plants, animals, climactic factors and weathered rock. It consists of mineral particles, decaying organic matter, water, air and living organisms. About half of soil is solely pore space; this is the pockets of air that allow roots to move and water to infiltrate. The other half of soil is made up of mineral and organic matter. Organic matter, however, only takes up between one and four percent of the soil’s substance. The majority of soil is a combination of clay, silt, and sand. Since these three elements play such a vital role in the breakdown of soil, soil classifications are determined based on how much of each element is in the soil. Each of these particles is unique. Sand is the smallest soil particle that can be seen with the naked eye. Because it is loose and single grained, it feels gritty when it is dry or wet. If a handful of sand is squeezed when it is dry, it will crumble as soon as pressure is released. If a handful of sand is squeezed when it is wet, it will form a cast. However, the cast will crumble the instant it is touched. Silt and clay particles are too fine to be seen with the naked eye. Silt feels smooth and floury when it is dry and greasy when it gets wet. Clay is very hard when it is dry and sticky and plastic when it is wet, usually forming extremely hard blocks or prisms when it dries. Clay exhibits properties of both cohesion and adhesion. When these three particles are combined, their characteristics are mixed to produce unique features. These three elements combine into eight soil types. Along with the different types of soil, there are various soil structures as well. The illustration shows how to identify soil types from the soil texture, and estimate the soil moisture from the look and feel of a soil sample. INFILTRATION RATE The textures of the soil dictate several different aspects of the soil in relation to water. The time it takes for soil to accept water, how much water the root zone reservoir will hold, the rate that water moves through the soil, and how much water is available to the plant is all determined by the type of soil. When soil first becomes wet, its infiltration rate is high, and then it lowers in a
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