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Soil consists of two components: minerals from weathered rocks, and organic matter from decayed organisms and animal wastes. The mineral content of the soil provides plants with nutrients, such as calcium, potassium, and phosphorus.
Organic matter improves drainage and helps prevent soils from becoming waterlogged, thus reducing the occurrence of diseases such as root rot. Organic matter also binds the mineral nutrients so they remain in the soil and are not washed away.
The spaces, or pores, between the tiny particles of minerals and organic matter are occupied by either water or air. Water holds vital dissolved nutrients, while air provides the roots with oxygen. Most plants do best in a soil in which half the pore space is filled with air and half with water.
Plants use nutrients obtained from soil to build the cells and tissues needed for growth. Nutrients that plants need in large amounts, called macronutrients, include oxygen, hydrogen, carbon, and an array of minerals. They also need micronutrients, or trace nutrients, which consist of cobalt, chlorine, boron, iron, zinc, molybdenum, nickel, manganese, and copper.
Soil texture, the size of the individual soil particles, affects how fast water drains, and how well plants absorb nutrients. The largest soil particles are grains of sand. Sand grains fit loosely together, with large gaps between them resembling marbles in a jar. The large pores let water (and the nutrients dissolved in it) drain out too quickly for most plants to absorb it.
Clay particles, on the other hand, are very tiny and packed closely together, resembling tiny beads in a jar. The pores between clay particles are so small that water drains very slowly. Slow drainage can lead to oxygen deprivation, because the water takes the place of air in the pores.
Another disadvantage of clay is that it binds water and some nutrients so tightly that most plants cannot absorb them.
A third soil particle is silt, which is larger than clay but smaller than sand. Most plants thrive in a soil type known as loam, which contains roughly 50 per cent sand, 25 per cent clay, and 25 per cent silt.
A loam soil drains water well, but not too quickly; and as a result, the plant can absorb nutrients more readily.
Exceptions include desert plants such as cacti, which do best in a sandy soil.
Plant nutrient absorption is also dependent on a soil’s pH, a measure of acidity or alkalinity. The minerals from which a soil is formed and the amount of rain that passes through the soil determine its acidity or alkalinity.
Pure water has a pH of 7.0, which is neutral; meaning, it is neither alkaline nor acidic. Soils with a pH of less than 7.0 are acidic; those with a pH greater than 7.0 are alkaline. Most plants absorb nutrients best in a soil with a pH between 6.5 and 7.5; however, plants such as roses prefer an acidic soil, while others — such as lilies — grow better in an alkaline soil. (Source: www.evergreenstudy.org.)
