How does type of soil affect plant growth?
Answers
Soil consists of particles and spaces. The particles can be organic or inorganic. The organic particles are decomposed organic matter, with bits of many different sizes; the inorganic particles are pieces of rock that have been reduced in size by natural actions such as erosion and weathering, as well as by the digestive action of organisms.
Soil type generally refers to the texture of the soil, determined by the relative amounts of the three basic sizes of inorganic particles - sand, silt, and clay. These three are descriptive of the size of the individual particles of soil, sand being the largest ( 2 mm - .05 mm,) silt next (.05 mm - .002 mm,) and clay (less than .002 mm.) Most soils contain a mixture of the three, with the most ideal mixture being termed loam.
Sandy soil, with mostly large sand particles, also have large spaces between the particles. In silty soils, there pieces are much smaller, fit together more closely, and the spaces are correspondingly smaller. With clay soils, the particles are so small that they fit together almost seamlessly, with hardly any spaces between.
The arrangement of particles and spaces determines the qualities of aeration, drainage, chemical activity and friability, which in turn affect plant growth by supporting or hindering the development of healthy roots. And of course, the heath of a plant is determined in large part by the health of the roots.
Aeration refers to the ease with which air can move into and out of the soil. In order to be healthy, roots have to breathe. Soil with plenty of sand - large particles - also has large spaces between the particles of soil, which allows air to move freely.
Drainage involves the movement of water through the soil. Water supports plant life both in itself, and as the basis of the mineral solutions the plants use in their various life processes. Soils with good drainage (those that have a good ratio of sand and silt) have just the right size of spaces to hold water long enough to give the roots time to absorb moisture and nutrients. Soils with too much sand allow the water to drain through too fast. Soils with too much clay have particles that are almost non-existent, so that water can’t run through, but stays in the soil so long that the roots suffer from being too wet, which promotes root rot and other diseases.
Chemical activity describes the extent to which the soil particles transport mineral ions and make them available to the plant roots in solution form. Clay particles are the most chemically active, so the right proportion of clay is important if the soil is to be fertile.
Friability involves the qualities of the soil that allow roots to penetrate easily as they grow. If the soil has too much clay, it’s too hard and the roots can’t penetrate easily. Again, the correct balance of sand, silt, and clay encourages the best growth.
Finally, the US Dept of Agriculture has also developed a taxonomy system that defines types of soil in terms of soil characteristics based on the physical, biological, and chemical properties that affect the uses to which land is put.