Describe the type of soil in which most plants do best. Include the proportions of sand, silt, and clay, and the amount of humus, that make for the most ideal soil conditions. Also, explain how each of these soil components contributes to the enhancement of plant growth.

Soils with the best oxygen and water penetration are loams, which have roughly equal proportions of sand, silt, and clay. Most plants do best in loam that contains between 10 and 20 percent humus, which is partially decomposed organic material.
Clay particles enhance the nutritive value of soils because they have a negative charge that attracts positively charged mineral ions in soil water. Thus, clay-rich soil retains dissolved nutrients that might otherwise trickle past roots too quickly to be absorbed. Sand and silt are also necessary for plant growth because they intervene between tiny particles of clay. Soils with too much clay pack so tightly that they exclude air-and the oxygen in it. Cells in a plant's roots, like cells in aboveground parts, require oxygen for aerobic respiration. Humus supports plant growth because it releases nutrients as it breaks down, and its negatively charged organic acids can trap positively charged mineral ions in soil water. Because humus shrinks and swells as it absorbs and releases water, it can also aerate soil by opening spaces for air to penetrate.