Garden experts routinely tell homeowners to amend their soil or to add nutrients to their soil because different plants need different soil types. In fact, the "Sunset Western Garden Book" editors advise that "an understanding of your soil is probably the most important" aspect of gardening. The ratios of clay, fine sand and larger sand particles make up a unique blend of soil in each and every garden.
Mineral and organic matter in clay soils are microscopic in size. They fit tightly together with little space in between for air or water. As a consequence, clay soil does not drain water quickly or easily. Plant roots in clay soil face the danger of sitting in a pool of water and rotting. Dense soil inhibits root growth. Furthermore, because there are few air pockets in clay soil, it warms up more slowly in the spring, thus delaying plant growth. While the clay does serve to retain more nutrients than other types of soil, gardeners must add pumice and vermiculite to aid drainage.
Sandy soils have larger particles. Sandy soil drains well and warms quickly in the spring because there are many spaces in between the large particles for water and air to flow through. However, those spaces also allow fertilizers to flow through the sand quickly before the roots have a chance to absorb the nutrients and the frequent watering that these soils require allow even more nutrients to wash away. Amendments to sandy soil should include materials that have a greater capacity to hold air and water, such as the large pumice or vermiculite minerals that break down and add nutrients very slowly. Organic amendments such as peat moss and leaf mold also break down slowly and provide nutrition as well.
Humus is soil is decomposed organic matter, like leaves, sawdust, bark or manure. Humus packs together tiny clay particles and improves both the drainage and aeration. The danger with humus is that the bacteria that carry out the decomposition process require nitrogen to survive and will take it from the soil if they run out of it from the organic material. For that reason, nitrogen is the most often added nutrient that gardeners add to their soil.
Loam and Silt
These two soils are variations of all the others. Silt refers to sandy soil with somewhat smallish grains of sand, while loam contains more organic material and is made up of 10 to 30 percent clay, 30 to 50 percent silt and 25 to 50 percent sand.