I learned many dirty words in the soil science classes I attended in college. We studied clay, silt, sand, loam and the various amendments used in the horticultural and agricultural industries. The only word that was too offensively dirty to ever use was “dirt”. In fact, I still do not know what that word means; only that one should not use it in reference to soil.

Garden enthusiasts in the Santa Clara Valley and the Santa Cruz Mountains do not need to be told of the importance of soil amendments. Although endemic soil is of exceptional quality, it may be too dense in most areas or too sandy in a few areas of the Santa Cruz Mountains. A few other areas may be very rocky.

Organic amendments are very useful for improving drainage and aeration in dense clay or adobe soil as well as improving moisture retention in sandy or rocky soils. As organic amendments decompose, nutrients leach into the soil, improving fertility. Microorganisms (fungi and bacteria) in the soil proliferate in organic material and convert the most important nutrient, nitrogen into nitrates which are easily absorbed by roots. (Nitrogen can not be absorbed directly.)

When selecting organic amendments, the most simple are usually the most suitable. Contrary to what some believe, the more exotic or more expensive are not necessarily better and may be wasteful. For example, peat moss is useful in very well drained sandy soil or for species which enjoy tannic acid. However, because peat moss retains so much moisture, it can actually be detrimental to very dense clay soil. Also, peat moss decomposes so rapidly, that is should be added more often than other amendments; and it is considerably more expensive.

I prefer to use simple redwood compost in our clay soil because it improves drainage and aeration without retaining too much moisture. It is also very inexpensive and decomposes slowly. We add a small amount to the vegetable garden every year to compensate for decomposition. In winter, I also add the oak leaves (valley oak) that the neighbor’s “gardener” blows into my front garden every Monday. Leaf litter does not last as long; but I can’t get any other amendment for a better price or delivered for free. If it is added early enough, it will be partially composted prior to planting in late winter.

Compost from garden waste is actually more appropriate as a soil amendment than non composted leaf litter and costs about the same. If it is well composted, it may be incorporated into the soil during planting. Other suitable organic amendments include ground timber byproducts and aged steer or horse manure.

Inorganic amendments such as perlite and vermiculite are also useful, but lack nutrients and should only be used for special situations and in conjunction with organic amendments. Although they last longer, they are expensive and only practical for small areas. Gypsum (calcium sulfate) is a chemical amendment which improves drainage and aeration by causing the clay particles to coagulate rather than bond together in a solid mass.

Potting soil is not intended to be used as a soil amendment and should only be used in containers. Although it contains several components which are used as soil amendments, most types also contain components which are not appropriate, such as peat moss. Potting soil is also very expensive in large quantities.