How To Improve Your Soil
You can't really change your soil's texture but you can improve it. However, it takes a large amount of effort and a long time to see any kind of permanent results. If your soil is extremely light (silty or sandy) you will probably want to mix in some fine-particle ingredients to make it less porous and more moisture-retentive. If your soil is very heavy (clay-like) you will want to loosen it, again making it more porous and better drained.
The ideal is, of course, somewhere between extreme lightness or heaviness. Such soils are called loams. Loam is a rather indefinite description as you can see, but implies enough large or coarse particles (like sand) so that porosity and drainage are good, and yet sufficient small particles (like clay or organic materials) to hold fertility, nutrients and moisture abundantly. Such a soil is almost automatically a good home for beneficial microorganisms.
One of the best ways to improve either a heavy or a light soil is through the addition of organic materials. This seems to be a contradiction, but as you'll see below, it's easy to understand why it works so well.
What To Do If Your Soil Is Too Heavy
Clay soils are sticky-slippery when wet, and bake hard when dry. They hold water and fertility well. However, the tiny clay particles have the shape of plates and nest so tightly that they practically eliminate pore space. Roots can hardly penetrate, and water drains poorly. Such soils are difficult to cultivate except at a certain stage of moderate moistness which may occur only a few days out of a year! Usually they are so slow to dry out in spring that you cannot do early planting. Then in summer they may cake and crack unmercifully. While this may not be too bad under a lawn, it is bad in a garden.
For your garden soil, try mixing two or three inches of organic material into the top six inches of a cultivated bed, to loosen it and improve its structure. Drainage can also be improved by loosening the soil through cultivation and by mixing in soil amendments. If the soil remains soggy much of the time, it may even be desirable to lay tile lines about two feet beneath the surface, to carry off standing water to a drainage channel.
There are several common misconceptions about improving clay soil. One is that adding sand to a clay soil will loosen it up and improve it. Adding sand to a clay soil will probably make it harder and more like cement. Another is that adding gypsum to a clay soil will improve it through a process called "flocculation." This is true, but only if you have a very sodic soil (high in sodium), a quite rare soil type not found on the North Coast. Gypsum is a calcium sulfate product sometimes used for pH correction. On our soils it is better to use lime for pH correction. While gypsum does not help raise soil pH as well as lime, gypsum does supply the nutrients calcium and sulfur to soil. Shredded wallboard is an ideal additive if your soil needs these nutrients - it is available free from Doug Lantelme, owner of Bayside Drywall; call 443-8728.
What To Do If Your Soil Is Too Light
Silty or sandy soils suffer the opposite troubles from heavy ones. They tend to be so porous that water and nutrients flush through, and this means overly frequent watering and feeding. But they can be cultivated and trod upon at any time of year without fear of compacting them--a concern with clays. You can increase their ability to hold moisture by adding clay-like materials or organic materials. About five percent clay thoroughly tilled six inches deep into sandy soil, or two or three inches of organic material similarly mixed in, should do the job.