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 (claylike) 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.

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.

Inorganic Materials as Helpful Additions

It is not always economical or convenient to use organic materials to improve your soil, and there are are other things that work well. Garden stores sell various products useful for loosening heavy soils or retaining moisture and nutrients in lighter ones. Among them are vermiculite, which is made up of particles of puffed mica, and perlite, which is pulverized volcanic stone.

These materials are often used in mixing greenhouse potting soils or for making golf greens. They may be helpful for containers or limited portions of the garden, but treatment of extensive areas becomes rather expensive because you may need as much as 50% in poorly-structured soil.

Recommendations for Lawn Soil

While it is a pleasant advantage to have really good soil for the lawn, lawns can usually make out pretty well with whatever soil you happen to have. Growing grass of itself improves soil; the fine grass rootlets finger between soil particles, leaving beneficial organic remains, since about half the grass rootlets die each year and are replaced by new growth. Prairie soils became black and rich because they grew grass through the ages.

Lawn grass loves fertility, but you can furnish this with a fertilizer. Roots penetrate deeper, and resist drought longer, if the soil is porous, and you can do something about this by cultivating the seedbed before planting, and by not squashing compactable soils with rolling or with heavy machinery.

If your soil is very sandy, you will have to fertilize and water more frequently than if the soil were heavier. But this is a lot easier than replacing the soil of your entire lawn.

If your lawn soil is sticky clay, it will hold fertilizer and water well, but you may have to soak such soils slowly, not run traffic over them repeatedly or try to cultivate them when wet. If a clay is so compact that it drains poorly and grass roots cannot grow deeply for lack of air, you may have to water more frequently to sustain the shallow-rooted turf. Such attentions are relatively simple, however, compared to the cost of bringing in or preparing a soil similar to that of a greenhouse potting mixture.

Facts on Acidity and Alkalinity

As if it is not enough to worry about the structure of a soil--and its fertility--there are other features which have an influence on both of these qualities and upon the type of organisms that thrive in soil. Acidity--alkalinity, measured by pH readings, is an important one.

A pH of 7 is neutral, neither acid nor alkaline. A pH higher than 7--such as 8 or 9--means that the soil is strongly alkaline. A pH less than 7 marks the soil as acid, around 5 quite so. A simple home soil test kit can be purchased in which you match the color of an indicator solution against a chart for the pH reading. Electronic devices that do this are also available.

Most plants do well when the pH is between 6 and 7, just a shade on the acid side. Most North Coast soils are naturally slightly acid and in this range. If your soil shows an extreme pH, it is best that it be brought close to this desirable range--at least for most plants.

Liming is the usual way to correct extreme acidity, or to make the soil "sweeter." The safest material to use for this is ground-up limestone rock, called agricultural lime. However, hydrated lime can be used. On a light soil, 50 pounds of ground limestone per 1,000 square feet should raise the pH about one unit, as from 5 to 6, but 75 to 100 pounds might be needed on a heavier soil for the same effect. It is best to take a pH test in order to determine whether liming is needed or not.

The other side of the coin is acidification of soils that are too alkaline. This is not normally a problem in this area. However, if needed, 40 pounds of agricultural sulfur per 1,000 square feet should reduce the pH nearly a point.

Back to where you started

Back one page: North Coast California Gardening Soils

Next: Humus, Composts and Mulches

This page last updated on November 3, 1996.

Copyright © 1996 Hans Koster