Home Soil Improvement
By: B. L. Holland, Master Gardener

In my part of the world the soil is not perfect. The topsoil is shallow and mixed with rock. Only a little deeper down is the bane of gardening - clay. Clay absorbs and holds lots of moisture. This is good, right? Not exactly, since most plants enjoy well-drained soil. They, like me, do not like wet feet. So, what does "well-drained" mean? To find out for yourself, deeply dig a hole where you intend to plant and fill with water. If the water is gone in 18-24 hours, drainage is okay. If not, consider raised beds. Gravel or sand in the hole will not improve drainage.

Garden plants also appreciate about 5 percent organic matter. Soil testing will indicate how much organic matter is already present in the soil. To improve the soil tilth as well as add nutrients, spread 1 to 3 inches of organic matter and mix to a depth of about six inches. Habitually adding compost, peat or rotted manure to soil before planting will make the soil a haven for roots. Just mulching plants with organic matter without mixing into the soil will not provide better aeration needed for healthy roots.

Holes for trees and shrubs should be shallow and wide. In heavy clay it is preferable to plant a couple of inches above the grade rather than sink the root ball into a loosened depression that will settle even deeper and accumulate excess water. Condition only a small amount of the backfill used when planting trees and shrubs. Roots may not want to grow into the surrounding unconditioned area and water movement may be affected.

Peat makes soil easy to work, improves water movement and aeration; but some types of peat may affect soil acidity.

Only well-rotted manure should be incorporated into soil. While it contains more nutrients than peat, its soil conditioning qualities will not last as long. Poultry manure contains more nitrogen therefore less should be used.

Wood products can improve soil; but they use up nitrogen while they decompose, so nitrogen should be incorporated on a regular basis.

Wood ash is strongly alkaline and its precise affect on soil pH is difficult to gauge. Use sparingly or not at all.

It takes fifty percent by volume of sand to improve drainage of clay and sand has no nutrient value. Adding less than fifty percent can actually make clay brick-like.

Lime can improve soil texture but affects pH so use it only if a soil test indicates the need.

Many agricultural by-products such as peanut shells, tobacco stems, rice hulls and corncobs make good mulch. Only when crushed and used with nitrogen do they make good soil conditioners.

And of course, compost is great for soil, improving soil texture and nutrients.

For details on the effects of numerous soil conditioners, see MU Guidesheet 6955.