May/June 2000

SmartGarden™

Second in a series highlighting AHS's new SmartGarden™ program SmartGarden™ - Organic Matter Feeding the soil instead of the plants

Is your soil poorly drained, or is it prone to drought? Does it feel sticky in your hands, or does it sift through your fingers like powder? Do the plants growing in it appear starved for nutrients? 

The solution to each of these common soil woes is the same-add organic matter. Organic matter-compost, leaf mold, grass clippings, rotted manure, or any material that was once alive-has near miraculous power to improve any soil. 

At first glance, it seems contradictory-the same material that improves the drainage of a heavy, clay soil can increase the capacity of a light sandy soil to retain water. It all boils down to soil structure, and regardless of soil type, organic matter improves its structure. 

Soil is made up of solid material and spaces between the solids-in roughly equal proportions by volume. About 90 percent of the solid portion of most soil is weathered rocks and minerals; these particles are classified according to size, from the smallest to largest: clay, silt, and sand. Most soils are a combination of particle sizes, with one or another predominating. 

The remaining solid part of the soil is organic matter. The pores between the soil solids serve as a reservoir for both the water and air necessary for healthy plant growth. The smaller the particles, the smaller the pores. 

Organic matter improves soil structure by facilitating the formation of soil aggregates-groups of particles bound together-and the resulting larger pore spaces between the aggregates. 

As soil organisms break down organic matter, they secrete a gummy material that binds particles into aggregates. Good soil aggregation results in a continuous network of pores allowing for better movement of water and air through the soil, and easier penetration by plant roots. It also improves the tilth or friability of soil. 

A soil with good tilth feels crumbly rather than powdery or sticky, and is easy to work. The best way to provide continuous, well-balanced nutrition for plants is to feed the soil with organic matter. As soil organisms digest organic matter, they release nutrients-in a usable form for plants. 

Unlike quick-release chemical fertilizers that offer a glut of nutrients that may be washed away with the next rain, the decomposition of organic matter is a continuous process; nutrients are released slowly over a long period of time. 

Of course, the amount of specific nutrients depends on the source of organic matter. Most manures are fairly rich in nutrients, while sphagnum peat moss contributes very few. There are many good sources of organic matter. 

Composted kitchen and yard wastes will in a few months yield a rich soil supplement. Leaves raked in fall become crumbly leaf mold, especially if they are chopped and mixed with a bit of soil to encourage their break down. Manure is often free from a nearby farm or stable, or it can be purchased from garden centers. Green manure is a cover crop that is sown, grown, and then turned back into the soil. Legume cover crops-such as clover and vetch-contribute additional nitrogen because of their symbiotic relationship with nitrogen-fixing bacteria. After organic matter has decomposed, it is called humus, which is dark in color, light in texture, and resists further break down. It continues to contribute to the soil environment by improving soil structure, holding nutrients against the force of leaching, increasing aeration, retaining water, and moderating a soil's acidity or alkalinity. 

How much organic matter should you add? A good starting point for soils that are worked each year is two to four inches of organic matter spread over the surface annually and then thoroughly worked into the soil. For more permanent plantings, the addition of an organic mulch such as shredded bark or leaves is helpful. The mulch gradually breaks down, and through the action of soil organisms and rainfall, nutrients become available to plant roots. 

Add organic matter whenever you plant, divide, or transplant perennials, shrubs, or trees. A soil well nourished with organic matter will sustain a healthy population of organisms, resulting in both improved soil structure and nourishment for your garden plants. This is recycling at its best.

Rita Pelczar, Associate Editor