News and Public Affairs

AUBURN, May 11--It is easy to take soil for granted. After all, it is almost everywhere. It may be sticky clay, gritty sand or powdery silt, but whatever type, the existing soil constitutes the planting medium for a garden on your property. To improve the soil's ability to support and nourish plants, gardeners strive to improve its structure and boost its fertility. Fortunately, there is an abundant, inexpensive ingredient that makes this job a lot easier. It is humus.

It is not a coincidence that humus is a part of every gardener's vocabulary, says Dr. Charles Mitchell, an agronomist with the Alabama Cooperative Extension System.

Humus changes sterile dirt into fertile soil. Derived from organic matter of all kinds, humus is the life support system of soil.

The presence of humus among mineral particles and air spaces enables soil to nurture plants two ways, Mitchell says. Humus creates a loose structure that simultaneously holds moisture and drains well. It also creates an environment that supports living organisms that convert soil nutrients into a form plant roots can use, building soil fertility. In short, humus brings soil to life.

In nature, humus is constantly introduced into soil as plant debris, dead animals and other organic matter that decompose on the ground. Humus permeates the top few inches of the soil through rains and the good services of earthworms and other

macroorganisms, where it continually revitalizes the soil around plant roots. This natural cycle is repeated over the seasons out in the wild, sustaining the great forests and other natural areas.

In woodland areas where there is much vegetation to decay and enrich the soil, the soil is rich in humus and very fertile. On the other hand, where there is little or no vegetation to provide organic debris, such as at the seashore or in the desert, the soil has little or no humus and is lean and infertile.

In developed areas, such as residential yards and gardens, where the natural vegetation has been removed or disturbed, this natural decay cycle is disrupted. Organic matter, such as leaves, dried plant parts, prunings, animal remains, manures and other debris, is routinely removed before it can recycle into the soil. Plowing of the soil combined with the mild, humid climate of Alabama further encourages the rapid decay of soil organic matter, leaving the soils of the South very low in humus.

Intensive planting of crops, turfgrasses and ornamental plants rapidly depletes soil of its existing humus content. Bare soil in garden beds is exposed to the harsh effects of sun, wind and hard rains, which further reduce its humus content and destroy its structure and fertility. To grow plants successfully, gardeners must constantly renew the soil by adding humus.

There is no such thing as perfect soil, says Mitchell. Every soil has problems in structure, texture, and/or chemistry that compromise its ability to nurture plants. The best way to confirm suspected soil problems is to submit a soil sample for laboratory analysis through the county Extension office. Extension specialists profile the soil content and structure, pinpointing deficiencies.

Fortunately, adding organic matter, or humus, can mitigate many problems. Here are six soil problems that can be addressed by adding humus.

Compaction: Good soil is loose and crumbly because it has lots of air spaces. Plant roots are able to penetrate soil deeply for extended drought resistance and stability. Air is also essential to the microorganisms that live on its organic content and process its nutrients to create fertility. Typically, soil in a home landscape is compacted, the air being compressed from it by the weight of foot traffic, construction, mechanical yard-care equipment and harsh weather. To reduce compaction, regularly add humus in the form of a topdressing to existing lawns. Spread a mulch of some organic on bare soil in beds and under trees and shrubs year around. Dig in compost, peat moss or the like into garden beds when planting to improve aeration.

Sandy soil: Sandy soil has large particles with large air spaces between them. Therefore, it drains so quickly that it dries out quickly. Also, water-soluble nutrients leach out rapidly before the plants can use them. Humus incorporated into sandy soil acts like a sponge, absorbing and holding moisture and any nutrients dissolved in it. Replenish the humus content of sandy soil at every opportunity.

Clay soil: Clay soils seem so thick because they have small particles with correspondingly small air spaces between them. They tend to stick together and cause water to fill up the air spaces. Since moisture does not drain from this soil well, plant roots rot. Adding humus to clay soils discourages the small particles from sticking so tightly. They aggregate into larger clumps creating larger spaces that drain more easily and hold air to improve soil texture.

Fluctuating pH levels: The acidity or alkalinity of soils, expressed as pH, affects how accessible their nutrients are to plants. Alkaline soils (pH higher than 7) inhibits the uptake of iron, boron, copper and other elements necessary for plant health. Excessive acidity (pH lower than 6) discourages plant absorption of other nutrients. Alter pH levels by adding either sulfur to increase acidity or limestone to reduce acidity in amounts indicated by soil test results. Because humus buffers soil against changes in its pH, adding lots of organic matter to the soil will help maintain desirable pH levels.

Infertile soil: Soil becomes sterile over time as its humus content is reduced by hot weather, removal of topsoil, or intense cultivation without replacement of organic matter. The number and activity of microorganisms in the soil is depleted. In their absence, the production of nutrients in the soil is severely curtailed and it becomes sterile. While fertilizer provides nutrients to plants, it does not solve a soil fertility problem. Supporting resident microorganisms in the soil is the long-term solution. Topdressing perennial beds with humus and incorporating it into cultivated soils every year provide a home for these organisms so they can assure soil fertility.

Pest and disease pathogens in soil: Soil rich in humus is alive. It supports active microorganisms to process nutrients and harbors beneficial macro-organisms such as ants and spiders that prey on soil-dwelling pest larvae and eggs. Humus creates a soil environment that supports beneficial nematodes and also bacteria such as milky spore that homeowners introduce into lawns to combat white grubs. Many other resident microbes attack and control disease pathogens that lurk in the soil. Topdressing and mulching lawns and gardens with organic material such as chopped leaves, compost or shredded bark products discourages soil pest problems.

SOURCE: Dr. Charles Mitchell, Extension Agronomist, Alabama Cooperative Extension System (334) 844-5489, and the National Garden Bureau