Life Support System for Soil
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
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
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
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.
organic matter, or humus, can mitigate many problems. Here are six
soil problems that can be addressed by adding humus.
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
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
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
SOURCE: Dr. Charles
Mitchell, Extension Agronomist, Alabama Cooperative Extension System
(334) 844-5489, and the National Garden Bureau