By Wayne McLaurin
Georgia Extension Service
Volume XXVII
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Productive soil is the single most important factor in
successful gardening.
Since few of us are blessed with perfect soil, we need to know
how to improve
what we have.
Improving the tilth
Adding organic matter to the soil improves the soil texture
(tilth). This improves
the environment for good root growth and the development of
soil microorganisms
that make nutrients more readily available.
Organic matter supplies some nutrients, too, but most forms
are lower in nutrients
than commercial fertilizers. To the plant, it makes no
difference whether the
nutrients come from organic or inorganic sources, since they
can use the nutrients
only in the basic inorganic form.
Nutrient availability
The difference is primarily in the availability. For
instance, nitrogen from
organic sources is released more slowly than nitrogen from
most commercial fertilizers.
Slow release of nutrients would be good in a soil already
adequate in nutritional
levels. Where soils are short of one or more nutrients,
though, it’s usually
best to add commercial fertilizers for the more quickly
available nutrients
to correct the deficiency.
It’s best to add both organic matter and inorganic
fertilizers. However, all
fertilizers added should be based on a soil test.
First, check your soil
Before adding fertilizers, first find out whether a problem
in growing healthy
plants is due to nutrition or a physical property of the soil,
such as poor
texture.
A plant in a poorly aerated soil may do poorly because the
root system can’t
use the nutrients, even though an ample supply may be
there.
Soil with a steep slope may have good surface runoff but poor
subsurface drainage
if the texture is high in clay or if underlying soils block
water movement.
Water is always held more tightly in fine soils than in
coarse, sandy soils.
A fine soil underlaid with compressed organic matter, sand and
even gravel won’t
drain well.
Uniformity, balance, drainage important
The best soils for growing plants are uniform in texture
throughout the root
zone, with a good balance of minerals, air and organic
matter.
One of the easiest ways around drainage problems is to make a
raised bed. Raising
the bed as much as 8- to 12 inches will allow proper drainage
from around the
root system.
Improving soil a continual process. It often takes 10 or more
years to make
a productive garden soil.
If your soil is too sandy or too high in clay, the solution
to both extremes
is essentially the same — add organic matter.
In a sandy soil, organic matter acts much like a sponge to
hold moisture and
nutrients. In clay, organic matter helps aggregate the finer
particles, allowing
for larger pore spaces, improving aeration and drainage.
It’s possible, especially in clay soils, to create a soluble
salt problem by
adding too much organic matter all at once.
Adding organic matter
The rule of thumb is to incorporate no more than 3 cubic
yards of organic matter
per 1,000 square feet per year. This is about 1.25 inches on
the soil surface
before it’s tilled in. All amendments should be thoroughly
tilled into the soil,
making it a uniform mixture.
The best organic amendments include relatively coarse,
partially decomposed
compost and aged barnyard manure. The type of manure isn’t
important. But it
should be at least one year old if you plan to plant soon
after amendment.
Never use fresh manure. Always compost it for at least 120
days before using
it. Because of high salts, avoid repeated use of manures
unless the salts can
be leached first. Dairy cattle manure generally is lower in
salt content.
Coarse sphagnum peat is a good amendment but is expensive
when compared with
manure or compost.
Many gardeners want to add sand to the clay to break it
apart. Don’t. Adding
sand to clay makes good bricks. If you are going to add sand,
you must also
add organic matter along with it — about 25 percent of each
by volume.
Remember, to build a good soil:
- Check for nutrients (soil test) and texture (ball
test). - Add organic matter (composted manure, compost, etc.).
- Dig and aerate only when the soil is workable (not too
wet and not too
dry). - Add nutrients according to the soil test with either
organic sources (slow
release) or commercial fertilizers (quicker release).
Texture Test
Roll some slightly moistened soil between your thumb and
forefinger. If it
forms a firm ball, feels smooth and becomes sticky when
moistened, it’s too
high in clay. It’s a better texture if you can’t form a ball,
the soil won’t
stay together and it feels somewhat grainy. If, on the other
hand, the soil
feels very coarse, it may be too sandy and won’t hold enough
water.
Subsoil Drainage Test
Dig a hole in the garden about 12 inches deep and the
diameter of a spade.
Pour water in the hole to the rim. Refill the hole a day later
and see how long
it takes all the water to soak in.
If it soaks in within a few minutes, the subsoil drainage may
be too good.
It may not hold enough water to sustain plant life and can
lose valuable nutrients
through leaching.
If the water takes more than one hour to soak in, the subsoil
drainage may
be poor. Plants may suffer from oxygen starvation (drowning)
in this soil.
Liquid Amendments
Liquid products break the surface tension of water around the
soil particle
and allow deeper water penetration. They in no way increase
the pore space of
a soil.
The liquid “conditioners,” then, can’t be considered soil
amendments. They’re
properly called “adjuvants.” At best, they may temporarily
enable water to penetrate
better. But they don’t break up clay soils as some claim.
They’re not substitutes
for amendments.
What about gypsum?
Gypsum is a salt (calcium sulfate) and does no more than
increase the calcium
and sulfur content. It doesn’t change the soil pH and isn’t a
substitute for
dolomitic lime.
To adjust the pH, always use dolomitic lime, since it also
adds magnesium,
an essential element. Many claims are made that gypsum breaks
up clay soils.
However, adding organic matter will break up clay soils far
better than any
other product while increasing the ever-important
microorganisms. In other words,
adding gypsum to a soil that doesn’t need it is a waste of
money.
