Improving Your Garden Soil
By Wilbur L. Bluhm
(Professor Emeritus, Oregon State University Extension Service, and Horticultural Consultant, Salem,
Oregon)
What can you do to improve your garden soil? The
question is as old as gardening itself.
From the time man first tilled the soil and planted seeds
he has tried to improve the performance of the plants he
grew. Most basic has been improvement of the soil.
Early settlers on the eastern shores of North America
received a good lesson when planting seeds and growing
plants. Native Americans showed them that by placing a
fish in a planting hole the plants would grow faster and
yield better. At the time neither knew what the fish in the
hole did, but it worked. They didn’t know the fish
carcass provided nitrogen, phosphorus, potassium,
calcium, sulfur, and other essential plant nutrients, often
deficient in many rain-soaked soils.
Our knowledge of soils and plant nutrition and growth
has come a long way during the past four centuries since
the fish-in-the-hole lesson.
The Soil That Is Ours
The soils we inherit from earlier generations are often
drastically altered from the original. Farming operations
before our homes were built began the soil’s change.
Native prairies and woodlands were converted to
agriculture. Or, if originally a forest, logging and other
operations had its impact on the soil. Both took their
toll, either in loss of soil structure and perhaps plant
nutrients, or compaction, or both.
Years later home construction became the second phase
in the change. Heavy equipment compaction layered the
soil below the surface. Existing soil was dug to make
room for basements or crawl spaces. This soil was then
spread over the existing, putting layer upon layer. Fill
soil was, and still is, often brought in, creating further
layering of the soil.
As a result few of us have the privilege of gardening in
native, virgin soil. The soil we have has commonly lost
much of its structure and ease of working. Some of us
are working subsoil, underlaid with good, fertile topsoil.
Often the soil’s fertility state has been changed, resulting
in nutrient imbalances, with increased acidity or alkalinity.
A redeeming feature is that there is much you can do to
improve the soil you have. A number of practices can
improve the soil and make it highly productive for you.
Soil Layering and Compacted Layers
First, let’s look at a compacted, layered soil that many of
us have. If at all practical, thoroughly mix the layers of
soil by spading or rototilling. If your lot or acreage is
sufficiently large, a deep-running plow or backhoe will
effectively break up compacted layers and mix the soil
layers. Farmers use “subsoilers” to break through
compacted soil layers, an alternative for larger lots.
The downside of equipment is that it also can create
compacted soil layers. Rototillers “beat” the soil, thus
compacting it, at the lowest extension of the tines.
However the compaction from rototilling is a “lesser
evil” than the needed breaking of a more severely
compacted layer and mixing of layers. Keep in mind that
a rototiller never tills as deeply as it appears. Therefore
the larger the rototiller the more effective it will be.
Plows and most other equipment also tend to create
compacted layers at the lowest depth of their
penetration. Again, the newly created compacted layer is
seldom as severe as the one for which the equipment is
used.
Soil, when in distinct layers, frequently has another
problem. Layering of soil, especially when done by man,
often creates drainage problems. Downward water
movement in soil is commonly impeded when soil is
artificially put into layers. Thus the importance of mixing
the layers of soil.
Soil Drainage
Poor drainage greatly restricts plant growth and what
plants you can grow. Test the drainage of your soil by
digging one or more holes to a depth of 2 to 3 feet. Fill
the hole(s) with water. The water should disappear
within 30 minutes or an hour. If it does not disappear
within 24 hours, only shallow rooted plants will survive.
Don’t confuse slope with good drainage. A sloping soil
may be poorly drained internally. Use the test above to
know for sure if your sloping soil has adequate drainage.
The poor drainage of many garden soils is due to layering
of soils and to compacted soil layers. Corrective
measures discussed previously can correct many such
problems.
Soil drainage can be improved by tiling, or sometimes
with “French drains,” ditches a foot or more deep filled
with sand and/or small gravel with an outlet for the water
to drain away. If you have a soil drainage problem
contact your local university Extension office, Natural
Resources and Conservation Service (USDA) office, or
business specializing in drainage problems for
information and assistance.
Adding Soil
Bringing in soil is sometimes a solution to problems of
low lying areas or providing a productive soil over one
that is filled with rocks, debris, or other materials. It
may help improve a soil of extremely poor structure or
heavy texture, high in clay and poorly drained. To avoid
creating layers, thoroughly mix the added soil with the
top six inches or more of the soil below.
Be sure to acquire good quality soil. Much available
“topsoil” is the topsoil from the bottom of a hole, and is
of poor quality. In most cases the best soil to acquire is
that which is most like what you now have, even with its
faults, to avoid layering. Topsoil from nearby
excavations for home, business, industrial, or other
construction is a good place to start your search for a
source of good soil. In any case, try to acquire soil that is
from the surface 12 inches, more or less.
Enhancing Your Existing Soil
“My garden soil is a poor, clay soil,” is an all too
common complaint. Often poor soil structure is mistaken
for high clay content. The soil is hard, crusty, and
difficult to work. Some soils are “loose” and seem to
have no body or structure. The solution is to add a good
soil amendment to improve the soil structure.
First, let’s allay some misconceptions about clay soils.
Few, if any, soils are pure clay. Some are high in clay,
and nearly every soil, except pure sand, has some clay in
it. Clay is a very important soil component. It imparts
many desirable qualities to the soil. Without clay a soil is
likely to be less fertile, even infertile, and added nutrients
are readily leached from the soil. Clay does this because
of its cation, pronounced cat-ion, exchange capacity, an
electrical process not unlike your car battery with its plus
(+) and minus (-) charges, that attracts plant nutrients and
holds them from leaching out of the soil.
In the minds of some the solution to a soil high in clay is
to add sand. This, too, is not without problems. Add the
right amount of sand and you will have made great
concrete. The right amount of sand has to be determined
for each site. There is no one easy answer to this. So, in
most cases, sand is not the answer.
How can you know if you have a clay soil? When the
soil is rather wet, form a ball an inch or two in diameter.
If the ball is rather sticky it may be due to clay. Then,
with the ball between your thumb and forefinger, make a
continuous thin ribbon about 1/8 inch thick. If the ribbon
sticks together its full length the soil clay content is
relatively high. If the ribbon breaks or crumbles after it
is an inch, more or less, long you probably do not have a
clay soil. It is likely a desirable silt loam, loam, or clay
loam soil. If it crumbles immediately as it leaves your
fingers, your soil is still lower in clay and higher in silt
and sand content. This procedure may take a little
practice before it works well for you.
Benefits of Organic Matter
Raw organic matter offers limited benefits. It’s in its
decomposition that the benefits occur. Many products
are formed within the soil that function to improve soil
structure, aeration, water-holding capacity, and enhance
plant nutrition. Partially decomposed organic matter is
often called “humus”, the merits of which are known by
many people.
Adding organic matter to your soil is an excellent solution
to poor structure, and a much better alternative than
sand. It also behaves much like the clay particles as
described previously, enhancing soil fertility, without the
structural problems of some heavy clay soils. Organic
matter has an exchange capacity that often exceeds that
of a clay soil. One of greatest values in mixing organic
materials into a soil is its effect on soil structure. Organic
material itself, or in its decomposition, binds soil particles
together to form larger particles. This is especially
important in heavier soils that are higher in clay. This
improves the soil’s drainage ability and air holding
capacity, both important for plant growth. The organic
material itself, and the resulting effects of its
decomposition on soil, improves the soil’s tilth.
Organic material has great water holding capacity. So,
while it improves drainage it also holds more water,
keeping the soil moist longer for plant growth.
In decomposition of organic matter the soil microorganism
– bacteria, molds, actinomycetes, etc. –
population increases manyfold, for it is they who are
responsible for the decomposition. This releases
nutrients from the organic matter, also benefiting plants,
and provides the many other benefits from using organic
matter.
Plants themselves contribute organic matter to the soil.
With a constant flux of new roots being formed and old
roots dying organic matter is added. However, it’s a
relatively low amount with continual tilling of the soil for
planting and replanting. Continuous grass growth over
many years may add 5% to the soil’s original organic
matter content, which is a significant amount.
Organic matter will benefit most soils, regardless of
texture or structure.
Sources of Organic Materials
Almost every community offers its unique types of
organic materials. In forested areas sawdust and
barkdust are readily available. Grain straw is available in
most communities. Sugar cane, coconut, rice, nut, and
other agricultural industries produce useful by-products.
Legume hays are not only good sources of organic
matter, but also of plant nutrients. Leaf mold, the
partially decayed leaves from your trees or from the
forest floor, has long been a favorite organic material.
Garden waste and other composts have become much
more available in recent years and, when well prepared,
are free of weed seeds and plant pathogens. Of course,
each of us can have readily made compost made from
our lawn clippings, tree leaves, garden and kitchen
wastes, and other materials.
Peat, aka peat moss, has been a standard organic source.
It is still the same good product as in the past, but may
occasionally contain weed seeds or pathogens, depending
upon method of harvesting. Peat decomposes more
slowly than most other organic sources and thus may
give its effects comparably longer.
Animal manures have for years been among the best
organic sources. While providing organic matter they
can also be a good source of plant nutrients. Partially
decomposed “old manure” is a good source of organic
matter but inferior to resh manure in plant nutrients. The
fear of “burning” with fresh manure is easily offset by
applying less of the better product. Unfortunately
manure may contain numerous weed seeds, including
those of noxious weeds.
When purchasing manure inspect carefully for unwanted
problems. One, garden centipedes (symphyllans),
introduced into a soil can cause tremendous damage for
years to come by feeding on plant roots. It’s best to
avoid manure stored for any length of time on the
ground. Older manures are more likely to come with
these problems.
Organic materials high in lignin, such as wood products,
straw, grass hay, and hulls, can deplete soil nitrogen
during their decomposition. Supplemented nitrogen,
supplied by a fertilizer relatively high in nitrogen, will
correct the deficiency. Wood products take up to four
years of decomposition for the “softwoods” – pine, fir,
spruce, etc. – before a nitrogen balance is reached in the
soil. Hardwoods – oak, maple, beech, etc. – take about
half this amount of time for decomposition. This is
usually not a problem with peat since it naturally contains
some nitrogen, or with composts, leaf mold, manures,
and other such sources.
Dealing With Sandy Soils
A truly sandy soil will feel gritty when rubbed between
the fingers. Most sandy soils have at least a little clay
and silt in them, but insufficient amounts of clay,
especially, to hold nutrients and water. Structure of
sandy soils may be either loose or, with an insufficient or
“wrong” amount of smaller particles and organic matter,
hard and crusty and impenetrable to water when dry.
They are inherently low in plant nutrients and good tilth.
Sandy soils can be much improved by “diluting” with
good soil and/or organic matter. The amendment(s)
should be thoroughly worked into the top six or more
inches of the soil, as deeply as possible.
It is wrong to assume that a sandy soil is well drained. If
the sand is underlaid by a compacted layer drainage can
be a serious problem.
Generally, in sandy soils fertilizer should be applied in
lesser amounts but more frequently to avoid loss by
leaching and contaminating ground water. This is more
important with water-soluble inorganic than organic
fertilizers.
Soil Testing
Testing your soil is a good way to determine the pH
(acidity or alkalinity), need for pH correction, and
nutrient content of your soil, with suggestions. A good
soil sample for testing is a composite of samples taken
throughout your garden, then thorougly mixed together
when sufficiently dry by pouring back and forth from one
clean bucket to another. Follow all instructions of the
testing laboratory to which you will submit your soil
sample. One thing you will not learn about your soil from
the test is the soil’s texture, that is, if it is a clay, clay
loam, loam, etc. A few labs, however, can make a
texture test for you for an additional fee.
Generally Speaking
Organic matter will do as much, or more, to improve a
given soil than most any other practice. Worked deeply
into the soil its effects can often be seen for years to
follow. The only real “downside” in using organic matter
is the nitrogen imbalance that high lignin materials can
present, and the potential for insect, weed, and disease
problems of some organic sources. Seldom do the
problems outweigh the benefits.
Keep in mind that working a soil when it is excessively
wet or when excessively dry should be avoided. When
too wet the soil structure is broken down, causing
puddling or “plastic remolding.” When too dry the soil is
ground or pulverized into dust, also causing a breakdown
of structure. In between lies a rather narrow range of
wetness for optimal cultivation of the soil.
Improving your soil – make it enjoyable and watch your
garden grow!
Tall Talk Article by Wilbur Bluhm - March, 2002 file:///C:/Data/WebPages/TBIS-web/tt0302b.html
1-11 of 11 1/10/2012 11:34 AM
These are not my words but are my thoughts :)
Have a nice day
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