Building Soil in Chatham
by Al Cooke, Extension Agent
Fall, 2005
Printable Version
Yes, you can! I get lots of questions that seem to assume if I can get just the right fertilizer in my soil, then everything will be fine.
Fertilizer is neither food nor medicine for plants. And in fact, fertility is often not a limiting factor for growth in local soils.
It would appear from my conversations that soil compaction might be the most limiting factor or at least the one that gives gardeners
the greatest difficulty. Let me say at the outset that alleviating this problem is neither easy nor quick. It usually takes either deep pockets
or a strong back or both plus a long time. So if you’re looking for a quick fix, the best I can suggest is to think small. Imagine an area of
soil that that you can easily work in an afternoon and save the rest for next year. If you try to do it all at once, it may be so frustrating that
you give up in despair. So start small and set yourself up for an early success.
Envision a pile of rubble. Think of small animals like mice or lizards moving in and out of that pile. For them, the significant
aspect of that pile of rubble is not the pieces that make up the pile; the significant part is the spaces among the pieces. Soil is similar and
roots need the spaces too.
If you break soil up into its components, soil scientists suggest that an “ideal soil” would be 50% particles and 50% pore space.
The size of the pore spaces affects water infiltration, drainage, and aeration. Curiously enough, the size of the pore spaces is relative to
the size of the particles. I won’t go into the numbers but clay and sand are defined by the size of the particles. And as you may know much
of the local soil includes a lot of clay.
Clay particles are very small. It follows that the pore spaces within the clay soil are also very small. Go back to our pile of rubble.
If you crush all the pieces down to the size of 2-inch gravel, then the spaces may prevent use by rodents. We’re down to insects or very
small lizards. And if the 2-inch gravel becomes pea gravel, the pore spaces again get smaller.
Very small clay particles allow only very small pore spaces. What are the implications of small pore spaces? Water infiltrates slowly
(often stands on the surface). Water drains slowly. Aeration is poor. This is not a good situation for growing plants.
Now let’s add another complicating factor. Perhaps you have in your yard or know of a footpath, dog run, or some similar area
subjected to regular traffic. Take a shovel and compare how easy it is to dig that compacted area with a lawn or similar area not
subject to foot traffic. Many of you will be able to detect a difference. The particles have been pressed tightly together further
reducing the amount of pore space available. That problem is called soil compaction.
Soil compaction is a factor that makes our soil difficult to work. It is likewise difficult for plants to grow in compacted soil.
Why is it compacted? In many cases soil compaction results from the activities associated with building the houses in which we live.
First comes the land clearing equipment, which may be bulldozers. Soon there will be concrete trucks. And delivery trucks.
Delivery trucks unload stuff that will be stacked on the ground – bricks, block, sand, etc. All of this activity tends to press the soil down.
And it doesn’t spring back. If you stop to look at a construction site, it’s difficult to imagine that anything will ever grow there. One of
my professors once suggested, “There is nothing native to this site.” For many of us it’s difficult to imagine that our home once looked
like that. But it probably did.
Between the construction activity and the initial purchase of the house, someone will need to make the place a little more presentable.
Sometime that involves what passes for landscaping or even professional landscape work. Some builders have a crew that puts plants
in the ground just like a crew that paints. Some builders hire landscapers and sometimes the homebuyer has the option to be involved
in the landscape process.
But unless you see what goes on, there’s little reason to assume they did anything to repair all the damage that was done to the soil.
That will explain why it’s so hard for you to dig. If it’s hard for you to dig, imagine how difficult it is for plant roots. What can you do?
Strategies for soil remediation: Following are several different approaches to improving the workability of your soil. There is no
one best way. Agriculture teachers focus on the concept of “appropriate technology.” Under that theory, I’m not going to suggest you
start with a big tractor and a subsoiler (although that may be an option for some of you). I’m going to trust you to figure out what is
appropriate for you.
Do nothing. This strategy is for those who have lots of time. Given a few years, the soil will likely sustain a few plants
(crabgrass, ragweed, dogfennel) whose roots will probe and gradually make the soil more suitable for other plants. Subterranean insects
(grub worms) or rodents (voles) should be considered a good sign. With ten to twenty years of neglect the soil may begin to be workable again.
Till – compost – till. This is the strategy for those who have more resources than time and want it done now (!). Either you have the
energy to manhandle a tiller or the resources to hire someone to do it. Before you till, consider the trees. Look up and imagine that
every tree you can see is falling toward you. Any tree that could hit the tiller probably has roots in the soil that the tiller is tilling.
The tree will not benefit from having its roots tilled. There are disadvantages to this approach, but you want it quick. If the trees
are more important than getting it done quickly, then you should choose another approach. If not, till everywhere that you would like
to plant and grow something – vegetable garden, orchard, lawn, flower beds, shrubbery. Till it all. You may never have this much
energy again.
Once the soil is loose, cover it with some decomposed organic matter. How much? No more than 50% by volume. How deep does
your tiller go? Probably only 6 to 8 inches. So apply 3-4 inches of compost and till again.
The compost does some wonderful things. One of them is to facilitate the formation of soil aggregates or clumps. Clumping of soil is
what we call structure. These clumps function as both large particles and small particles in the soil. Remember soil particles and pore
spaces? These clumps function as large particles and create large pore spaces. Large pore spaces are good for water infiltration and
drainage. Within those clumps we have small particles and small pore spaces. These small spaces are good for water retention.
Voila! We have good water infiltration, good drainage and good water retention all in one ingredient! It’s amazing but true that organic
material – dead and decomposing – is a good addition to poorly drained clay soil as well as excessively drained sandy soil. It’s like magic.
Good stuff.
Organic material also provides good retention of nutrients when you fertilize. But that’s another article.
Compost – wait – till. This is the option for those who have more time and some resources of energy and material. It is a curious
thing that nature makes compost for us. If you can just get some of the raw material and put it on the soil surface, it will break down right there.
The breakdown is the result of action of worms, insects, fungi, bacteria, and other microorganisms in the soil. The big ones feed on big pieces,
digest some, and deposit the rest. Eventually they die, and it’s all deposited. Smaller life forms work on the smaller pieces until you can’t tell
any more what the original material was. That’s a good definition of finished compost – when you can’t tell what it used to be.
At this point you have improved the environment for living organisms in the soil. And indirectly that was your goal: a living soil.
People often report finding earthworms where there were none 6 months to a year after organic topdressing. That’s a good sign.
If earthworms can live there, the soil is workable. Plants can live there.
At this point you may or may not decide to till. The tilling will certainly be easier, because the living things have already done the
hard work. And actually, this is about as close to natural as gardening gets. People have called this strategy “lasagna gardening” or
“sheet composting” as if we invented it. But the process is older than gardening, older than man.
One further thing about organic matter – leaves, grass clippings, compost, manures, etc. You never add enough, and it’s always
going away. It continues to break down. The amount of organic matter in the soil is largely a function of climate. The breakdown
process is regulated by temperature and moisture. When you quit adding it, the soil will stabilize. But if anything is growing, then
there is always some newly dead material to be recycled.
Cover crops. This strategy is for people with plenty of time and not so much energy or other resources. It may also be for people
who have places where they don’t care what the neighbors think. Some guides for establishing cover crops sound as difficult as
growing anything else. But it can be fairly simple. Scratch the surface, broadcast seeds, wait a few months, and mow. You may
or may not till.
The most difficult task may be the initial soil scratching. It may be possible to do it with a hard tooth rake. You may need to use a
tiller just to break the crust. You may be able to do it with a rented verticutter. Scratch the surface enough to allow you to broadcast
some seed and rake them in.
What kind of seed. Cover crops are traditionally grouped as grasses or legumes and warm or cool season. Legumes have the
capacity to take nitrogen from the air and “fix” it in the soil. Grasses and legumes are often used together or in rotation. Warm
season covers are grown in summer and cool season covers in winter. (Duh!) For most of us, the crop selected is much less
significant than actually planting something!
Warm season grasses include several millets and sorghum-sudan grass. Warm season legumes include lespedeza, clovers,
cowpeas, garden peas, and soybeans. There’s also buckwheat, which is neither grass nor legume.
Cool season grasses include annual ryegrass, cereal rye, oats, and wheat. Cool season legumes include crimson clover,
Austrian winter pea, and hairy vetch.
Don’t make a career out of deciding which cover crop to plant. Remember you’re not expecting to harvest anything from these crops.
You’re using them for their capacity to improve your soil. There’s a lot to managing them including when to cut (before they set seeds)
and whether to till or plant through the stubble. Do find out how tall the cover crop may grow; you may need to mow it more than once
to keep it within the capacity of your mower. But you will usually find the soil much more readily workable after the cover crop.
And you have a new supply of mulch or organic soil amendment.
You can learn a lot more about cover crops from
Sustainable Practices for Vegetable Production in the South
Raised beds. I guess the primary reason for raised beds is to improve drainage. Sometimes it’s a way to introduce workable soil
on top of something like rocks or concrete. Because the beds are raised, there is a height differential that allows excess water to
drain out. It’s useful to remember, however, that just as a sponge can absorb water from below, a soil may hold more water than is
desirable – even against the pull of gravity. A soil with small particles and small pore spaces may still be poorly aerated – because
the pore spaces are still small and retain a lot of water. So elevation alone is not sufficient for good drainage.
And that raises the question of what you use to raise the bed and how. It’s a good idea to first till, or at least loosen, the soil beneath
the area to be raised. Usually you’re going to need a working area where you stand or walk around these beds. Soil from these areas
can be robbed which helps create your height differential. The bed is raised and the path is lowered. Organic material such as compost
can also be added to increase the volume and improve aeration. All of this can be retained with the building material of your choice.
Wood, rock, brick, concrete block. If you choose, you can make a seat around the edge. Such a seat makes raised beds excellent for
people with bad backs or legs. Sit down gardening! My day is close.
Working with your soil. I have developed an affection for my
garden fork. If you are not familiar with this tool it resembles a
pitchfork, but a pitchfork will not do the work it does. It has a short straight handle. The tines, usually 4, are about as wide as your
finger and just as thick. Viewed in cross section the tines are nearly square.
This tool is stuck into the soil as far as you can get it with your foot or your full weight.
Then rock it back and forth. Don’t try to
dig with it like a shovel.
Just pull back on the handle and watch the soil rise and crack.
(Now you see why the pitchfork won’t do
this; and you can see that the cheaper fork with flat tines won’t last long either.)
I use this tool to completely loosen a bed when necessary. A small bed is preferable depending on age and energy level. If the
soil is compacted and has never been worked, it may take some effort for small result. OK, it will take some effort regardless.
And the amendments to use. Almost any organic amendment is suitable for soil incorporation as long as you can’t tell what the
original material was. Garden compost, commercial compost, rotted manures, leaf mold… The one that I don’t use in clay soils
is peat. Peat has its place. In container media it is good because it helps to hold moisture. If you have clay soil, you already have
good water retention. Drainage will be more helpful. Peat is also slow to absorb water. You can float it on a bucket of water
perhaps for hours. Clay soil is also slow to absorb water. So the characteristics that peat brings are not those that are useful in
many clay soils. It’s great for improving water retention or reducing excess drainage in sandy soils.
There’s also a non-organic amendment that you may find useful. It’s not sand; they add sand to clay to make brick as some have
discovered the hard way. But you can add pea gravel, “Chapel Hill grit,” or some similar product sized about 1/8 to 3/8 inch.
These materials provide the big particles that go along with big pore spaces. They improve water infiltration and drainage.
Whatever material you add to your soil it must be incorporated. You don’t want 3 inches of something on top of what you already
have. You may find water still doesn’t move readily into the clay beneath. Roots won’t either.
And if you’re buying “topsoil” out of the back of a truck, take what you get. There is nobody (nobody!) mining and selling topsoil.
People do clean ditches and things like that and often sell the byproduct. And some compost processors call what they sell “topsoil.”
It may be compost or mostly compost. If it’s a bagged product, read carefully to see what is in it. What you already have is possibly
topsoil. It may just need to be reinvigorated.
This page created and maintained by
al_cooke@ncsu.edu
Created July 13, 2007
Last Modified July 16, 2007
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