Know your Garden Soil


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The more you know about your garden soil, the more successful and productive your garden will be.

Are you planing to grow a garden? Do you already have one? A lot has been written about "sustainable gardening" recently, but my grandparents and great grandparents practiced this technique years and years before the phrase was ever spoken. To them it was just common sense. You only get out of your garden what you put in to it.

If the soil in your garden plot is poor and depleted, never fear, there are a number of things you can do to improve soil quality. Here are some pieces of information that can help you understand what makes up soil and how to improve it if needed.


garden soil before any amendments
The same garden soil AFTER amendments of compost & sand










The Physical Parts of Garden Soil

Soil is divided up into two basic layers: topsoil and subsoil. Soil (including your garden soil) is continually being created at the surface by decomposition of organic matter when it mixes with mineral matter. As new soil is being created at the surface older soils down in the soil profile are being depleted of organic matter and mineral nutrients. This buried and nutrient depleted layer is called sub-soil. With this information it is easy to see why preventing erosion of topsoil is very important. Depleted subsoil makes lousy garden soil. Soil is made up of components that include particles of rock matter (minerals) of various sizes, water, air, and organic matter.

Mineral Matter makes up about 45% of a good garden soil by volume. There are three general divisions of mineral matter: Sand, Silt and Clay. Each component is on average, about 100 times smaller that the previous. Ratios of each component are used to define the classification of soil type by soil scientists. Sandy Loam is the most desirable soil type for gardening, and is roughly 40% sand, 40% silt and 20% clay.

-Sand is the largest mineral component of soil. Size ranges between 2mm and 0.02mm. Sand is primarily quartz (silica), and has very little nutrients. The primary benefit of sand in soil is that is allows the soil to break up and be worked for planting (a term called "tilth"). It also helps prevent crusting which can, at it's worst, prevent seeds from breaking the surface.

-Silt is the intermediate size of mineral components ranging between 0.02mm and 0.002mm. Silt is also primarily quartz, and provides mainly the same benefit to soil as sand.

-Clay is the smallest size of mineral components with sizes of 0.002mm (2 microns) and smaller. Clay contains mineral components that provides nutrients to plants. Due to it's small size and shape (typically flat or plate shaped), more surface area is available for those nutrients to be dissolved and made available for plants to use.

Water and Air make up about 25% each of a good garden soil. Water and air fill the empty spaces and voids in soil between the mineral particles. Without enough water and air in the soil profile, most plants cannot survive. Too much water, and the air is driven out of the soil. Soil that is too wet will "drown" most plants. Too little water, and most plants can't survive either. Of course there are exceptions to the rule - Water Lilies and Cacti for example have VERY different requirements that most vegetable plants.

Organic Matter makes up the remaining non-living components of soil. A good garden soil will have around 5% organic matter. Organic matter is basically plant and animal matter in various states of decomposition in the soil. It contributes nutrients to the soil that minerals generally do not. Organic matter also improves the workability of a soil. If your garden soil is not as good as you would hope, adding organic matter is the easiest way to improve it.

Living Things are the final component of healthy garden soil. Earthworms, certain insects, bacteria, fungi, and other living things actually contribute to soil quality be helping decompose organic matter into a form that is usable to plants. Burrowing creatures also help improve soil quality by aerating it. Using pesticides (although needed on occasion) should be used only sparingly in your garden. Remember - it kills the good critters as well as the bad ones. Unless you have some kind of infestation that can't be controlled in any other way, try to avoid pesticides, as it can directly and adversely affect the quality of your soil.


Garden Soil Chemistry

Soil Chemistry generally refers to whether your soil is acidic, alkaline, or near neutral. This is referred to as pH (the potential of Hydrogen to react...) pH is measured on a scale of 1 - 14, with anything below 7 being acidic, and anything above 7 being alkaline (basic). A pH of 7 is neutral.

Most vegetable plants and herbs require neutral to slightly acidic soil ranging from pH 6.5 -7.0 The more acidic or alkaline a soil becomes the less mineral nutrients are available for plant use. pH is easy to test and you can buy an inexpensive test kit at a local garden center, or you can send soil samples to a local extension agent. They will test the soil for you at little or no charge. You can also buy an electronic tester that tell you not only soil pH but also the level of nutrients and moisture available in the soil.

The most common problem here in southern Indiana is soil that gradually acidifies over time. The old timers called this sour soil and fixing this condition was called sweetening the soil. This condition is counteracted by adding ground limestone, also known as agricultural lime, to the soil to bring it closer to a near neutral pH. Work with a local garden center, nursery or county extension agent to test, evaluate and improve your soil chemistry.

Two things determine the chemistry of your soil.

The first is the type of rock in your area. Native rock breaks down over thousands of years to create sand silt and clay The most common rock in my area of Southern Indiana is limestone and sandstone. Most bedrock here is limestone which is a carbonate of calcium and magnesium and is made up of the remains of ancient marine life. Limestone is neutral to basic in nature. If you live in an area where granite or volcanic rock are your main rock type, your soil will likely be acidic.

Decomposition of organic matter can contribute to soil chemistry as well. We have many caves in this region, including the worlds largest cave system just to our south - Mammoth Cave in Kentucky. These caves were carved over time by rain water passing through leaf litter on the forest floor and as a result becoming slightly acidified. The acidic water slowly followed cracks in the rock and dissolved it's way into becoming caves. This same acidity can affect the pH of your soil. Especially if you live in a area where pines and other conifers are common, because their needles tend to be more acidic than leaves from most hardwood trees.


Amending your Garden Soil

The most common type of soil amendments are compost and sand. I add compost to my garden whenever I have some available in the compost pile. Compost added to garden soil will improve the workability, as well as adding nutrients.

You can also add fresh organic material to your garden in the spring or fall. Grass clippings, leaves, sawdust, manure (fall only), are just a few examples. I don't recommend adding straw or hay to your garden. Not because it won't improve the soil, but because the long fibrous strands are hard to work in. They tend to wrap around the tines of a tiller and bog it down, instead of breaking up and working in. Hay and straw are better off if you compost them, then add them to the garden later.


A ugly but effective compost pile

Plant residue from the garden itself can be worked back in at the end of the growing season. The only possible exception being cornstalks, as they tend to behave like hay and straw.


High quality compost straight from the center of the pile

If your garden drains poorly or tends to be clayey, you can add sand to it, to improve the structure, but it should be relatively coarse sand. Add sand to a depth of about of 1"-2" and till in deeply and completely.

Sand and organic matter added to your garden soil can help prevent crusting. when you first plant your garden, then get rain, the soil can crust over on top. This can prevent seeds from germinating properly or at all, especially very small seeds like lettuce, turnips, and radishes.


Mulch and Garden Soil

Most vegetable plants can benefit from a layer of mulch. Mulch is essentially a covering on the soil that serves four purposes:

1 - It helps retain moisture in the garden soil around your plants, by reducing the soil's exposure to evaporation. This in itself is enough justification for mulch.

2 - It helps keep the garden soil from overheating in the summer sun. Mulched garden soil can be as much as 20° cooler than bare garden soil. This prevents plants from being overstressed from the heat.

3 - It helps control weeds around the base of vegetable plants that might otherwise compete with them for valuable nutrients and moisture.

4 - Mulch will break down into compost and improve the quality of the garden soil over time.

Note in the image below that grass clippings have been used as a mulch around the base of the tomato plant.  Grass clippings are a very effective mulch IF they are allowed to dry before collecting them and placing around your garden plants.

Grass Clippings used as a mulch under a tomato plant

There are many things that can be used for mulch. Newspapers, grass clippings, compost, leaves, peat moss, and commercial bark mulch all serve the purposes listed above.

Hilling soil around certain plants can also serve as a mulch. Potatoes and corn are two of the most commonly hilled plants.

If you're in doubt about the effects of mulch, try an experiment of your own. Grow two identical plants side by side in your garden. Put generous amounts of mulch on one and leave the soil bare around the other. By the end of the growing season, see which one looked healthier and produced more.


Know your garden soil, and take care of it, and in the long run, it will take care of you in return. The more you know about your soil, the more productive your garden will be. Plants grow bigger, better and faster in good quality soil than in poor depleted soil. The more productive your garden is, the more self sufficient you have the ability to become.


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