Understanding Soil

There's more to it than a test kit and fertilizer.

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iStockPhoto.com/Greg Nicholas
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Three soil profiles show the differences in slightly eroded vs. severely eroded soils in Iowa; the dark, highly productive soil is on the left. courtesy Natural Resources Conservation Service/Lynn Betts
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Avoid tilling when the ground is wet to maintain peds. iStockPhoto.com/Kary Nieuwenhuls
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This soil profile in central Iowa shows root penetration through the topsoil. courtesy Natural Resources Conservation Service/Lynn Betts
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Preparing your garden with a fork helps create soil structure that makes planting easy. Shutterstock.com/Sharon Kingston
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Preparing your garden with a fork helps create soil structure that makes planting easy. iStockPhoto.com/Rich Legg

Synthetic fertilizers and off-the-shelf enhancers have made it easy for folks to get by without knowing much about the ground that grows their garden. According to that formula, all it takes for success is a soil test, a bag of fertilizer and plenty of water. Physical characteristics of the growing medium itself are often ignored.

However, physical structure, texture and organic matter content all contribute to a soil’s tilth or ability to support plant growth, and they need to be considered to get the most of your precious inputs. When texture, structure and organic matter are in balance, the soil is said to have good tilth. Soils with good tilth not only produce healthy plants, they also minimize the need for inputs.

Loam is lovely

Soil texture is the first, and easiest, characteristic to recognize. Push a trowel into your soil in a place where there aren’t many rocks. Does it go in easily? Does the soil break into little pieces as it runs through your fingers? If so, you have soil that is easy to work, but it might be low in nutrients and dry out quickly after watering. If, on the other hand, the soil resists your trowel and breaks into large chunks, then it might be hard to work, but probably contains more nutrients and holds water even during dry weather.

These differences in soil texture are due to the relative amounts of sand, silt and clay in your soil (See “Looking for Loam”). Loam, the most desirable soil texture, contains approximately equal volumes of sand and silt and about 25 percent clay. Loam has enough sand to keep it loose so plants can easily push down roots and find a firm foothold, enough silt to allow water and the nutrients it carries to percolate, and enough clay to hold water during a dry spell. But texture is only part of the equation.

Aggregate engineering

Soil structure or the way those textural components interact with one another is critical. When you run a loamy soil through your fingers, some of the particles are individual grains of sand and silt, but others are aggregates of smaller grains cemented together by sticky clays and organic compounds. Taken out of the soil, we might call one of these aggregates a dirt clod, but a soil scientist calls it a ped. The size and shape of the peds determine the soil structure. Why are peds important? Because the spaces between them are the conduits through which water and nutrients travel, at least in a fine-grained soil.

Soils that haven’t been disturbed for many years often have well-developed structures that readily absorb water and keep the root zone aerated. Disturbing a nicely structured soil with routine plowing or rotary tilling is an easy way to alter its structure – and not necessarily for the better. Cultivation can break down the peds and improve the structure of the soil for planting, but doing so when the soil is too wet will destroy the peds and create a hard, impervious mass. Compaction from too much heavy traffic also destroys the structure with similar results.

Humoring with humus

The third important soil characteristic is organic matter content. When organic material decomposes, carbon atoms combine with oxygen from the air to form carbon dioxide, which is lost to the atmosphere. If decomposition is incomplete, as in the case of compost, then a carbon-rich material called humus results. The molecular surface of humus attracts, holds and slowly releases nutrients, which can be used by plants.

Because humus is black to brown, the amount of organic matter can easily be estimated by soil color: dark colors indicate lots of humus and light colors indicate a deficiency. Organic matter is very important to the nutrient balance of the soil because it holds fertility that might otherwise be lost to leaching.

Nutrients and the soil

The three major plant nutrients commonly discussed with respect to plant growth are potassium (represented by the chemical symbol K), phosphorous (P) and nitrogen (N). In many soils, potassium is extracted directly from the clay and is not often deficient unless clays are absent. Phosphorous, on the other hand, may be in short supply because the natural extraction process is very slow and the amount available in native soils is generally very small. Most naturally occurring phosphorous is recycled by the soil; it can also be added organically in manure and compost.

Nitrogen is the most difficult of the three nutrients to manage. Although the air we breathe is rich in nitrogen, we cannot use it, nor can most plants. Instead, nitrogen is extracted by certain bacteria that form nodules on the roots of leguminous plants like peas, beans, alfalfa and clover. Once the plants die, the nitrogen can be lost in one of two ways: It can dissolve and be carried away by groundwater, or it can turn back into a gas and be lost to the atmosphere. To be retained in the soil, nitrogen must be held by humus, so it is not surprising that a soil with low organic matter will quickly lose nitrogen. In fact, if a nitrogen-rich fertilizer is applied to a low-humus soil, more of it is lost than is used by the plants, necessitating almost continual reapplication.

Tips for good tilth

Once you understand the physical characteristics of your soil you can work to make it a better growing medium for plants. If it is too sandy, add clay; if there is too much clay, add sand. If there isn’t enough organic matter, add compost or peat.

Avoid compaction of the growing areas by using dedicated paths through the garden. No matter how tempted you are, avoid tilling when the soil is too wet. If the soil’s structure is already compromised by cultivation, then treat it as gently as possible when digging. If the soil is compacted, aerate it with a fork or other tined tool.

Compost will improve just about any soil’s structure, and it is the best way to increase organic matter content – so add it freely. And since oxygen constantly attacks the carbon atoms in humus, be sure to work the compost into the soil to minimize its exposure to air.

Jeff Walker teaches geology and environmental studies at Vassar College. His research focuses on sustainability, agriculture, and on the environmental writings of the 19th-century nature writer John Burroughs. His most recent book is an edited reissue of Burroughs’s Signs and Seasons (Syracuse University Press).