Understanding Soil

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

| January/February 2008

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.

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