Lawrence F. London, Jr. - Dragonfly Market Gardens
 mailto:london@sunSITE.unc.edu - mailto:llondon@bellsouth.net

EMAP-Agricultural Lands

A Beginner's Guide to Soil Quality

What do you mean by soil quality? Isn't dirt just dirt?

Soil is more than just dirt! Soils allow us to produce crops and timber. They're responsible for the rich plant life we see around us and help provide the scenic beauty we all enjoy outdoors. Soil is like an underground ocean -- full of life and activity! (Oh, and by the way, when we talk about the ground, we say "soil" -- "dirt" is just soil where you don't want it, like on your carpets!)

Soils don't just take up space, they do something -- many things! They provide space and support for plants to grow, and places for little creatures to live. A good quality garden soil only contains about 50% solids. The rest is air, or pore space -- which allows plants to spread their roots out. Plants are happiest when about half the pore spaces have water in them. Good soils can hold water when it gets there and then release it to the plants when they need it. Most of your average plant is made of water, which comes mostly from the soil!

Soil also has to be able to hold and release nutrients (things like the nitrogen, phosphorus, and potassium, or "N-P-K", in plant food or fertilizer) -- there are fourteen essential nutrients, in addition to the carbon, hydrogen, and oxygen, that plants need. What's more, the nutrients have to be in a form that the plants can use. Just as an egg has to be broken open for use in cooking, soil nutrients have to be broken down into a form that the plants can use. This is done by a process in the soil called nutrient cycling. Little creatures called soil organisms have a vital role to play in nutrient cycling, by "cracking the egg" and getting the nutrients into a form that is appetizing and usable for plants. It's important for a soil to support a healthy population of soil organisms, so that nutrient cycling can go on. These organisms also keep essential nutrients from leaching away.

Soil organisms are also important because they provide the glue to help soil particles stick together, which gives soil a nice crumbly structure or tilth. A soil with good tilth will have plenty of air for the roots to breathe and will allow water to soak into the soil and eventually get to the roots. A soil without organisms and roots has poor soil tilth and is susceptible to crusting and erosion. Water doesn't soak into a crusted soil very well. Instead, water flows over the smooth surface, taking some of the nutrient-rich topsoil with it. Soil erosion is not only bad news for the crop -- which is losing its footing -- but is also bad for surrounding areas -- which can choke on the eroded dirt. Soils on hills and bare soils are particularly susceptible to erosion.

Soils can also act as filters and protect our environment (especially groundwater) by holding on to and breaking down wastes, chemicals, and pollutants. Soil organisms are important in this respect, too.

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