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A Soil Primer
by Paulette Mouchet

Originally published in "American Rose".

The basis for successful organic gardening is excellent soil. The better the soil, the better the plant. What is excellent soil? There are three aspects to defining soil: texture, structure, and porosity.

Classification of soil texture by percentage of clay, sand and silt
Classification of soil texture
from the USDA

Soil texture is essentially the percentages of clay, sand, and silt. The National Gardening Association's Gardening for Dummies has a simple test for approximating soil texture. Take a handful of moist soil and squeeze it into a ball. Work the soil into the shape of a ribbon by pressing and rolling it between your thumb and forefinger. Stand the ribbon straight up.

If you can't make a ribbon without it falling apart, the soil is at least 50% by volume sand with very little clay. If the ribbon is much less that 2" long before it breaks, the soil has about 25% clay. If the ribbon is between 2" and 3.5", it has about 40% clay. If it is still together and longer than 3.5", then it is at least 50% clay.

Soil structure is the way the clay, sand, and silt particles join together with organic matter to form aggregates or clusters of particles.

Tilth is a term used to describe soil with structure that is good for cultivating. Soil with good tilth is loose, well aerated, and is easy for roots to penetrate.

Soil porosity is determined by the size and shape of the soil aggregates, which in turn determines the permeability of the soil to water and air.

Soil type is defined by texture, structure, and porosity. The term sandy soil describes soil that has at least 70% sand particles. The remaining 30% of the soil is made up of clay, silt, organic matter, and open spaces. Sand particles form aggregates or clusters that are large and boulder shaped so the structure of sandy soil is very open making it very porous.

Clay soil has at least 35% clay particles and the remaining 65% of the soil is made up of sand, silt, organic matter, and open spaces. Because of the nature of clay particles, they form small, flat aggregates (think tiny pancakes). Thus clay soil structure is very dense with little porosity.

Loam is considered the ideal garden soil. Loam contains approximately 35-45% mineral particles (clay, sand, and silt in equal proportions), 5-15% organic material, and 50% pore spaces. These proportions result in a soil with good tilth and porosity. Plant roots can easily grow through it to find food and water. Loam soil has a nice, earthy smell and is dark in color. A handful will form a loose ball that is slightly crumbly.

Precious few gardeners have ever seen loam soil, let alone had the opportunity to garden in it. Mostly, we are stuck with something a lot less than ideal. So how do we improve the soil we have?

Short of bringing in a backhoe and hauling out truckloads of your existing soil, there's not much you can do with the basic texture of the soil; that is, the percentage of clay to sand to silt. Nor can you directly change porosity, which is determined by the structure: the way the clay, sand, silt particles join with organic matter to form aggregates. The one thing you can change is the percentage of organic matter.

When you increase organic matter in the soil you reduce the overall percentage of minerals (clay, sand, and silt) and you change the composition and shape of the soil aggregates, which then changes porosity.

worm compost
Rainbow's End grown organically in soil amended with worm compost and redwood compost
Photo by George Mouchet

Organic matter has profound effects on the soil. It:

Fred Magdoff, professor of soil science at the University of Vermont and chair of USDA's regional Sustainable Agriculture Research and Education program, divides organic matter into three types: living, dead, and very dead.

1) The living portion includes bacteria, insects, nematodes, fungi, earthworms, and plant roots, and typically makes up 10 to 20% of the total organic material.

The living portion is vital to soil structure. Soil bacteria make a "glue layer" that glues clay, sand, silt, and organic matter together to form rock-shaped aggregates. Fungi such as mycorrhiza produce a sticky substance that stabilizes the aggregates. Microarthropods and bacterial feeding nematodes create the pore spaces. The end result is good soil structure aggregates with air spaces.

Beneficial soil organisms are easily killed by chemical insecticides and herbicides. Chemical fertilizers cause problems for these good guys by creating acids that cause localized die-off and by disrupting the balanced ecosystem of organisms.

2) The dead portion, which typically makes up 10 to 20% of the total is, "the organic matter that decomposes fairly easily." Organic mulch, which is generally defined as uncomposted plant material (ex. grass clippings and fresh manure), and fresh or unfinished compost are examples of this kind of organic matter.

Beneficial soil organisms (the living portion) decompose this dead organic material to make food for themselves and your plants. The process results in well-rotted compost and humus (see below).

3) The very dead portion of organic matter is the humus and well-rotted compost. Humus is often described as the 'life-force' of the soil. The following from Wikipedia is a good summary of its benefits.

Benefits of Humus

So what does all this mean to the home gardener?

Your first thought might be to run out and buy some humus for your garden, but there is an easier and cheaper way to increase humus in your soil. Remember, humus is created by beneficial soil organisms as they break down fresh organic matter. The best way to improve your soil is by adding lots of dead organic matter (fresh and unfinished compost) on a regular basis. This will maintain a healthy population of beneficial soil organisms, which in turn will provide the nutrition your plants need.

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