Garden Soil Information
What is soil?
When you walk on soil, it’s reasonable to think that you’re walking on something solid. After all, it holds you up. However, you might be surprised to learn that about half of the volume in a typical soil is empty space.
As it turns out, this empty space is critical to the function of soils, especially where plant growth is concerned. This empty space holds water after a rain (or after you water), and because water and soil interact in complex ways, at least some water stays in the soil and available to plants long after the force of gravity would otherwise have forced it to move down and out.
The water-holding capacity of soil is important not only because plants need water, but also because plants “drink” their nutrients. Plants rely on nitrogen, calcium, iron, and other elements to move from soil into water, which then carries the nutrients to special, element-specific gates in the cells of plants’ roots. Without water, plants are both thirsty and hungry.
The empty space in soil is also important because it allows air to move through the soil. Plant roots need to take up oxygen and get rid of carbon dioxide just like you and I do, and if there’s no air movement through the soil, roots die. Soil conditions are ideal for most plants to grow when half of the empty space in the soil is filled with water and the other half with air. Walking or driving on top of soil can compress the soil such that there is less empty space per unit volume – this is called “compaction,” and compaction reduces a soil’s potential for supporting plant growth.
So if half of the soil is empty space, what’s the other half? Well, basically, the earth is a big ball of rock. Over millions of years, water and wind and freezing and thawing have broken the surface of this big ball into small pieces. Soil scientists group these pieces into three categories by size. Sand particles are the largest, ranging from 2.1 millimeters (1/12 in) down to 0.05 millimeters (1/500 inch) in diameter. Silt particles are next smallest, and clay particles are even smaller – so small that individual particles cannot be distinguished even under a fairly powerful microscope. Together, the sand, silt, and clay accumulated in a particular place may or may not be abundant. Some parts of the world have no soil at all (they’re just bare rock), while other places have soil that’s hundreds of feet deep (usually because it’s washed or blown there from elsewhere). Most of the world, including southern Wisconsin, is somewhere in between, with soils 30-200 cm (1-6 feet) deep.
Theoretically, a soil could be all sand, all silt, or all clay, but most soils are made of at least some of each of these three particles. Soil scientists group soils into types depending on how much of each size of particle they contain. The soils of Eagle Heights Gardens and University Houses Gardens consist largely of what’s called “silt loam.” This means that these soils have all three sizes of particles, but that silt is the most abundant. Silt loam soils tend to be good for agriculture, though they can be easily eroded.
Between them, sand, silt, and clay make up 90 to 99 percent of the half of the soil that’s not empty space. The last little bit – between 1 and 10 percent of the total weight of the soil – is made of what’s called “organic matter.” Organic matter is made of formerly living things (stems, roots, worms, bacteria, etc.) that are in various stages of breaking apart. When these creatures were alive, they concentrated the nutrients that living things need in order to function. As these creatures start to come apart after death, the nutrients their bodies contain are released, and released much faster than the same nutrients are from sand, silt, and clay. As a result, even though organic matter is scarce, it’s critically important to a soil’s ability to support life. The silt loam in our gardens is 2 to 5 percent organic matter.