Soil is simply a mix of minerals, air, water, and organic material that combine to form a growing medium for plants. A few days ago I mentioned soil tilth. (In "Developing New Garden Beds" on Feb 20, 2011). Tilth is all about the soil's ability to support a plant's root growth. Root growth directly correlates to plant growth; poor root growth equals poor plant growth.
There are three properties that define soil tilth: soil texture, soil structure, and soil fertility.
Texture refers to the size of the particles that make up the soil. Sand, silt and clay identify relative sizes of soil particles with sand being the largest particle and clay being the smallest particle. To visualize a relative comparison, think about golf balls. Imagine that one golf ball represents one grain of sand. Compared to a golf ball, a particle of clay would be smaller than a single grain of salt. Clay can be 10,000 times smaller than sand. As all those varying particles join together to form soil, the prevailing component defines it. If sand is the major component you have a sandy soil, if everything balances well you have a loamy soil, and if clay is overwhelming you have a clayey soil.
|Sandy soil in one of my gardens.|
Structure is how those particles of clay, silt, and sand fit together to create soil. An important part of that structure is the little air pockets that exist when two particles join together. Those air pockets are defined as pore space. Think about golf balls again. When you hold six or seven golf balls in your hand you can easily identify the big air spaces between the individual balls. Sandy soil has lots of air, lots of pore space. Now pour salt into your hand. Can you see the air pockets when the sand grains stick together? It's not as easy. Clay soil has very little air space between particles, very little pore space.
|Pore space between soil particles.|
Fertility refers to the capacity of the soil to supply nutrients to plants. Organic matter in the soil, microorganism activity in the soil, and chemical elements present in the soil all influence its fertility. Soils are primarily mineral based and depending which minerals are present, different elements are available to plants as nutrients. The primary nutrients are nitrogen, phosphorus, and potassium; those are the same three nutrients you see available on almost every box or bag of fertilizer. There are a total of 17 elements that plants need for growth. Oxygen, carbon, and hydrogen are supplied through water and air and all of the rest come from the soil.
|16% nitrogen, 16% phosphorus, 16% potassium|
Good soil, or a soil with good tilth, has good texture, good structure, and good fertility. In good soil only about 50 percent of it is solid; the remaining 50 percent is pore space. That pore space is filled with air, water, or a combination of the two. Roots grow through the pore space seeking the air and water.
If a soil has too small pore spaces, like in a high-clay (clayey) soil or in a compacted soil, roots have a hard time growing. Clay soils do well in retaining water and nutrients inside the pore spaces, but at the cost of air. The lack of large pore spaces means the water has a hard time draining out and air has a hard time seeping in. There are plants that do relatively well growing in clay soils, but many can be drowned when the soil gets wet.
If a soil has too large pore spaces, like in a high-sand (sandy) soil, the opposite problem holds. There is an over abundance of air while water and nutrients drain quickly through the pore space. Roots can easily grow through the air space, but without water retention they'll quickly dry out.
The key is to moderate the soil texture and improve the structure. This is most easily accomplished by adding soil amendments. The best soil amendment to improve structure is organic matter. Think of these soil amendments as little moist chunks of sponge as compared to the golf balls and salt grains discussed earlier.
When the large pore spaces between the sand particles are filled with little sponges, the water is soaked up as it drains through, retaining moisture and nutrients. When those same sponges are in clay, the tiny particles join together and surround it and as the organic matter decomposes it leaves behind a little pocket of air, creating new pore space.
Adding organic matter to a poor soil won't correct all of its deficiencies overnight, but it will begin to improve right away. Regular applications of amendments over a long period of time is often necessary to make good soil, and it is worth the effort. Organic material is good for all three soil tilth properties.
I'll discuss amendments and organic matter in another article soon. If you think your soil is too sandy or too clayey to grow a garden don't despair. Help is on the way with the addition of organic matter.
Link to "Developing New Garden Beds"