Soil serves many purposes as a medium for plant growth. It works as an anchor, a water holding tank, stores and supplies plant nutrients, influences fertility and that’s just to name a few things. The impact of soil is far reaching and is a key component of successful growing.
We know of its importance, but what is “soil”? To help describe & manage soil there are many properties that should be examined.
Soil is composed of both minerals (derived from the rock under the soil or transported through wind or water) and organic matter (from decomposing plants and animals). The mineral portion of soil is identified by its texture, which is the proportion of sand, silt and clay in the soil.
Most soils contain all three particle sizes. Loam is a specific textural class of soil that contains somewhat equal amounts of sand, silt and clay. Most of our topsoils are loams. Loam can vary from a rather equal mixture of the three textural sizes, to a mixture dominated by one particle size or another. As a gardener, you should inspect loam before purchasing it, because these variations affect your management practices.
Soil composition is categorized into three groups by size – sand, silt, and clay. These three terms refer only to particle size: sand is familiar to most of us, and is the largest textural soil size. Sand grains can be seen with the naked eye or with a hand lens. Clay particles are so small that they can only be seen through an electron microscope. Silt is sized between sand and clay; individual silt particles can be seen through a lower-power microscope. Sand has excellent aeration and drainage. It tills easily and warms up rapidly in spring. However, it erodes easily, and has a low capacity for holding water and nutrients.
Clay has poor aeration and slow drainage. It is difficult to till and warms up slowly in spring. However, it tends to erode less quickly, and has a high capacity for holding water and nutrients. Silt has intermediate characteristics compared to sand and clay. Most soils are a combination of the three. The ratio between the three particles are what give soil its texture. To give you an idea, clay loam texture soil, is almost composed of equal parts of sand, slit, and clay.
The triangle pictured above is from Soil.org and is used so that terms like “clay” or “loam” always have the same meaning. The textures correspond to specific percentages of sand, silt, or clay. Knowing the texture helps to manage the soil.
The way soil particles are arranged is defined as soil structure. Soil particles bind together to form peds or aggregates. The conditions that the peds have endured create a specific shape. They could be granular, blocky, columnar, platy, massive or single-grained.
Soil mineralogy is the primary influence on its color. By taking note of the soils color we can readily identify its composition. The color of the soil is also valuable because it can be used as an identifier of soil behavior. We know that a brightly colored soil will drain well and that one with patterns of gray, red and yellow will tend to be soggy and wet.
There are three elements essential to plant growth: carbon, oxygen and hydrogen. These elements are provided through air and water. The other elements that play a role in plant growth are typically referenced as plant nutrients and are provided by the soil or through fertilizers. Plant nutrients whether macro or micro are brought to the plant through the root system.
The vast majority of soils have residual nutrients. You can assess the amount of residual nutrients in your soil with testing. If you are fertilizing without the results of a soil test you may very well be exacerbating the problem you’re trying to eradicate. It also worth noting that nutrients are often present in sufficient supply but are unavailable due to pH which is too high or low. Thankfully, these issues can also be identified with soil testing and are resolvable.