The simple answer, though not quite completely true, is that topsoil is the top layer of the soil which has gone through a years-long natural process in which the topsoil gets completely saturated with nutrients creating a soil which is so healthy for plants that people are willing to pay for it. The topsoil soaks up nutrients from grass, leaves, tress, twigs, and many other things that decompose on the soil. When you purchase bags of topsoil, you are paying for this substance which is supposedly loaded with nutrients. This is not always the case, however, since real topsoil is a limited resource. So what is that stuff you buy in bags that you think is topsoil? Here is some information to help you determine that, and even test the topsoil you buy to ensure that it is indeed topsoil.
Soil is differentiated into different layers. To keep things simple, we will call the surface layer topsoil, and the other layers we will call subsoil. Topsoil is usually darker in color than the subsoils. This dark color can be attributed to organic matter, which is also responsible for other desirable physical attributes including its ease of tillage, texture, and water retention.
Topsoil tends to have lower levels of clay, salt, and lime than subsoils. These components contribute to an end product which cannot sustain life properly. Subsoils generally have higher levels of these components which explains why they make for a poor substitute for real Topsoil. Depending on the region, topsoil is usually no more than two to eight inches thick. Many topsoil vendors do not sell real topsoil and actually sell these subsoils in place of real topsoil. Many of these subsoils are acceptable replacements for real topsoil if certain criteria are established. As a consumer it is important to know what you are buying and if it will be adequate for whatever you are trying to plant.
There are five main chemical properties that can have an influence on the quality of topsoil. They are soluble salts, texture, organic matter, pH, and sodium adsorption rate. It is vital that topsoil meets certain guidelines regarding these ingredients, or the topsoil delivery will be completely worthless with regards to sustaining life. Each of these five properties must be within certain levels in order to be suitable for planting. If you discover that any of these components are too high or too low, it is imperative to replace the topsoil or, when possible, fix it up and make it suitable for planting. If you plant in topsoil that is not suitable for planting, you are just wasting your time. It is well worth the time and the small, one-time expense, when applicable, to avoid wasting much more time and money in the future when you have to replace all of your topsoil and all of your dead plants.
Getting the proper tests done on topsoil would cost no more than thirty dollars or so, and is well worth the investment. This way you will know that you have good soil in which to plant. If, for whatever reason, getting these chemical tests done is not possible, there are a number of things to feel for in addition to things to look for in order to test the quality of the topsoil. This is obviously not as good as actually having the chemical tests performed. This is simply because the pH level, for example, may be too high or too low, but you cannot tell that it is simply by looking and/or touching the topsoil. However, if chemical testing is impossible, these physical tests will at the very least be able to help you determine if there or are not certain problems.
If there are light colored deposits on the surface of the soil, this is usually a good indicator that there is too much salt in the soil. You can also check for signs of salt crusting or crystals on the surface of the topsoil. This topsoil should only be bought if the chemical tests determine that pH levels and salt levels are not too high.
