By Trish Grenfell, UC Master Gardeners of Placer County
Q: Many stores carry bagged topsoil but they all seem to vary in color from light brown to ash. How do I know which is a good soil to use for planting?
A: Topsoil is supposed to be the top layer of the soil which has gone through a very long natural process in which the topsoil gets completely saturated with nutrients from grass, leaves, twigs, and organic matter that decomposes on the soil. Some store-bought topsoil might not be this premium type of soil at all. How organic it is really depends on where the company that sells the topsoil gets its topsoil from.
The first thing you should do is read the fine print on the bag and purchase the correct product for your project. There are potting soils for container plantings, planting mixes for transplanting and other “topsoil” blends for topping off landscape beds.
If you see a ripped bag in the pile, check out its contents. Make sure it is uniformly dark in color. Soil with good organic content is dark brown as opposed to light brown. If it is light-colored, it probably has a high amount of sand in the mix. A sniff test will tell you if the soil is rich and full of organic matter. If any unpleasant odor is present, give it a pass. Rub a bit of the contents between your fingers. If it crumbles, it is a good sign. If it stays hard and is difficult to crumble, it could have low organic matter. You can wet some of the dirt and feel if it feels too gritty or too sticky. Ideally, it will feel like an equal combination of each. Try to avoid any bagged soils that are full of bark and/or sand.
Bagged compost typically cost about the same as top soil at big discount lawn and garden centers. Mixing some with the bagged top soil increases the likelihood of achieving an excellent soil mix for your plants. Whichever bagged soil or amendment you choose, be sure to mix it well with the existing soil.
Remember that compost is the single best way to beef up the soil you already have, and you can generate that at home—in surprisingly large quantities.