Potting Soil, Topsoil, Compost, Seed Starting Potting Mix – What’s the difference?

You find yourself standing in the store, staring at dozens of different bags of soil trying to figure exactly what you need to start a garden – what’s the difference between all of these “soils”? Typically, you will find different types of soils or soil amendments, such as mixes, topsoil, and composts. There are many subtle differences in soil mixes such as the amount of sand, clay, etc., and they are often designed to cater to different plant needs like those developed for flowers and those for cactus. Mixes are normally specific to the plants and/or use that you have in mind. Topsoil is simply the top couple of feet of soil excavated from a particular location. It should be dark and rich in organic matter, for best results. Topsoil tends to be dense, and can have debris in it like rocks, twigs, etc. Because it is so dense, it can become waterlogged. Topsoil is usually best in areas with little to no actual soil – especially areas where you want to plant landscape plants in locations with a lot of fill or marle. Compost may have a little soil with a small amount of organic matter that has been augmented with additional organic matter (like food scrapes, mulch, grass clippings, etc.) left to decompose for long periods of time. Compost can be made at home and is usually used to amend, or improve soil, by mixing it in to your existing planting area. Many folks mistake compost for soil or fertilizer. Compost is an organic matter amendment (which is great for plants) but it is not a nutrient fertilizer. You’ll still need to fertilize your plants even if you use compost. (For more information on how to make compost at home, visit http://edis.ifas.ufl.edu/ep323) Potting mix (usually soil-less) is typically a mixture of compost, peat moss, and perlite. This provides a light weight soil that doesn’t get waterlogged yet but retains moisture well. Potting mix is better for most container gardens, as waterlogged soil can lead to root rot and other problems. Potting soils and mixes are your best bet for most potted plants. A seed starting potting mix is an even lighter weight and airy soil with higher percentages of perlite and/or vermiculite. It is made this way so that the mix has a lot of air spaces, so when seeds germinate they can easily develop roots. For more information on how to make your own potting mix, visit http://solutionsforyourlife.ufl.edu/lawn-and-garden/homemade-potting-mix/. There are many types of soils and potting mixes to choose from. Consider experimenting with your own mixes to suit your needs or give us a call for suggestions.
Eden Santiago-Gomez
Community Gardens Program Assistant
UF/IFAS Pasco Extension

Whitney Elmore, Ph.D.
County Extension Director & Urban Horticulture Agent
UF/IFAS Pasco Extension

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