Topsoil and potting soil both offer good growing support for plants, but local soil conditions will determine which is best for vegetables in a specific area. The USDA County Extension program publishes information and helps gardeners obtain soil testing to understand the benefits and challenges of existing garden soil. Learn more about your soil to produce healthy vegetables in your growing area.
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Soil Vs. Mix
Before choosing between potting soil and topsoil, it is important to distinguish between potting soil and potting mix. Ideal for seed-starting and frequently recommended for container gardening, potting mix contains no soil. The mixture, which can include peat moss, perlite and sand, is designed to allow fragile seedling roots to expand without encountering obstacles in the growing medium. It cannot provide enough nutrition to sustain growing plants and must be nutritionally supplemented if used for container gardening. Potting soil is sometimes recommended for raised-bed gardening, like vegetable growing, but not for starting seeds.
Potting soil contains soil. It can also contain peat moss, perlite, organic matter, fertiliser and other nutrients. If this seems confusing, it is because there are no regulations, federal or state, governing exactly what a potting soil mixture must contain. Working with your County Extension agent to test your soil, combined with close label reading, will help you determine what potting soil mixture to add to your garden soil.
Topsoil content is an even larger challenge than potting soil. Purchased at nurseries, bagged top soil is likely to be a consistent product, governed by the company's intent to obtain repeat customers. Commercial top soil is likely to meet general standards, able to sustain lawn or garden in the company of regional soil amendments required for more than minimal results. Non-commercial top soil can be whatever its purveyor says it is -- or not. Loaded with clay, full of rocks, bristling with wood chips and stems, non-commercial topsoil may even be contaminated with substances that do not support plant growth. Results can be disappointing without being illegal. Close examination is the customer's best protection. Reputable vendors will have little or no problem with your having soil samples tested, which can at least give you clues to chemical and nutrient content.
Potting Soil Vs. Topsoil
Choosing the best soil for growing vegetables in your area means finding out about the tilth, drainage and nutritive capacities of your existing soil plus those of the soil you plan to add. One way to improve your cultivation is to test garden soil both before and after you have added more soil. This will help guide you on quantities still needed and other amendments that will strengthen your soil even further. In general, potting soil is light enough to improve drainage in soggy soil and topsoil can add some substance to dusty, sandy or easily eroded soil.
When Neither Will Do
County Extension agents in the Miami-Dade area of Florida are blunt in the extreme: Provide your own soil because it's almost impossible to dig into their rocky, alkaline version. Further, agents point out, diggable soil may contain nematodes and plant diseases that will hamper growth. One strategy suggested in Extension materials is complete soil replacement. Raised beds begin with a heavy local soil barrier and end with pure compost as a growing medium. Raised beds extend the concept of container gardening to make room for larger-than-container crops.
A Regulated Alternative
One of the most encouraging elements of organic gardening is the monitoring of products used for growing. The National Sustainable Agriculture Information Service provides suggestions for obtaining organically acceptable soils and soil amendments, along with recipes for making your own. While you may not regard your garden as organic, following recommendations can improve the quality of your garden soil.
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- The Daily Freeman; BEYFUSS: Tips for Buying the Right Topsoil; Bob Beyfuss; April 2011