The dirt on Buying Topsoil: Soil Science and Preparation

by Don Boekelheide

Simplifying, your options boil down to topsoils vs. soil 

(or soil-less) blends.  Avoid using pure compost as your sole planting material.  And never confuse mulch, such as bark chips, with a planting medium. 

Technically, the term “topsoil” should be used only for natural topsoil, even if marketers routinely violate this rule.  Commercial “topsoil” might indeed be rich soil nobly rescued from a farmer’s field destined for condos, but caveat emptor—in a recent case locally, the excavated rubble from under a new bank tower, mixed with biosolids from a sewer plant, was being sold as “topsoil.” 

To be reasonably sure of what you are getting, buy only from a trusted company with a good reputation.  “Topsoil” is best purchased in bulk.  Plastic bags of “topsoil” generally range from disappointing to disgusting.   

When shopping for container and planter soil keep these points in mind:

•  For large “raised bed” planters, you’ll need a lot of mix, so be ready for potential expense and transportation issues.  A vegetable planter 1 meter wide by 2.4 meters long by 30 cm tall (about 3 feet x 8 feet x 1 foot) holds 720 liters of soil mix (24 cubic feet).  

•  Always work with a firm you know and trust.  Ask other gardeners, and share information.  

•  For vegetables, be ready to add compost and fertilizer, particularly when working with soil-less blends that contain no fertilizer.  

•  Since you are creating a closed, human-controlled system, you bear responsibility for all the inputs that nature provides in natural soils.     Know that your mix will naturally shrink over time as it packs down and as organic matter is lost to decomposition.  For a while, you can add additional mix and compost as needed, but at intervals you will want to replace old soil mix with a fresh batch, a necessary step whenever a disease or pest gets established, salts build up, or yields drop.  

•  You may be able to recycle small amounts of soil for containers by pasteurizing it in the oven (though it stinks up the house), but many gardeners simply recycle old potting soil through the compost pile and eventually into parts of the garden that have not been boxed in any way.  

• In short, containers and planters have been used since ancient times and have advantages and disadvantages.  Many gardeners, including me, use and enjoy these techniques.  However, to be successful, you need to pay scrupulous attention to the soil (or whatever it is) you put in the box.  

The alternative to topsoil is a commercial planting blend, widely available both in bulk and in bags.  These products are generally made from a percentage of soil (though see the next paragraph) combined with materials such as municipal compost (from yard waste, or in a few cases biosolids), bark, sand, lime (to correct pH), and occasionally more expensive products such as PermaTill.  Again, quality and price vary widely, though you can usually find information on a given blend’s percentage of ingredients, pH, and so on. 

You can also find soil-less blends (or “potting soil” or “potting blend”) made from peat, bark, perlite, vermiculite and other materials.  These are widely available, particularly in bags, but sometimes in bulk.  Better quality products offer formulations for specific purposes, such as for growing seedlings or indoor tropicals.   

Although it is certainly not impossible to mix your own potting or planter soil, it’s rarely done these days on a home scale.  ​© Don Boekelheide 2010 all rights reserved ​

In the series of three articles that accompany this soil science topic we’ll take a closer look at active zone soil preparation: the nuts and bolts of digging a vegetable bed, doing transplant preparation and dealing with buying soil.  (Go to www.mastergardenersmecklenburg.org, Garden Articles, and find the articles in this Soil Science & Preparation series.) ​© Don Beokelheide 2009 all rights reserved