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Index : Soils and Fertilization
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- 5. When it Comes to Topsoil - Buyer Beware! - Top
- Purchasing top soil, especially in bulk, can be a real crap shoot for the home gardener. There are as many types of topsoil as there are suppliers; and that's a lot!
Normally, most people think of topsoil as being like a rich garden-loam with lots of organic matter. If you were in the prairies of Illinois, where rich organic topsoil goes down three feet deep, that might be the case. But here in Arizona, topsoil could be anything from a heavy Rillito river-bed clay to a sandy, gravelly foothills type material. Neither would be very good for growing plants.
There really is no true definition of topsoil, it all depends on where the topsoil comes from. If the soil is taken from former farm land there is always a chance that the soil will be contaminated with excessive salts or weed killer. Either can prevent the growth of healthy plants. I have seen many instances where people have purchased topsoil only to find that they literally can't grow anything in it!
If you are thinking about buying bulk topsoil, regardless of the supplier, it's a good idea to see the product ahead of time. Visually, the soil should look like soil, that is, not excessively rocky and gravelly or as dense as modeling clay.
Ask the supplier where the soil comes. If the topsoil is a mix, made by the supplier, ask what it's composition is. It's also wise to have the soil tested. The most important testing should be for levels of salt. Salt, even in low concentrations, can make it impossible to grow certain salt-sensitive garden and landscape plants.
Soil salinity is determined by measuring the electrical conductivity of a solution extracted in a lab from a water-saturated soil paste. Salinity is abbreviated as EC with units of decisiemens per meter (dS/m) or millimhos per centimeter (mmhos/cm). Both are equivalent units of measurement and give the same numerical value.
In general, soils with a salinity of 0 to 2 (dS/m) are fine for growing all plants. Levels of 2 to 4 may restrict the growth of salt-sensitive plants. Levels of 4 to 8 will hinder the growth of many plants. Levels of 8 to 16 will prevent all but the truly salt-tolerant plants to grow.
Most fruits will not tolerate salt levels higher than 2 (dS/m). Most vegetables will not accept salt levels higher than 2.5 (dS/m). Flowers like geraniums will be damaged at salt levels of 2, but roses are tolerate of levels up to 4 (dS/m). Most landscape plants will grow successfully with soil salt levels up to 3 (dS/m). But even the highly salt-tolerant bermudagrass cannot take levels above 9 (dS/m). Some topsoils are well above this level!
Testing for salt levels in the soil can be done at private labs here in the Tucson area. For lab locations you can check with the soil supplier or at the Pima County Extension Office by phoning ph. 626-5161.
As for weed killer residue, that may be present in topsoil coming from farms, there is no good way to lab test for these products. Laboratories must know the exact chemical they are searching for and look specifically for that chemical. They cannot just look randomly for anything and everything. However, a sample of the topsoil to be purchased could be acquired and tested to see if seeds will germinate and grow in it. Just a cup or so of soil is all that's needed to grow some fast germinating seeds like radish and ryegrass. Both will germinate and send forth seedlings in less than a week. If after keeping the soil moist for 10 days the seeds don't germinate, or sprout but the seedlings look sick and die, that soil is likely contaminated.
As for bagged topsoil that can be purchased in garden centers, there is less concern over salts and weed killers as the source of these products is consistent. Bagged topsoil is more like potting soil. It is commonly high in peat and can be used for vegetable, flower, and ground cover beds. It often is too heavy and dense to use in containers. The high amount of peat makes for poor drainage and the promotion of root rot.
Finally, if your purchasing topsoil, do it from a reputable company that has been in business for a considerable length of time. Although visually looking at the soil and possibly testing it is still warranted, the chances of the product being unusable is much less.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161. - Updated: November 9, 2003
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