For Release On Or After 05/21/10

By Dan Gill
LSU AgCenter Horticulturist

A number of misconceptions surround how we prepare soils for planting. Sometimes the advice may sound reasonable, but it may not provide the benefits you expect.

Gypsum’s not the answer

I have often seen and heard recommendations to apply gypsum – calcium sulfate – to heavy clay soils to “loosen” them and make them easier to work. Sometimes recommendations are to spread gypsum over hard, compacted soil in a lawn to loosen it. This is supposed to be accomplished by the gypsum improving the structure of the compacted clay soil.

According to Tom Koske and J. Stevens with the LSU AgCenter, however, with the exception of the arid western United States and, perhaps, some of our coastal areas where clay soils can be high in sodium, adding gypsum as a soil-softening amendment is not generally beneficial and will not loosen the soil. So in the overwhelming majority of soils Louisianans gardeners garden in, it is pointless to add gypsum unless you need to increase soil calcium.

Calcium is deficient in soils in many parts of Louisiana. But generally when soil calcium is low, the pH is also low. In this instance, lime – calcium carbonate – can be added to the soil to raise calcium level and raise the pH to a more desirable level. If the soil is also low in magnesium, you should use dolomitic lime.

Some situations arise where the soil is low in calcium but the pH of the soil is high enough. In those cases, adding lime would make the pH unacceptably high, and gypsum is the perfect solution. Gypsum is calcium sulfate and has a neutral reaction in the soil. By adding gypsum, you can raise the calcium level without raising the pH. How do you know if you soil needs calcium and what the pH is? Contact your local LSU AgCenter office and request a soil test kit. A standard soil test costs $10.

So adding gypsum to compacted sandy soils or to clay soils low in sodium, which is typical in Louisiana except right on the coast, is a waste of money and natural resources and can even have negative impacts. For instance, excessive calcium in the soil can tie up phosphorus, an important nutrient.

Add some sand

Heavy, clay soils that are difficult to work and garden in are not uncommon in Louisiana. When I moved to Prairieville just south of Baton Rouge, I was shocked at how much more difficult it was to garden. Since I knew gypsum wouldn’t help, I decided to improve the soil with sand.

The feeder roots of plants such as shrubs, vegetables and flowers are in the upper 6 to 8 inches of the soil, so that’s the critical zone to change by increasing the amount of sand. But this must be done properly to work.

The important thing to remember when adding sand to a heavy clay soil is that it takes a lot. An inch or two spread over the surface and worked in will simply not do the job. For sand to substantially change the nature of the clay, it must be at least 50 percent of the soil. So changing the upper 6 inches of soil means 6 inches of sand must be worked into it. This would be accomplished by tilling the soil at least 6 inches deep, spreading 3 inches of sand over the area, working that in, and then spreading another 3 inches of sand and working that in.

Along with the sand, you’ll also add organic matter – composted, finely ground pine bark is ideal for heavier soils. The addition of organic matter alone will improve and loosen clay soils. But organic matter decomposes, and the benefits are reduced in a year or less. The sand will permanently change the texture of the soil.

Don’t make a “bathtub”

Another solution often used to deal with bad soil involves digging out soil a foot or more deep and replacing it. In new subdivisions, the soil is often truly terrible. Contractors, who are more interested in providing a suitable base for the house than the landscape, often fill the lot with dense, heavy subsoil. Landscape plants understandably will not thrive when planted into this environment.

But digging out the soil and replacing it with a loose, high-organic matter soil mix – blended topsoil or garden soil – is not the solution. When it rains, rainwater will flow across the heavy soil and penetrate down into the loose soil in the bed. When the water hits the heavy clay bottom and sides, the bed will fill up like a bathtub. Plant roots can literally drown in these circumstances – so this isn’t the best solution.

Instead, go up. Build a raised bed about 12 inches high on top of the existing soil. The raised nature of the bed will provide for excellent drainage, and a 12-inch depth will allow for strong root systems.

Rick Bogren