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 really not provide the expected.

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. Some recommendations are to spread gypsum over hard, compacted soil in a lawn to loosen it up. This is supposed to happen because the gypsum improves the structure of the compacted clay soil.

With the exception of some of our coastal areas where clay soils can be high in sodium, adding gypsum as a soil-softening amendment is generally not beneficial and will not loosen the soil, according to LSU AgCenter soil scientist J Stevens. So, in the overwhelming majority of soils Louisianans 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 calcium is low, the soil acidity or pH is also low. In that instance, lime (calcium carbonate) can be added to the soil to raise the calcium level and raise the pH to a more desirable level. If the soil is also low in magnesium, dolomitic lime should be used.

But some situations arise where the soil is low in calcium but the pH of the soil is already high enough. Adding lime would make the pH unacceptably high. In this case, 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 extension 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 (typical in Louisiana except right on the coast) is a waste of money and natural resources, and can even have negative effects. For instance, excessive calcium in the soil can tie up phosphorus.

Add some sand

Heavy clay soils 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 in clay soils common there. Because 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 in a bed by increasing the amount of sand. But this must be done properly to be effective.

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 soil, it must be at least 50 percent of the soil. So to change the upper 6 inches of soil, 6 inches of sand must be worked into it. This can be done 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 over the area and working that in.

Along with the sand, you should also add organic matter, of course. 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. Sand, on the other hand, will permanently change the soil texture.

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 of blended topsoil or garden soil is not the solution. When rains come, water 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 is not 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.

These beds at the LSU AgCenter Hammond Research Station were created by piling garden soil on top of the existing soil and spreading it out to a depth of about 12 inches. Photo by Dan Gill

Mounding soil to a depth of 12 inches or more over hard clay soils will provide drainage for attractive garden beds. Photo by Dan Gill