MANAGING ALKALINE CLAY SOILS

The Texas blackland prairie is a tornado-shaped area that sweeps down through the heart of Texas, beginning as a broad band just south of  the Red River and tapering to a point at the city of San Antonio.  Its soils are a black, alkaline, calcium-saturated, smectite clay, sometimes referred to as "gumbo," which developed from the Austin chalk formation and the volcanic ash that adulterated those carbonates as they were deposited in the shallow Cretaceous-age seas.  From a gardening standpoint, the overall area also includes the very similar clay soils that developed from the Taylor marls and shales along the eastern flank of the blackland prairie and the Eagle Ford shales along the western side.

According to the U. S. Department of Agrculture, the alkaline clay soils within this broad prairie were originally some of the world’s finest and most fertile agricultural soils. Why aren’t they so today? Because we ruined them with single crop farming, neglect, grading, compaction, erosion, and a failure to maintain a sufficient level of organic matter.

Soil Preparation: Restoring and maintaining that excellent garden soil is not at all difficult.  All you need do is to stir up the native soil and establish and maintain an appropriate level of organic matter. That is to say, to prepare our native clay soils for planting -- be it for a lawn, a vegetable garden, or a flower or shrub bed -- till or dig in a layer of finished compost to at least the rooting depth of your plants, striving for a mixture of one part compost to no more than 3 or 4 parts of native soil. (If you’re planting acid-loving plants such as azaleas, you’ll need almost 100 percent compost.) And then, primarily by mulching with additional compost, add at least another 10 pounds of organic matter per 100 square feet every year to maintain an adequate level.

If you make your own compost, then use that.  But, if you must purchase compost, nothing works better with these alkaline soils than cotton burr (cotton bole) compost.

And be patient.  Don't expect overnight success.  Soil properties result from and are modified by a combination of on-going physical, chemical, and biological processes -- all influenced by temperature, moisture, and both the native and added raw materials.  Depending on its original state, you may be able to develop an "ideal or perfect" soil in six months time.  Or it may take a year, two years, or even longer.  But, if you keep adding that organic matter, it will get there.

Organic matter will also provide most of the nutrients your plants need, and the limestone, shale, and clay from which the soil formed will provide the remainder, with very little supplemental feeding required for most plants.

In that regard, after allowing the newly modified soil to mellow for 4 to 6 months, have a soil analysis made. Then you can add any missing nutrients.  And you can lower the pH, if necessary, into the range of 6.5 to 7.0 by adding horticultural sulfur at the rate of 1 pound per 100 square feet every 6 months.  Then, after a year or so, you'll find this is no longer necessary because a continuous 2 or 3% concentration of colloidal organic matter is maintaining the soil pH within the proper range.

In any area with calcium-saturated clay, don't add any mineral or organic fertilizers that are high in calcium or phosphorous until you have the results of a proper soil analysis.  And that includes such products as bone meal, soft rock phosphate, mushroom compost, chicken manure, etc.  Most soils in the Texas blackland prairie and its vicinity already have too much calcium because of the underlying limestones.   And most urban soils have too much phosphorous from previously applied lawn fertilizers.  (Nitrogen and potassium leach out of the soil, but phosphorous doesn't.)

Checking the Drainage: If there's any question about whether you need raised beds for proper drainage, make a percolation test before you make any soil modifications.  Simply dig a hole about the size and shape of what used to be a 2-pound coffee can.  Fill the hole with water and let it drain completely.   Then fill it again and measure how long it takes for the water to drain out this second time.  If it drains in an hour or less, you have excellent drainage.  If it requires 1 to 4 hours, the drainage is average to poor, but acceptable.  If it takes more than 4 hours, you need a raised bed -- or a French drain or some other type of artificial drainage system.

Manures: Don't add manure of any kind (or organic fertilizers containing appreciable manure) to any clay-based soil more often than once every three or four years.  Use of manure more often than that can cause a buildup of harmful salts.  Sandy soils don't have this problem because the salts are leached out by rain and by irrigation water.

Sand: Never, under any circumstances, add any type of sand to a clay-based soil—unless you're prepared to go all the way. And that means tilling in at least 600 to 1,000 pounds per 100 square feet. This is the minimum amount required to establish a sand content of 10 to 15 percent to a depth of only 6 to 8 inches. Without copious amounts of organic matter, anything less than that risks turning the soil into adobe.