Use and Misuse of Gypsum and Lime

Never use gypsum, lime without prior research

Brian Jervis: Master Gardener

Saturday, November 26, 2016 12:00 am

Q: When is it best to apply gypsum and lime to my lawn? D. A. Tulsa

A: You should never use either one without specifically documenting a reason. Gypsum has been used in the past to improve the drainage of clay soils, and lime is used to treat overly acidic soils.

Gypsum is calcium sulfate and was once thought to be able to loosen heavy clay soils. Science-based studies have shown this not to be true.

Clay particles are so small and flat they pack down and present a barrier to water, nutrients and plant roots. Tactics to loosen these soils have been researched thoroughly and gypsum, along with other materials, has been shown not to be useful. The OSU handbook, E-1003, “Oklahoma Homeowner’s Handbook for Soil and Nutrient Management,” explains the origin of the gypsum myth.

Gypsum is appropriate to treat soils with calcium or sulphur deficiency, but this is not likely in our area. Gypsum is also useful to treat “sodic” soils, or soils that have been exposed to excessive amounts of sodium containing salts. These soils need to have the salts removed to be plant friendly. The calcium in gypsum will displace and correct the salt excess.

The ideal approach in dealing with heavy clay soils is to use raised beds. If raised beds are not an option, regularly tilling large amounts of organic material will improve the tilth of clay soils as it does with all other soil types.

The use of lime is useful but should always be based on the pH (measurement of the level of acidity) result of a soil test.

Lime, which is calcium carbonate, will raise the pH in soils, making it less acidic. The pH of soil is a result of several factors, one of which is the amount of rainfall. Rain filtering through soils tends to remove calcium and add hydrogen. Hydrogen in soil increases acidity; calcium reduces it. The more rainfall, the more likely the soil will be acidic.

Often people who are from states east of us will assume liming is needed regularly, as it was often needed there, but that is not the case in most of Oklahoma.

Average yearly rainfall in Oklahoma drops steadily from our eastern border to the panhandle. Because of this, on average, there is less soil acidity from east to west.

Most plants perform best in neutral to slightly acidic soil; this includes most turf grasses. According to the USDA, the optimum pH for fescue and Bermuda is acidic, in the range of 5.5 to 7.0. Our average pH in the Tulsa area, based on more than 1,000 soil test results performed by OSU is 6.8, which is favorable to turfgrass and other plants. So, on average, no lime is needed and if used might be harmful to your grass. A soil test will inform you of a need for lime.

Garden tips

§  Fertilize cool-season grasses like fescue with 1 pound nitrogen per 1000 square feet. This should be the last fertilization for fescue until next spring. Do not fertilize Bermuda or Zoysia until green-up next April.

§  Spring-flowering bulbs like hyacinth, narcissus and tulip, which are sold for “forcing,” can be potted indoors for a colorful winter display.

§  Tulips can still be planted outdoors through this month.