Understanding Limestone

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Many Texas soils are acid soils and certainly almost all of east Texas soils are acidic. If you remember your basic chemistry class from high school, acidic soils have a soil pH that is less than 7.0. More technically, pH is a measure of hydrogen ion (H+) concentration in the soil solution.

Soil acidity is caused by various environmental, climatic, and cultural factors. The most common of these factors are the following three factors.

First is the parent material from which the soil is derived. The rocky subsoil gets the entire process started.

Second is leaching by rainfall or irrigation that removes basic elements such as calcium, magnesium, and sodium from the soil profile leaving acidic hydrogen, aluminum, and manganese.

Lastly, cultural practices such as nitrogen fertilization, removal of harvested crops and associated basic elements, and soil erosion, which results in a loss of basic elements.

We care about soil acidity because of the way it affects plants and their growth. Some plants thrive in acidic soils and other cannot tolerate and will prefer a basic soil.

Since plants are more sensitive to acid soil conditions than others, it is important to understand which species are more sensitive to soil acidity. And to reduce soil acidity and help neutralize soil acidity, we add the natural stone limestone in a fine granular or powder form.

Again, speaking the language of chemistry, limestone is primarily composed of calcium carbonate, but many contain small amounts of clay, silt and dolomite. Dolomitic limestones come from natural deposits which contain both calcium and magnesium carbonates. The magnesium content of limestone is especially important where soils are deficient in this essential plant nutrient. If a soil test indicates low magnesium, dolomitic limestone can be used to correct both the nutrient deficiency and pH.

To be clear, the addition of limestone is done primarily to reduce acidity but may also provide some nutrients!

The value of limestone is in its ability to neutralize soil acidity. It’s a little-known fact that all limestone is not created equal. Limestone’s properties differ considerably, and these differences influence the limestone’s ability to neutralize soil acidity.

Its effectiveness depends on the purity of the liming material and how finely it is ground. Pure limestone has a calcium carbonate equivalence (CCE), or neutralizing value, of 100%. When you purchase any limestone product, you should always look for the CCE.

All other liming materials are compared with this (100%) standard. The CCE of commercial limestone products should be available through the vendor. The bulk agricultural limestones that we’ve used in hay meadows and pastures for years in Angelina County was somewhere around 65% CCE. Recently, the more frequently used “super fine lime” is up closer to 90% CCE.

So, let’s make this practical and applicable. At a pH below 5.5, the concentration of soluble aluminum increases and becomes toxic to plant root growth. Furthermore, optimum nutrient uptake by most crops occurs at a soil pH near 7.0. Honestly, in a perfect soil, pH would be at 6.5.

I hope I’ve convinced you of the need for limestone, but the question remains, “just how much should I apply on my garden, lawn, or pasture?”

I don’t have a clue, but I know how to find out.

For specific recommendations you’ll need a soil test. For a $13 test you’ll find out your pH, the amount of limestone needed (if any), and the availability of nutrients such as nitrogen, phosphorus, and potassium, calcium, magnesium, and sulfur.

The time to take a soil sample is long before you need it. Ideally, several months in advance of crop growth provide time for pH adjustment. Soil pH fluctuates during the year becoming lowest in the fall.

The Texas A&M AgriLife Extension Service in Angelina County will be presenting a seminar entitled “Alternative Summer Forages” on Monday, May 21 at the Angelina County Office. This is an evening program starting at 6:30 pm and should be over by 8 pm. Participants will receive 1 CEU for their pesticide license. Cost is $10.

Call 936.634.6414 ext. 0 or email cw-sims@tamu.edu for more information.

Author

Cary Sims
Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu

Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.
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