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Soil pH By Mario Villarino

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As temperatures start warming up, ranchers and farmers start planning crops and crop cycles for the summer season. A very important part of the planning process should include soil testing to determine current soil conditions before planting. According to Dr. Redmon, Texas A&M AgriLife Soil Scientist, various environmental, climatic, and cultural factors can affect formation of acid soils. The most common factors are: Parent material: Soils formed from acidic rocks have a lower pH than those formed from basic or alkaline parent material. Leaching: Rainfall/irrigation leaches basic elements such as calcium, magnesium and sodium from the soil profile, leaving the acidic elements hydrogen, aluminum and manganese. Cultural: Various management practices can affect the formation of acid soils, including: Soil erosion can cause the loss of basic elements thus increasing soil acidity. Removal of harvested crops depletes basic elements from the soil. Nitrogen fertilization can lead to the formation of acid soils when ammonium is converted to nitrate by soil microbes releasing hydrogen ions. Anhydrous ammonia, urea, and ammonium nitrate each produce about one-third as much acidity as ammonium sulfate. Soil pH is a measure of hydrogen ion activity in the soil solution. However, a buffer-pH test should be used to more accurately predict the limestone needed to raise soil pH to a desired level. The soil pH scale extends from 0 to 14; thus soil pH in the range of 6.6 to 7.3 is rated neutral. Soils are considered slightly acid between pH 6.5 – 6.1, moderately acid between 6.0 – 5.5, strongly acid between 5.5 – 5.1, very strongly acid between 5.0 – 4.5, and extremely acid below pH 4.4. Soils with pH values above 7.4 are rated as alkaline. Although a decrease in soil pH from 6.0 to 5.0 does not appear significant, there is a 10-fold increase in soil acidity for every whole unit change in soil pH.Proper detection of soil pH can be achieved using a soil test. The most important benefit of liming acid soils is a reduction of the potentially toxic elements hydrogen, aluminum and manganese. Hydrogen ions only become toxic to plants in extremely acid soils (pH<4.0) and at very low calcium levels. As pH drops below 5.5, the concentration of soluble aluminum increases and becomes toxic to plant root growth when it exceeds 1.0 part per million (ppm). Below pH 5.2, the concentration of manganese can become toxic. Optimum nutrient uptake by most crops occurs at a soil pH near 7.0. The availability of fertilizer nutrients such as nitrogen, phosphorus and potassium generally is reduced as soil pH decreases. Phosphorus is particularly sensitive to pH and can become a limiting nutrient in strongly acid soils. Thus, reduced fertilizer use efficiency and crop performance occurs when

soil acidity is not controlled. Soil pH also affects the types, concentrations and activities of soil microorganisms. As pH drops below 5.5, the population of soil microbes changes and is reduced due to aluminum and manganese toxicity and lower nutrient availability. Naturally occurring soils in Hopkins County can have a pH below 5.0. An effective method to correct acid soil pH is using agricultural lime. Agricultural lime is a product of limestone quarries. All limestones are not the same and may react more or less efficiently based on the particle size and neutralizing value of the limestone material. Smaller particles have more surface area, react more rapidly to change soil pH, and thus have a higher efficiency rating (ER). Particles larger than 0.080 inches in diameter (about the size of #9 shotgun shot) do not react with the soil to effectively change pH. As particle size decreases, the rate of reaction increases and reaches 100% for particles less than 1/1,000 inch in diameter. The ability of a limestone to neutralize soil acidity also depends upon its calcium carbonate equivalence (CCE) or neutralizing value, which is expressed as a percentage. Pure calcium carbonate is the standard and has a CCE of 100%. All other liming materials are compared with this standard. Dolomitic limestones

contain both calcium and magnesium carbonates. If a soil test indicates low magnesium, dolomitic limestone can be used to correct both the nutrient deficiency and pH. Limestone can be applied at any time, however, several factors should be considered when planning an application. Soil moisture is critical to the reaction of limestone, thus rainfall patterns in the area should be used as a guide. Soils should be sufficiently firm to support heavy equipment and minimize compaction. If subsoil pH is low, a long period will be required for the limestone to effect a change in the soil pH with depth. In addition, coarse limestone reacts more slowly and, as with all limestones, is most effective when incorporated into the soil. When a more rapid and longer lasting pH adjustment is needed, the use of finely ground, high ECCE limestone is advisable. Dry bulk limestone is typically applied using fertilizer spreader trucks. A moisture content of 7 to 9% in fine limestone is needed to minimize dust and achieve a uniform spreading pattern.

For more information on this or any other agricultural topic please contact the Hopkins County Extension Office at 903-885-3443 or email me at [email protected].


Mario Villarino DVM, Ph.D.
Hopkins County Extension Agent for Ag and NR
1200B Houston Street
Sulphur Springs, Texas 75482

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