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Correcting Soil pH with Calcitic and Dolomitic Limestone

By Achille Correggia, B.Sc. (Agri.) Agronomy, CCA-ON

Soil acidity can be considered a variable that affects nearly all soil properties - chemical, physical, and biological. Expressed as soil pH, this property is a major factor in determining which trees, shrubs, or turf species will dominate the landscape. Soil pH changes can occur naturally through the weathering of rocks and the organic matter decay process, or it can occur artificially through the application of fertilizer and/or limestone. In nature, soil acidification is the result of a complex series of reactions as organic matter decomposes into carbonic acid, strong organic acids or strong inorganic acids. Fertilizers containing ammonium can also cause soil acidification.

Knowing which soils need liming and how much lime to apply is dependent on both the soil pH value and the buffer pH value, both are available on most soil testing reports. The soil pH value determines which soils need to be limed by measuring the concentration of hydrogen ions in the soil solution, the higher the hydrogen ion concentration the lower the soil pH. However, it is the buffer pH value (the concentration of hydrogen ions on the soil particle) that determines the amount of lime required to reach your desired soil pH level. Most turf species grow well in soils with a pH between 5.5 and 7.5.

If your soil tests are indicating low pH levels (acidic soils) one corrective measure is the addition of a liming material. Liming materials made of crushed limestone are generally divided into two groups, calcitic or dolomitic, based on their content of calcium and magnesium. Calcitic limestone is almost pure calcium carbonate (calcite) containing 40% calcium, while pure dolomite (dolomitic limestone) contains 21.7% calcium and 13.1% magnesium. On acidic soils dolomitic limestone provides a good source of magnesium, in addition to its neutralizing ability.

The quality of both forms of limestone is determined by the neutralizing value and fineness rating. The neutralizing value indicates the amount of acid that a liming material will neutralize when it is completely dissolved. This value is expressed as a percentage of the neutralizing value of pure calcium carbonate, which has a value of 100. A limestone that neutralizes 90% of an acid when compared to pure calcium carbonate (calcitic limestone) has a neutralizing value of 90. The second factor, fineness rating indicates the reaction speed of the limestone once applied to an acidic soil. Limestone that maximizes surface area (having a low SGN) will provide an improved rate of dissolution. When limestone particles become too large (above 1.65 mm) they neutralize soil acidity so slowly that it is of little value to the turf manager.

These two factors, combined with soil test results, assist turf managers in determining the amount of lime that needs to be applied to correct any acidity problems. When the neutralizing value is multiplied by the fineness rating and then divided by 100 the Agricultural Index (A.I.) has been calculated.

The agricultural index is an indicator of limestone quality which combines the neutralizing value and fineness rating into a single factor. The average agricultural index for Ontario Limestone is 75, therefore limestone requirements in Ontario are based on an A.I. of 75. If a turf manager is using a limestone material that has an A.I. that’s above or below 75 it will result in less or more limestone being required to reach the desired soil pH.

Example: A soil testing report indicates that a golf course superintendent’s 16th fairway has a soil pH of 5.5 and a buffer pH of 6.0. Based on a target soil pH of 6.0, the recommended limestone requirement for this soil would be 6 tonnes/ha of a limestone with an A.I of 75. The limestone material that the superintendent has purchased has an A.I. of 90. The below calculation indicates the amount of liming material required to reach the desired soil pH.

How about gypsum?

Gypsum (calcium sulphate) is used as a source of calcium or sulphur and as a soil conditioner; it will not affect soil pH. When used as a soil conditioner, it’s applied to rectify sodic soils which are typically a problem in the Prairies. Sodic soils contain levels of sodium that are sufficient to interfere with the growth of turf. Soil base saturations with a sodium percentage that is greater than 0.5% could have a detrimental affect on both germination and root growth.

The removal of sodium cations from the soil exchange site is done most effectively by the addition of calcium. Providing calcium in the form of gypsum is the most practical way to bring about this exchange. When gypsum is added to a sodic soil it breaks apart into calcium and sulphate ions when dissolved in water. The calcium displaces the sodium on the soil exchange site; the displaced sodium will react with the sulphate ion in the soil solution forming sodium sulphate which is leachable from the soil.

When deciding on a liming material, a turf manager can choose between a liming material that only corrects soil pH (calcitic limestone) or a liming material (dolomitic limestone) that can correct both soil pH and magnesium levels. This decision depends on whether his/her soils have a magnesium deficiency. Once a decision has been made between either calcitic or dolomitic limestone, the material should have an agricultural index that is seventy five or above to provide reliable results. Liming material with an agricultural index below seventy five typically will result in more required tonnage of lime, which could take longer to neutralize acidic soils, costing the turf manager both more time and money.

If high sodium levels (sodic soil) are a concern rather than pH, gypsum should be used to correct this soil problem due to its ability to remove sodium ions and not affect soil pH.

Reference:

Ministry of Agriculture, Food and Rural Affairs Publication 611 (1998). Soil Fertility Handbook. Toronto, Ontario, Canada: Queen’s Printer for Ontario.
Brady, N.C., Weil, R.R. (1996). The Nature and Properties of Soils. (11th Edition). Upper Saddle River, New Jersey, USA: Prentice Hall.
Ministry of Agriculture, Food & Rural Affairs Publication 384. (2000). Turf Management. Toronto, Ontario, Canada: Queen’s Printer for Ontario

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