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Soil pH Gradients near Calcite and Dolomite Particles

Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824

R. W. Blanchar^*

School of Natural Resources, 144 Mumford Hall, Univ. of Missouri, Columbia, MO 65211

* Corresponding author.

ABSTRACT

The quality of liming material is normally judged by its neutralizing power, particle-size distribution, and mineralogy. Historically, three methods have been used to evaluate these factors: (i) observing the effect of a liming material on crop yield, (ii) studying changes in soil pH with time after liming, and (iii) measuring unreacted carbonate. This study used a new method, glass pH microelectrodes, to evaluate the reactivity of limestone in soil. The effect of limestone particle size, mineralogy, and water leaching on the amount of unreacted carbonate and the soil pH around limestone particles was investigated. The reaction rate of calcite and dolomite was proportional to the surface area of the particles. Calcite reacted approximately twice as fast as dolomite. Reproduceable pH gradients exist around limestone particles in soil. In static systems, soil pH varied as a linear function of the reciprocal of distance from the particle surface. These gradients were generally independent of limestone particle size; however, in some cases pH was higher near larger particles. The soil pH next to dolomitic limestone particles was lower than near calcitic particles. Soil pH gradients around leached limestone particles were independent of the direction of water flow. The amount of limestone reacted in leached soil systems could be predicted from the number of particles and the shape of pH gradients around them in unleached soil systems.

Contribution from the Missouri Agric. Exp. Stn. Journal series no. 11412.

Received for publication June 3, 1991.