Influence of Laboratory Soil Pulverization and Historic Limestone Quality On Soil pH Measurements.

The importance of limestone quality has historically been directed to neutralization efficiency following application to soil while the larger particles of calcium carbonate have been considered inert materials.  The lack of any limestone law, regulation, or limestone quarry survey in Texas has resulted in limited producer knowledge toward limestone quality and differences between limestone or other liming sources.  As a result, many producers have historically applied poor quality limestone that was dominated by coarser particles larger than 2 mm in diameter.  These larger particles have been considered too low in surface area to provide any significant acid neutralization capacity, however these particles maybe creating significant under-estimation of the true soil pH and acidic growing conditions.  An increasing number of differences in water pH measurements determined by different soil testing laboratories, on field split samples, suggested that differences in soil pulverization methods between these sample laboratories might be the culprit.  A replicated laboratory incubation study was undertaken to address how soil pulverization methods and intensity impacted water pH of acid soils incubated with coarse limestone particles.  Soil pulverization methods included hand sieving of the soil, use of two different hammer-mill style pulverizes, a disk mill set at 1 and 2 mm openings, and direct grinding of the soil with a ring and puck mill.  Following soil pulverization, the soil water pH of each sample was determined using a standard 2:1 DI water:soil slurry using a combination pH electrode.   All pulverization methods, with the exception of hand sieving, impacted the soil pH, however delta difference in soil pH from the zero limestone addition control and hand sieved method was directly proportional to the aggressiveness of the pulverization method and the soil texture.   In each case, newly milled limestone particles temporarily increased the water pH of the soil slurry, although with time this the water pH dropped, likely due to continued exchange of reserve soil acidity.  The influence of soil texture, time of pH equilibration and pulverization intensity will be presented in detail.  The results of this study indicate that soil testing laboratories must reconsider the impact of pulverization when addressing soils which unknown limestone quality/quantity applications.