(70) Improved Reactivity Indices for Horticultural Limes

Abstract

The objective was to develop indices to describe reactivity of different lime particle size fractions with respect to pH change in horticultural substrates. Particle size efficiency (PSE) was calibrated from pH responses for separated six lime particle size fractions (>850, 850 to 250, 250 to 150, 150 to 75, 75 to 45, and <45 μm) from three calcitic limes, and seven dolomitic limes, based on their increase in substrate pH relative to reagent grade CaCO3 when mixed in a sphagnum peat substrate at 5 g CaCO3 equivalents per liter of peat. The fineness factor (FF) was calculated for a liming material by summing the percentages by weight in each of the six size fractions multiplied by the appropriate PSE. The effective calcium carbonate equivalence (ECC) of a limestone was the product of the FF and the acid neutralizing value (NV) in CaCO3 equivalents. Reliability of the parameters for FF and ECC were then validated in two experiments, using 29 unscreened carbonate and hydrated lime sources, including the 10 calibration limes. In one experiment, 1 L of peat was blended at 5 g of lime (i.e., not corrected for differences in NV between limes). In the second experiment, 5 g CaCO3 equivalents for each lime, corrected for NV, were blended with 1 L of peat (a different peat source), using the same 29 lime sources. Both FF and ECC were positively correlated with the corresponding substrate-pH changes, with P < 0.001 and r2 from 0.87 to 0.93. This calibration of PSE, FF, and ECC can improve limestone selection and application rate for the short term response and fine limestone sources used in horticulture.