Effects of Biological Nitrogen Reactions on Soil Lime Requirement Determined by Incubation

Abstract

Incubation of soil with CaCO 3 is generally considered a reliable method to determine the lime requirement (LR) of acid soils. Because of their considered reliability, these incubations are often used to calibrate buffer methods; however, one study reported that the use of room temperature incubation with CaCO 3 overestimated the actual LR determined by field testing. The objective of this study was to compare the pH change following CaCO 3 incubations for 60 d with those following 3‐d incubations with Ca(OH) 2 and to determine the possible role of soil N reactions causing any differences in pH change. Seventeen soils were incubated with either CaCO 3 for 60 d at approximately 85% field capacity or for 3 d with an equivalent amount of Ca(OH) 2 solution plus water to maintain a 1:1 soil/solution ratio. Both were incubated at room temperature (23 ± 2°C), followed by measurement of pH (1:1 in water). Ammonium‐N and NO 3 − –N were analyzed at Days 0 and 60 of the incubation. Soil pH was lower following the 60‐d CaCO 3 incubation than after the 3‐d incubation with Ca(OH) 2 The analysis of N transformations indicated that positive values of H + (more H + was produced than consumed) were generated from nitrification after 60 d of incubation in 14 out of 17 soils. Furthermore, incubations with soils that have been air dried produced a flush of nitrification that increased the ionic strength and decreased pH even further. These effects from long‐term incubation would erroneously increase the LR. Incubation with Ca(OH) 2 for 2 to 4 d avoids these errors.