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

Incubation of soil with CaCO3 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 CaCO3 overestimated the actual LR determined by field testing. The objective of this study was to compare the pH change following CaCO3 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 CaCO3 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 NO3--N were analyzed at Days 0 and 60 of the incubation. Soil pH was lower following the 60-d CaCO3 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.