Assessment of soil pH and lime requirement methods and recommended lime rates for six reference soils across US land grant institutions

Soil pH and liming recommendations that address the soils, crops, and liming materials have been developed and adopted by land grant universities since the early 20th century. We inventoried land grant institution soil pH and lime requirement (LR) measurement methods for 1980 and 2020 and examined differences in lime rate recommendations for six reference soils using a survey developed by members of the Fertilizer Recommendation Support Tool initiative. Laboratory analysis for six acidic soils with a range of properties was shared with scientists requesting a lime recommendation for each, assuming a 0- to 15-cm soil depth, 6.5 target pH, and lime material having 100% effective calcium carbonate equivalence. Soil pH methods, LR methods, and lime rate recommendations were documented for 48, 41, and 34 states, respectively. The most widely used pH method was a 1:1 soil–water ratio (34 states, 71%). Thirty-one states use one or more buffer solutions to determine LR with the most widely used being the Sikora (10 states), Mehlich (10 states), and Shoemaker, McLean, and Pratt (nine states) buffers. Forty lime rate recommendations from 34 states for each soil were summarized with median rates ranging from 2242 to 9079 kg ha⁻¹ and coefficients of variation ranging from 41% to 73%. The reasons for high LR variability are likely due to different calibrations as no strong trends for LR method or region were observed. Efforts are needed to develop and harmonize lime recommendations to provide accurate and transparent guidance, especially for states sharing common soils and boundaries.