Comparison of Methods to Determine Crop Lime Requirement Under Field Conditions

Abstract

Little field research has evaluated the effectiveness of pH buffer methods free of hazardous chemicals, such as Sikora and Mehlich methods, for estimating lime requirement (LR). This study assessed how Shoemaker–McLean–Pratt (SMP), Sikora, and Mehlich buffers, titratable acidity (TA), and soil properties relate to the LR under on-farm conditions. Experiments were established in 2.54- to 6.42-ha areas of 14 Iowa fields. Replicated treatments were a control and 6.7 Mg ha−1 of effective CaCO3 equivalent. Soil samples (19–48 per site, 15-cm depth) were collected before liming and after a crop harvest (6–12 mo after liming). Soil pH ranged from 4.60 to 8.05 across all samples. Mean SMP, Sikora, and Mehlich buffer pH values differed (P < 0.05) and were 6.43, 6.36, and 5.77, respectively. The mean difference between SMP and Sikora buffers was explained by small differences in six of eight loess-derived soils and increasing difference with increasing soil organic matter (OM) and clay. The Mehlich buffer showed the best relationship with LR (r2 = 0.28), which was followed by Sikora (r2 = 0.18) and SMP (r2 = 0.12). Sikora and Mehlich buffers performed better than SMP or TA and include no hazardous chemicals. Consideration of OM increased the proportion of LR variation accounted for by the Mehlich or Sikora buffers to r2 = 0.32 and 0.24, respectively. The improvement, although small, can be easily implemented for routine estimation of LR because OM is measured together with pH and buffer pH in samples submitted to laboratories.