Acidity in upland and hill soils: Cation exchange capacity, pH and lime requirement

Abstract For 30 acid upland and hill soils, with initial pH from 3.5 to 5.8, and loss on ignition from 14% to 94%, cation exchange capacity varied with pH according to the relationship CEC = m pH+ C , where m and C are characteristic of each soil. The slope m depended on loss on ignition (LOI), and is given by: m =Δ CEC/Δ=0.22 LOI+1.92. Because of these relationships, CEC measured at the natural pH of the soil (in 0.025 M MgSO 4 solution) is more relevant to field conditions than a conventional measurement at pH7. An improved method for estimating lime requirement (LR) of these soils was developed, based on the relationship found between LR and the difference in CEC (ΔCEC) between that at the natural pH of the soil and that at the target pH of 5.5: LR=1.12 ΔCEC+1.48. Lime requirement can therefore also be estimated from loss on ignition using the earlier relationships, and is given by: LR=(0.25 LOI+2.15) Δ+1.48, where Δ is the difference between the natural pH (in 0.025 M MgSO 4 ) and the target pH. Predictions based on this agreed well with results for a further 10 soils.