Abstract

In many parts of the world notably the US Midwest and Europe, soils have been limed to a particular target pH (pH_H2O 6.5–7.0) according to the crop. Because plants do not directly respond to the activity of H⁺, it is pertinent to enquire why this approach to liming has enjoyed such widespread popularity. Among the reasons, the original near-neutral pH of many of the soils was no doubt a consideration and the use of acid-sensitive legumes to supply N in rotations in the early stages of agricultural development also played a part. The introduction of the pH meter at about the same time as N fertilizers found widespread popularity replacing legumes in rotations facilitated the measurement of soil acidity and removed the focus from the real problems of soil acidity, namely, toxic levels of Al³⁺ and Mn²⁺ and deficiencies of nutrients such as Ca, Mg, N, S, P and Mo. Even after legumes disappeared from the rotations, the high target pH values were retained. Liming experiments throughout the world reveal that, with very few exceptions, all crops including legumes cease to respond to lime above pH_H2O=5.5–5.8, provided that the factors (Ca, Mg, Mo, B, P, etc) negatively impacted by soil acidity are optimized. In fact on highly weathered soils, liming to near neutrality can have disastrous effects on yields of many crops. Many examples are presented as illustrations of the few benefits accruing to liming to neutrality and the many benefits of farming with levels of acidity somewhat more intense than has normally been the case. Among the latter benefits are increased profitability from higher nutrient efficiencies, reduced diseases and pests, slower nitrification with less water pollution, improved soil tilth, improved availability of many metals and P.