A comparison of strategies for ameliorating subsoil acidity: I. Long-term growth effects

Subsoil acidity is an important yield-limiting factor. Mechanical procedures of deep lime incorporation and surface applications of gypsum have been shown to be beneficial, but no long-term comparisons of these strategies have been published. Without such information it is difficult to make appropriate management decisions. The work reported here was conducted toward this end. In a long-term study with maize (Zea mays L.) on a strongly acidic Plinthic Paleudult, conventional moldboard incorporation of lime (15 Mg ha(-1)) was compared with (i) deeper incorporation of the same quantity of lime,vith plowing and subsoiling operations, and (ii) treatments where large additional quantities of lime were similarly introduced below normal plow depth. The efficacy of gypsum was tested by adding 10 Mg ha(-1) to conventionally limed plots. For 11 seasons, the average grain yield benefit ranged from 5 to 17% in the case of mechanical strategies and was 25% in the case of gypsum. Yields were increased only marginally by extra lime applications and segmental (slotted) amelioration proved inferior to deep-plowing procedures. The gypsum treatment proved profitable only in the fourth season, but by the eighth season had proved more profitable than the best mechanical procedure; and by the 11th season, the gypsum treatment had resulted in a cumulative yield advantage of 3.8 Mg ha(-1). Long-term superiority of the gypsum treatment was unquestionable in this study, but gypsum is often unavailable and acidic soils are frequently unresponsive to gypsum. In such situations, deep plowing should not, as is often the case, be discarded as impractical.