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Acid‐subsoil amelioration is complicated by differences in the efficacy of lime and gypsum across the diverse soil environments in which the problem occurs. This study was conducted to explain long‐term growth responses to lime and gypsum on a Plinthic Paleudult of mixed clay mineralogy. In a 10‐season experiment that monitored treatment effects on profile chemical properties, we compared the effects of (i) incorporating 15 Mg ha−1 of lime to different depths, (ii) incorporating 25 Mg ha−1 of lime to about 0.5 m, and (iii) conventionally incorporating 15 Mg ha−1 of lime plus 10 Mg ha−1 of gypsum. Even at the highest application rate, lime had minimal effects on acidity below the depth of incorporation. Gypsum, however, markedly improved the rooting environment to a depth of 0.75 m. Sulfate sorption against extraction with dilute CaCl2 was accompanied by pHw increases of ≈0.4 units, by similar increases in ΔpH (pHw − pHs), by depressions in exchangeable acidity of as much as 1.5 cmolc L−1, and by decreases in acid saturation of more than 30%. The rate of subsoil amelioration was, however, much slower than that reported in more intensely weathered soils of similar texture. Only in the sixth season were benefits evident in the 0.60‐ to 0.75‐m horizon, and acidity in the 0.75‐ to 0.90‐m horizon actually increased significantly. It is speculated that this resulted from NO3 accumulation and ionic strength–induced dissolution of interlayer Al. These findings indicate that acid‐subsoil amelioration in soils with Al‐hydroxy–interlayer minerals requires greater quantities of gypsum than soils that are dominantly kaolinitic.

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 with 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.

In many highly weathered soils crop exploitation of subsoil moisture reserves is severely curtailed by toxic levels of Al. Since vertical movement of lime is usually extremely slow in such soils, specialized mechanical and/or chemical procedures are required to overcome the problem. A field experiment with maize (Zea mays L.) on a strongly acidic Plinthic Palcudult examined the effects of surfaceincorporated gypsum on yield, root development, and profile chemical properties for four cropping seasons. The effects of gypsum (10 Mg ha ') were time dependent, but by the fourth season had resulted in a cumulative grain yield gain of 3.4 Mg ha '. Progressive depressions in the level of exchangeable Al were accompanied by increases in subsoil Ca, Mg, and SOj-S. Water pH increased markedly in the zone of maximum SO 4 -sorption/precipitation, but pH determined in KC1 remained unchanged. By the fourth season the effects of gypsum on subsoil root development were striking. These results indicate that surface incorporation of gypsum is an economically viable approach to subsoil amelioration on soils such as that studied here.