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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.

In many high potential cropping areas, particularly of the tropics and subtropics, crop exploitation of subsoil moisture reserves is prevented or severely curtailed by high levels of exchangeable Al. Since the effects of surface incorporated lime on subsoil acidity are minimal in most highly weathered soils, specialized mechanical and/or chemical procedures are required to overcome the problem. This work was conducted in order to assess the practicalities of mechanical profile modification and to compare the efficacy of several possible procedures. In a field experiment with maize (Zea mays L.) on a strongly acidic Plinthic Paleudult conventional moldboard incorporation of lime was compared to lime incorporation using a deep moldboard plow, the Wye‐double‐digger, a modified subsoiler capable of incorporating vertical bands of ameliorated topsoil to depths of 0.7 m, and a deep limer designed to ameliorate vertical bands of soil to a similar depth. All the procedures tested proved superior to conventional liming, the average response to profile modification ranging from over 1 400 kg ha 1 in a season in which severe moisture stress was experienced to approximately 400 kg ha−1 in an exceptionally high rainfall season. Yield responses were related to increases in rooting volume and to changes in root configuration. Segmental liming proved as effective as deep plowing or double digging.

Notwithstanding a voluminous body of literature on the subject there is still poor agreement regarding the relative merits of lime requirement indices based on exchangeable Al and those based on pH measurements. The work reported here was conducted in an effort to obtain greater clarity on the subject. Lime × P × Mg factorialized pot experiments were conducted on Mollisols, Ultisols, and Oxisols using corn as an indicator crop. Plant tissue was analyzed for several essential elements plus Al and the soils were tested for exchangeable cations, P, and pH. Yield responses fell into three categories: (i) those in which liming neither promoted nor depressed growth, (ii) those where growth was initially promoted by liming with no depressive effect and (iii) those where liming to neutral pH values depressed yield. The results showed that corn response to lime was essentially similar over a wide range of soils when Al saturation of the soil was used as the index of lime requirement. Corn response was poorly related to pH in water but the relationship improved when salt pH was used. As nearneutral pH values were approached, the uptake of a number of nutrients was reduced, suggesting that the long‐held view that such pH values have beneficial effects on nutrient availability warrants re‐investigation.

Due, in part, to the relative paucity of published comparisons based on field generated data, there is still poor agreement regarding the relative merits of lime requirement indices based on exchangeable AI and those based on pH. The objective of this study was to compare such indices using results obtained from long-term field experiments. Data were obtained over 22 site-years from lime trials conducted on clay (Typic Haplorthox) and sandy loam (Plinthic Paleudult) soils differing widely in organic carbon content. Relative maize (Zea mays L.) yields were used to compare the prognostic value of soil pH with indices obtained using exchangeable A1 and exchangeable acidity (A1 + H). Both within and across soils, pH proved to be markedly inferior to A1 based indices. Exchangeable acid saturation of the effective cation exchange capacity, a readily obtained and popular index of lime requirement in some countries, proved as effective as less easily acquired indices based on exchangeable A1 per se. The findings reported are consistent with those of many glasshouse studies and support the viewpoint that indices based on AI or acid saturation should replace pH as a measure of lime requirement.