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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

A Comparison of Strategies for Ameliorating Subsoil Acidity

II. Long-Term Soil Effects

^a 27 Drew Ave., Howick 3290, South Africa
^b KwaZulu-Natal Dep. of Agric., Cedara College, Private Bag X9059, Pietermaritzburg 3200, South Africa

farina{at}nitrosoft.co.za

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 pH_w increases of 0.4 units, by similar increases in pH (pH_w - pH_s), by depressions in exchangeable acidity of as much as 1.5 cmol_c 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.

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