Residual Value of Lime and Leaching of Calcium in a Kaolinitic Ultisol in the High Rainfall Tropics

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Abstract
A long‐term experiment was conducted on a highly acidic (pH 4.6), coarse‐textured Ultisol (Typic Paleudult; loamy, siliceous, isohyperthermic) in the high rainfall region of southeastern Nigeria in order to evaluate the requirement for and residual value of lime [Ca(OH)2] to a continuous crop rotation, and to determine the fate of applied Ca in the soil profile. The initial lime rates used were 0, 0.5, 1, 2, and 4 t of Ca(OH)2 per hectare. Maize (Zea mays) was planted in the first season and cowpea (Vigna unguiculata) in the second season under a no‐tillage, stubble conservation system.Results from the six‐year experiment indicate that relatively low rates of lime are adequate to sustain yields in a continuous maize‐cowpea rotation system. Liming at a rate of 0.5 t/ha maintained maize yield near maximum for 2 years after application. Sustained maize yields for 5 years or more were possible with a lime rate of 2 t/ha. The critical level of exchangeable Al required for 90% maximum yield of maize was about 35%. Cowpeas performed well and showed strong tolerance to soil acidity when planted as a late second‐season crop after maize without additional fertilizer application. The critical level of exchangeable Al ranged from 25 to 55% depending upon rate of chemical fertilizer as well as cowpea variety used.Leaching losses of Ca from the surface soil during the first 3 years were 2‐equivalents in the 0‐ to 2‐t/ha treatments. But an equivalent of 1.6 t/ha of lime was leached from the surface soil of the 4‐t/ha treatment suggesting that the mobility of Ca in this soil is a consequence of the hydrolysis of the pH‐dependent charge in the presence of acidifying nitrogeneous fertilizer.Exchangeable‐Al saturation in all subsoil layers of all treatments 3 years after liming exceeded 40% and soil pH (H2O) was <4.3 indicating that lime was leached as neutral Ca salts and had little effect in ameliorating subsoil acidity. Soil profile analysis after 3 years showed that nearly all Ca applied may be found within the 90‐cm depth. However, recycling of the leached Ca from the subsoil would require deep‐rooting species tolerant to high Al levels.