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

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 <0.5 t/ha of Ca(OH)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.