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Liming as Related to Solubility of P and Plant Growth in an Acid Tropical Soil¹

ABSTRACT

The reactions between soil and fertilizer P at six levels of liming were investigated on an acid tropical soil. An acidic soil of pH 3.8 was limed to pH 7.0 in stages. Four levels of phosphorus were added to the soils.

For any level of phosphorus, liming decreased soluble P and labile P until the pH reached about 6.5. The phosphate potentials did not lie consistently on known solubility isotherms, but the potentials approached hydroxyapatite at pH values between 6.9 and 7.0. A labile pool of P appeared to govern mainly the concentration of P in the soil solution. Limed soil had a higher maximum adsorption capacity for phosphorus than the unlimed soil. Adsorption of P by the freshly precipitated Fe and Al hydroxides apparently caused the greater inactivation of added phosphorus in the limed soils.

Rye (Secale cereale L.) and millet (Setaria italica L.) were grown in the greenhouse at six levels of lime and four levels of phosphorus. The yields and percent P of rye were significantly increased by P without lime, but the response to added P was greater in the presence of lime. Lime with added P caused larger differences in yield and percent P of millet than in rye. Liming increased the yields of millet with added P until a pH of about 5 to 6 was reached, but further additions of lime decreased the yields. The percent P of millet increased with the initial applications of lime, but further additions resulted in a slight decrease of percent P. Plants were healthier and their root system better developed on the limed soil, which probably caused a higher percent P in the tops.

¹ Contribution from the Colorado Agr. Exp. Sta. and the ARS, USDA. This work was supported in part by the Rockefeller Foundation.

² Graduate Assistant, CSU (present address: Agriculture Research Station, Dep. of Agriculture, Maha Illuppallama, Ceylon); and Soil Scientist, USDA, Fort Collins, Colorado, respectively.

Received for publication February 5, 1973. Accepted for publication April 25, 1973.