MANGANESE RETENTION BY SELECTED CALCAREOUS SOILS AS RELATED TO SOIL PROPERTIES

Calcareous soils often need supplemental manganese (Mn) to support opt imum plant growth, but some reports show that the apparent recovery of applied Mn is very low in such soils, i.e., nearly all of the applied Mn is retained in the soil. This experiment was conducted to find the relationship between the retained Mn and selected properties of calca reous soils. Eleven surface (0-20 cm) soil samples with pH ranging fro m 7.7 to 8.1 and calcium carbonate equivalent (CCE) ranging from 20 to 50% were used in the Mn adsorption study. Two-gram subsamples of each soil were equilibrated with 20 mt of 0.01M CaCl2 solutions initially containing 10 to 200 mg Mn L-1. The Mn that disappeared from solution (after 6 h shaking at 25 degrees C) was considered as adsorbed (retain ed) Mn. The adsorption data showed a highly significant fit to Freundl ich and also to the two-surface Langmuir adsorption isotherms. The coe fficients of both isotherms showed significant positive correlations w ith cation exchange capacity (CEC), organic matter (OM), and CCE of th e soils indicating that OM and calcium carbonate are the sites of Mn r etention in calcareous soils. Comparison of the adsorption data of thi s experiment with those of plant Mn uptake of the same soils (publishe d earlier) shows that as the Langmuir second surface adsorption maxima (maximum retention capacity) of the soils increase the plant Mn conce ntration and uptake decrease.