Manganese retention by selected calcareous soils as related to soil properties

Calcareous soils often need supplemental manganese (Mn) to support optimum 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 calcareous soils. Eleven surface (0-20 cm) soil samples with pH ranging from 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 (retained) Mn. The adsorption data showed a highly significant fit to Freundlich and also to the two-surface Langmuir adsorption isotherms. The coefficients of both isotherms showed significant positive correlations with cation exchange capacity (CEC), organic matter (OM), and CCE of the soils indicating that OM and calcium carbonate are the sites of Mn retention in calcareous soils. Comparison of the adsorption data of this experiment with those of plant Mn uptake of the same soils (published earlier) shows that as the Langmuir second surface adsorption maxima (maximum retention capacity) of the soils increase the plant Mn concentration and uptake decrease.