USE OF SORPTION ISOTHERMS FOR EVALUATING THE EFFECT OF LEONARDITE ON PHOSPHORUS AVAILABILITY IN A COAL MINE SPOIL1

Abstract

The effect of leonardite (surface coal) application on P availability was studied in Glenharold [USA] spoil material. P sorption isotherms of leonardite and spoil material indicated greater P sorption by leonardite as compared to spoil material. The sorption maximum (P sorption capacity) of leonardite calculated from the linear form of the Langmuir equation was twice that of the spoil. The addition of 10% leonardite to the spoil increased sorption by 21%. A growth chamber experiment was conducted to evaluate the effect of leonardite on plant growth and the utilization of applied P. P was applied as calculated from sorption curves to establish 7 levels of P in equilibrium solution. Thickspike wheatgrass (Agropyron dasystachum) was grown for 5 wk in pots containing spoil material, and spoil + 10% leonardite. Marked response was obtained to applied P. The yield increased with increasing levels of solution P up to 0.2 ppm in the case of spoil material, and up to 0.145 ppm for spoil + leonardite. Leonardite application depressed grass growth at all P levels. At the optimum P level (0.2 ppm), the yield was decreased by 20% of the maximum. Uptake of P by plants followed trends similar to yields. Addition of leonardite significantly reduced P content of, and P uptake by, thickspike wheatgrass. Desorption of P from the sorbed phase was low in the presence of leonardite. Apparently high P sorption by leonardite and its subsequent low desorption to replenish depleted P in solution resulted in reduced plant growth and lower uptake of P by the grass. After the harvest of thickspike wheatgrass, the pots were replanted with alfalfa (Medicago sativa) without further additions of P. Three crops of alfalfa were obtained. The yield increased in relation to the amounts of residual P remaining in the pots. Addition of leonardite improved the growth of alfalfa without affecting the uptake of P. A substantial increase in the utilization of indigenous Zn and Mn was also observed with leonardite. Greater availability of native Zn and Mn in the presence of leonardite may be the reason for the improved growth of alfalfa. The spoil material is apparently acutely deficient in plant-available P. The use of leonardite as a soil conditioner will, however, need further investigation with different plant species.