SOIL AMENDMENTS FOR REDUCING PHOSPHORUS CONCENTRATION OF DRAINAGE WATER FROM HISTOSOLS

Recently, the Everglades Agricultural Area (EAA) has been scrutinized by groups concerned with the impact of P-enriched drainage water from organic soils (Histosols) on the ecology of adjoining wetlands. The objective of our research was to evaluate the effectiveness of soil amendments in reducing the P concentration in drainage water from an organic soil. Plastic cylindrical columns were filled with soil collected from a Pahokee muck (euic, hyperthermic Lithic Medisaprist). Three soil amendments were added to the soil columns: (i) a municipal drinking water purification facility waste material (''residual''), which is composed of CaO, Al-2(SO4)(3), and a starch-based polymer; ail commercial agricultural dolomite [CaMg(CO3)(2)]; and (iii) commercial agricultural gypsum (CaSO4.2H(2)O). The amended columns were saturated for 60 d with an aqueous solution containing 5 mg P L(-1) and then drained. The columns were then subjected to four P desorption cycles by saturating with distilled water and draining. Leachates were collected and analysed for total dissolved P (TDP). Residual and dolomite amendments reduced soil affinity for P sorption and resulted in a decrease in the quantity of soil P retained after repeated leachings. The net result from gypsum amendment was a small increase in P sorption capacity combined with an increased soil affinity for P. The reduced leachate TDP concentrations resulting from gypsum amendment were long lasting and were dependent on gypsum amendment rate. The effectiveness of gypsum amendments in reducing drainage-water TDP concentrations needs to be evaluated under commercial agricultural field conditions.