680 (PS 5) SORPTION ISOTHERMS INDICATE THAT CALCINED CLAY AND ALUMINUM AMENDMENTS INCREASE PHOSPHATE RETENTION OF SOILLESS SUBSTRATES

Abstract

Soilless container substrate components such as sphagnum peat moss and pine bark have very little PO4 adsorption capacity; thus, one way to reduce PO4 leaching during container production is to increase the PO4 retention of container substrates. Laboratory experiments have been conducted to create sorption isotherms at 26 C for arcillite, a calcined product of the 2:1 clays montmorillonite and illite, and a medium of 70 peat moss: 30 perlite. Thirty ml of Ca(H2PO4)2 solutions at rates of 0, 100, 300, and 500 μg P/ml were added to 5 cm3 samples of the test materials. Both materials were amended with aluminum at rates of 6.4 and 19.2 mg Al/cm3 test material from Al2(SO4)3 solutions. Adsorption isotherms confirmed that unamended medium does not retain PO4 and indicated that arcillite is effective at adsorbing PO4 as it retained 1900 μg P/cm3 test material at the 500 μg P/ml rate. The isotherms also showed that Al amendments increase PO4 retention in both arcillite and the medium. Desorption isotherms suggested that the unamended arcillite and Al-amended medium and arcillite will continue to release low levels of PO4 for an extended period of time.