EFFECT OF DRYING OF SOILS ON THE ADSORPTION AND LEACHING OF PHOSPHATE AND 2,4-DICHLOROPHENOXYACETIC ACID

Surface samples of an allophanic (Patua silt loam) and a non-allophanic (Tokomaru silt loam) soil were used to examine the effects of drying on the adsorption and leaching of phosphate and 2,4-dichlorophenoxyacetic acid (2,4-D). Phosphate and 2,4-D adsorption isotherms and adsorption kinetics were determined for field-moist, air-dried and oven-dried soil samples using KH2PO4 and C-14 labelled 2,4-D. In a separate experiment, the leaching of a pulse of phosphate or 2,4-D was examined using soil columns. The Patua silt loam adsorbed 4-7 times more phosphate and 2,4-D than the Tokomaru silt loam. Compared with field-moist soil, both air-dried and oven-dried soil increased (2-5 times) the adsorption of phosphate, and oven-dried soil decreased the adsorption of 2,4-D. The adsorption kinetics indicated that there was no effect of drying on the rate of adsorption of either phosphate or 2,4-D. In column experiments, there was less leaching of phosphate and 2,4-D from the Patua soil than from the Tokomaru soil. In both soils, there was no effect of drying on the leaching of phosphate, whereas oven-drying increased the leaching of 2,4-D. Solubilization of organic matter during drying increased the accessibility of P to adsorption sites on the mineral surface and thereby increased the adsorption, whereas the water-soluble organic matter bound 2,4-D and thereby decreased the apparent adsorption onto soil and increased leaching.