Effect of water potential, temperature, and soil microbial activity on release of starch-encapsulated atrazine and alachlor.

Starch encapsulation has been proposed as a method for controlling the rate at which pesticides are released into the soil. Relatively little is known about what environmental factors influence controlled release. A series of laboratory studies were initiated to improve our understanding of how water potential, temperature, and soil microbial activity influence rate or release of starch-encapsulated atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)-acetanilide). Water potential, imposed using polyethylene glycol, significantly influenced swelling of the starch matrix and rate of release of both herbicides. At a water potential of 0 MPa, complete release required 21 d for atrazine and 7 d for alachlor. As water potential declined, so did rate of release. At a water potential of -1.5 MPa, less than 50% of the encapsulated atrazine and less than 80% of the encapsulated alachlor had diffused out of the starch matrix after 28 d. Temperature also influenced rate of release of both herbicides. At 35 degrees C nearly three times more atrazine and two times more alachlor was released from starch granules than at 15 degrees C at all sampling times. Soil microbes increase the rate of release. After 21 d there was a twofold increase in the percentage of atrazine released from starch granules applied to nonsterile soil compared with granules applied to sterile soil. Effect of soil microbes on rate of alachlor release was apparent only at early times. After 5 d there was a 20% increase in the percentage of alachlor released from starch granules when microbes were present compared with release from starch granules applied to sterile soil. After 14 d essentially all of the alachlor had been released from starch granules applied to either sterile or nonsterile soil. The different influences that water potential, temperature, and soil microbes have on rate of release between atrazine and alachlor are likely due to differences in water solubility of atrazine (30 mg L-1) and alachlor (240 mg L-1).