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Field Study of Solute Movement in a Highly Aggregated Oxisol with Intermittent Flooding: II. Picloram¹

ABSTRACT

Most of the applied herbicide picloram (potassium salt of 4-amino-3,5,6-trichloropicolinic acid) was retained in the top 40-cm depth of the Molokai soil with cumulative applications of 48 to 74 cm water following application of the chemical to the soil surface. Distribution of picloram in the soil profile one week after each irrigation was determined by in situ sampling of soil solution with porous ceramic cups placed at several depths. Although movement of the picloram peak was retarded more than was anticipated from equilibrium adsorption measurements, a significant fraction of the applied herbicide moved ahead of the peak deep into the profile (down to the 143-cm depth) after a single application of 24-cm water. Rapid flow through macropore sequences combined with temporary retention of picloram in aggregate micropores appear to be responsible for the unusual pattern of movement. Published data for nitrate movement in the same field plots of Molokai soil were compared with picloram movement data to provide a measure of the relative mobility of these solutes. A comparison of the results reported here with published field data of solute leaching in other soils indicated a greater retardation of solute peak movement in Molokai soil than in less aggregated soils.

Key Words: water quality • ground water • pesticides • herbicides • irrigation • tropical soils

¹ Journal Series Paper No. 1581 of University of Hawaii, College of Tropical Agriculture, Honolulu. This research was conducted under Western Regional Research Project W-82.

² Graduate Assistant, Professor, Graduate Assistant, and Professor, respectively. Department of Agronomy and Soil Science, University of Hawaii, Honolulu 96822.

Received for publication January 19, 1973.

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