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Field Study of Solute Movement in a Highly Aggregated Oxisol with Intermittent Flooding: I. Nitrate¹

ABSTRACT

Movement of applied nitrate (a concentrated solution sprayed uniformly over the soil surface) under intermittent flooding was investigated in Molokai silty clay (Typic Torrox), an important soil for sugarcane (Saccharum officinarum L.) and pineapple (Ananas comosus) production in Hawaii. Nitrate distribution in the profile was characterized under an extended irrigation scheme by in situ sampling of soil solutions with suction probes 1 week after each water application. The nitrate peak moved only one-third as far as would have been predicted by a piston displacement model. Equilibration of applied nitrate solution with the aggregated surface soil for 1 week prior to flood irrigation further retarded nitrate leaching. Apparently, equilibration involved bulk flow and diffusion of nitrate into aggregate micropores where the solute was less susceptible to transport in water being conducted mostly in the macropores during and soon after irrigation. Prediction of nitrate distribution based on dispersion (diffusion plus convection) equation approximated experimental results for successive irrigations reasonably well when equation parameters were obtained by fitting one set of experimental data. The drawback of this theoretical model is discussed.

Key Words: water quality • pollution • fertilizers • irrigation • ground water

¹ Journal Series paper no. 1510, University of Hawaii, College of Tropical Agriculture, Honolulu.

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

Received for publication September 7, 1972.