CHLORIDE LEACHING IN THE ROTHAMSTED DRAIN GAUGES: INFLUENCE OF RAINFALL PATTERN AND SOIL STRUCTURE

Abstract

Summary Calcium chloride giving 253 kgClha −1 was applied to the 20‐inch and 40‐inch 1/1000 th acre drain gauges after drainage re‐started in 1974. It was tagged with 30 Cl in the 40‐inch gauge. After two preliminary peaks in each gauge, chloride concentration was related to cumulative drainage in the 20‐inch gauge with an overall pattern of peak, decline and tail, interrupted by subsidiary peaks, whereas in the 40‐inch gauge it declined very slowly with marked fluctuations. Total chloride and 30 Cl behaved similarly. A simple computer model simulated the main features of the relationship well for the 20‐inch gauge, but less satisfactorily in the 40‐inch gauge and only if water was assumed to move in a very small proportion of the clay in the lower half of the gauge. These simulations and some for model rainfall regimes suggested that the shapes of the relationships were greatly influenced by rainfall pattern and by retention of soil solution in fine pores during movement in larger pores. Possible effects of soil structure on leaching are discussed.