SOLUTE MOVEMENT DURING INTERMITTENT WATER-FLOW IN A FIELD SOIL AND SOME IMPLICATIONS FOR IRRIGATION AND FERTILIZER APPLICATION

The movement of a bromide tracer in response to intermittent water flow was investigated in the field. Two experiments were conducted. The first involved application of a 5 mm pulse of dilute potassium bromide solution to pasture plots of contrasting initial water content. All plots then received a further application of 50 mm of water. Twenty-four hours later core samples of soil were collected and the distribution of water and bromide measured. The final bromide distributions were found to be dependent on the initial water content of the soil. Bromide applied to initially dry soil was very resistant to movement by subsequent leaching water. The second experiment also involved the application of a 5 mm pulse of potassium bromide solution followed by some leaching water. The time scale was however much greater. There was a 12 day period between solute application and final sampling, with a leaching treatment applied at different times. Coupling a mobile-immobile variant of the convection-dispersion model with a description of the transient water flow and root water extraction provided a mechanistic model. This model could successfully describe the main features of the solute movement under the four different regimes of water application. The assumed depth of water extraction by roots strongly affected the predicted final solute distribution. Some practical implications for the scheduling of fertiliser applications and irrigation events are discussed.