Nutrient Availability under Trickle Irrigation: I. Distribution of Water and Bray no. 1 Phosphate

The distribution of water and Bray no. 1 phosphate was measured in the surface soil of a Chromic Luvisol that had been under trickle irrigation for 5 years. The region of the soil reaching field capacity after 8 to 12 h of irrigation extended horizontally to at least 65 and up to 90 cm from the outlet and was no deeper than 12 cm from the surface. After irrigation had ceased soil water decreased with time but usually did not become significantly different from field capacity until after about 24 h. Where the soil had not been fertilized, Bray no. 1 phosphate decreased from 42 to 19 ppm over a 25-cm depth immediately below the trickle outlet, while 65 cm away it decreased from 78 to 16 ppm. Banding fertilizer 50 to 80 cm from the outlet increased phosphate at the surface to 113 ppm but did not increase phosphate at depth. These results imply that trickle irrigation caused both horizontal and vertical movement of native soil phosphate near the outlet, and that phosphate fertilizer applied 50 to 80 cm away from the outlet remained near the soil surface and above the root zone. Irrigation of an 8- to 12-h duration usually increased the concentration of Bray no. 1 phosphate by an average of 63% in a region of radius 30 cm from the outlet and about 12 cm in depth. The phosphate concentration remained above pre-irrigation levels for between 6 to 23 h after the end of irrigation but fell to pre-irrigation levels as the soil dried out. A series of linear regression equations of similar slope were found to describe the relationship between Bray no. 1 phosphate and soil water content for all 300 samples collected in the multicycle irrigation experiment. Differences in intercept values were related to the variable phosphate distribution with depth at various distances from the trickle outlet. Phosphate release probably resulted from reduction of amorphous iron phosphates during the anaerobic phase of the irrigation cycle. These results suggest that trickle irrigation leads to cyclic release of both native and applied phosphate during each irrigation cycle. Leaf analysis showed that under these circumstances there was no significant response to applied phosphate fertilizers at rates up to 78 g of P as single superphosphate per plant. Over the period studied trickle irrigation appears to be an agricultural management system with a much lower requirement for the application of phosphate fertilizer when tree crops are grown.