Phosphorus Associated With Sediments in Irrigation and Drainage Waters for Two Large Tracts in Southern Idaho

Abstract

Abstract Phosphorus was measured in irrigation and surface drainage waters for two large irrigation tracts, and inorganic, water‐soluble PO 4 ‐P and total P inputs and outputs were computed. The present irrigation practices on both irrigation tracts conserve P by removing more P from the Snake River in irrigation water than is returned in drainage water. Even greater P conservation could be attained by implementing new practices. Approximately 90% of the P in waters diverted to irrigate the Northside Tract remained in the tract. About 50% of the amount diverted remained in the Twin Falls Tract. Particle size segregation takes place in some drainage streams and the finer sediments returning to the river contain higher total and NaHCO 3 ‐extractable P concentrations than the soils from which they were eroded. The importance of defining sampling methods and procedures for P analyses is illustrated by comparative data on irrigation and drainage waters. The PO 4 ‐P concentration in samples filtered through a 0.45 µ m membrane filter was independent of the sediment concentration in waters, whereas the total P concentration of unfiltered samples was proportional to the sediment concentration.