Water‐soluble NO 3 ‐Nitrogen, PO 4 ‐Phosphorus, and Total Salt Balances on a Large Irrigation Tract

Return flow from a 82,030‐ha (202,700‐acre) tract of calcareous silt loam soils irrigated with water diverted from the Snake River in southern Idaho increased the downstream total soluble salt and NO 3 ‐N loads, but decreased the downstream PO 4 ‐P load. Under the existing water management practice, 50% of the total input water returned to the Snake River as subsurface drainage. Net total soluble salt output was 2.4 metric tons/ha (1.0 English ton/acre) and, on the average, was considerably greater than necessary to maintain a salt balance. Net NO 3 ‐N output was 33 kg/ha (30 lb/acre). Only about 30% as much PO 4 ‐P left the tract via drainage water as entered the tract in irrigation water. As water passed through the soil, PO 4 ‐P was removed by chemical reactions in the soil, thus decreasing the concentration in the subsurface drainage water and decreasing the downstream PO 4 ‐P load. Applied P fertilizer was not leached into the drainage water.