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Water-soluble NO₃-Nitrogen, PO₄-Phosphorus, and Total Salt Balances on a Large Irrigation Tract¹

ABSTRACT

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 NO₃-N loads, but decreased the downstream PO₄-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 NO₃-N output was 33 kg/ha (30 lb/acre). Only about 30% as much PO₄-P left the tract via drainage water as entered the tract in irrigation water. As water passed through the soil, PO₄-P was removed by chemical reactions in the soil, thus decreasing the concentration in the subsurface drainage water and decreasing the downstream PO₄-P load. Applied P fertilizer was not leached into the drainage water.

¹ Contribution from the Northwest Branch, Soil & Water Conserv. Res. Div., ARS, USDA; Idaho Agr. Exp. Sta., cooperating.

² Research Soil Scientist, Agricultural Engineer, and Soil Scientist, respectively, Snake River Conserv. Res. Center, Route 1, Box 186, Kimberly, Idaho 83341.

Received for publication August 20, 1970. Accepted for publication December 29, 1970.