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