RELATIONSHIPS BETWEEN SOLUBLE AND EXCHANGEABLE Ca, Mg AND K LOST FROM AGRICULTURAL WATERSHEDS

Abstract

Seven agricultural watersheds in southeastern Ontario and southwestern Quebec with varying land uses and soil types were sampled 14 times between 7 May 1974 and 21 May 1975. Discharge volume, suspended sediment concentrations and soluble and sediment (NH4oAc exchangeable) Ca, Mg and K concentrations were measured each time. Significantly decreased pK – 1/2p (Ca + Mg) values occurred in watersheds with more impermeable soils and greater corn and small grain land uses. The relatively higher stream K concentrations were attributed to K-enriched surface runoff and leaching loss from K fertilization of corn and small grains. Seasonal variations in the Gapon exchange constants or exchangeable cations associated with suspended sediment probably related to variation in surface soil erosion. Estimated annual soluble cation losses varied by watershed but were highest from watersheds with the largest proportions of corn, cultivated land and calcareous soils.