Potassium adsorption ratios as an indicator for the fate of agricultural potassium in groundwater

Fertilization of agricultural land in groundwater infiltration areas often causes deterioration of groundwater quality. In addition to nitrogen and ph osphorous, potassium deserves attention. The fate of potassium in the subsu rface is controlled mainly by cation-exchange. Use of the Potassium Adsorpt ion Ratio (PAR), analogous to the Sodium Adsorption Ratio (SAR) allows one to distinguish between leaching of agricultural potassium to groundwater an d desorption of potassium due to increased hardness associated with increas ed manure spreading on a calcareous soil. Two regional field studies in The Netherlands indicate that groundwater below infiltration areas with agricu ltural land use, has increased PAR values compared to pristine groundwater in seepage areas and groundwater below infiltration areas with forest. Down ward transport of agricultural potassium is retarded compared to conservati ve chloride due to cation-exchange. In one field study silicate weathering is found to be a potential source for K in pristine groundwater when the re sidence time is long and easily weatherable silicates, such as glauconite, are present. (C) 2001 Elsevier Science B.V. All rights reserved.