The ceramic cups of soil moisture access tubes were found to contribute excessive amounts of Ca, Na and K to solutions drawn through them. They also adsorbed significant amounts of P from solutions containing P. Leaching the cups with 1 N HCl reduced the contamination of Na and K to acceptable levels. Calcium levels were greatly reduced but significant amounts of Ca contamination continued. Phosphorus adsorption was reduced to an acceptable level.
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Grover et al. (1970) studied this question.
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