Factors Affecting Adsorption of Cations by Plant Roots
1953
Mehlich, A.
The adsorption of Ca, Mg, K and Na by metabolically inactive H-roots of wheat and alfalfa in relation to the nature of the anion has been studied. Titration curves and fH values showed wheat roots to have a weaker acid character than alfalfa roots. A high retention of H⁺ over metal cations was indicated. As a consequence, only small amounts of cations were adsorbed when offered as nitrates, but large amounts were adsorbed when offered as the bicarbonates. The order of increasing adsorption in either case was: Na, K, Mg, Ca. When Ca, Mg and K were offered in exchangeable form on a soil colloid with Ca/Mg and Ca/K ratios of 4, the Ca/Mg ratios on the roots were 4 and 8, and the Ca/K ratios were 10 and 24 for the wheat and alfalfa roots, respectively. The corresponding distribution of cations on the wheat roots were: Ca, 73%; Mg, 19%; K, 8%; and for alfalfa roots they were: Ca, 85%; Mg, 11%; and K, 4%. It is concluded that the adsorption of cations by metabolically inactive roots is a neutralization reaction involving principally Ca. The group of compounds involved in this reaction have been designated as RCOOH. Selective adsorption of cations involving exchange reaction, notably Mg and K, through RH is restricted to metabolically active roots. It is suggested that the proportions of K to Ca entering the plant is related to the concentration and proportion of RH to RCOOH of roots.
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