Barley plants containing radioactive K retain this isotope against distilled water but release it to salt solns. and clay suspensions. For equal amts. of cations in the nutrient media, the colloids greatly exceed the single-salt solns. in removing radioactive K from plant roots. Expts. with barley plants containing radioactive Na also show that colloidal clay suspensions are more efficient depleting agents than are salt solns. of corresponding cation concs. These relationships are reversed if direct contact is prevented by placing semi-permeable membranes between the roots and the clay particles. For plants containing radioactive Br anions the clay suspensions remove smaller amts. of the isotope from the roots than do the corresponding salt solns. Various expts. are reported which indicate that contact depletion of roots by clay particles is not the result of root injury. The intake of ions by roots is not a uni-directional process; ions of the same species may move into the root and out of the root at the same time.