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The effects of soil humic matter on the uptake of ions by oat roots and on the (Mg²⁺+K⁺) ATPase activity, responsible for energy transduction for ion transport at cell membrane, were studied. Four-day-old roots treated for 8 h with unfractionated humic solution (50 μg org C × ml⁻¹) took up K⁺ and SO₄staggered⁻² ions at rates, respectively, 33% and 106% higher than the controls. Similar effects were caused by humic and fulvic acids, and by the hot-acid soluble and hot-acid insoluble fractions prepared from humic acids.

At org C concentration higher than 1 μg ml⁻¹, the humic extract inhibited both Mg²⁺ dependent and K⁺-stimulated ATPase activities of microsomes isolated from roots. At 0.33–1 μg org C ml⁻¹, the Mg²⁺-dependent activity was inhibited, whereas the activity stimulated by K⁺ increased. Humic acids and the acid-insoluble fraction inhibited the ATPase activity more than fulvic acids and the acid-soluble …