The effect of commercial humic acid on tomato plant growth and mineral nutrition

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Abstract
The effects of humic acids extracted from two commercially‐available products (CP‐A prepared from peat and CP‐B prepared from leonardite) on the growth and mineral nutrition of tomato plants (Lycopersicon esculentum L.) in hydroponics culture were tested at concentrations of 20 and 50 mg L‐1. Both the humic acids tested stimulated plants growth. The CP‐A stimulated only root growth, especially at 20 mg L‐1 [23% and 22% increase over the control, on fresh weight basis (f.w.b.), and dry weight basis (d.w.b.), respectively]. In contrast, CP‐B showed a positive effect on both shoots and roots, especially at 50 mg L‐1 (shoots: 8% and 9% increase over the control; roots: 18% and 16% increase over the control, on f.w.b. and d.w.b., respectively). Total ion uptake by the plants was affected by the two products. In particular, CP‐A showed an increase in the uptake of nitrogen (N), phosphorus (P), iron (Fe), and copper (Cu), whereas, CP‐B showed positive effects for N, P, and Fe uptake. The change in the Fe content was the most appreciable effect on mineral nutrition (CP‐A: 41% and 33% increase over the control for 20 mg L‐1 and 50 mg L‐1 respectively; CP‐B: 31% and 46% increase over the control for 20 mg L‐1 and 50 mg L‐1, respectively). Increases in Fe concentration in the plant roots were especially pronounced (CP‐A: 113% and 123% increases with respect to controls for the 20 mg L‐1 and 50 mg L‐1 treatments; CP‐B: 135% and 161% increases with respect to the control for 20 mg L‐1 and 50 mg L‐1 treatments). On the basis of the current experiments and from evidence in the literature, reduction of Fe3+ to Fe2+ by humic acid is considered as a possibility to explain a higher Fe availability for the plants.

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