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

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.