Effect of humic substances in nutrient film technique on nutrient uptake

An experiment was conducted to find out how humic substances affected nutrient uptake of plants. The test plants, oregano, thyme, and basil, were grown in nutrient film technique at two pH levels (4.5 and 6.5), in two substrates (peat and perlite), and at three levels of humic substance that was a peat extract (control, low, and high concentration). Nutrient uptake of potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) were determined by elemental contents in aerial parts of the plant and its weight. Humic substance had no effect on K, Ca, and Mg uptake but lowered the uptake of Fe, Mn, Zn, and Cu for the three test plants, more pronounced with perlite than peat and more at low pH than at high pH. The lowering of the uptake might be caused by complexation of Fe, Mn, Zn, and Cu by the humic substance and the lower availability of these metals in complexed form than as a cation or as EDTA‐chelate in the case of Fe. It is not clear why the effect of the humic substance on micro‐element uptake is larger at low than at high pH. The complexation is expected to be stronger at pH 6.5 than at pH 4.5. At low pH, the high concentration of humic substance caused a low fresh weight of the shoots, perhaps caused by a toxicity of the humic substance at low pH. This was less pronounced at high pH.