Nutrient acquisition and eucalyptus growth affected by humic acid sources and concentrations

Abstract

Humic acid (HA) can increase eucalyptus growth due to its biostimulant effect and improvement in plant nutrient uptake. Magnitude of these stimuli varies according to the source, chemical composition, presence of bioactive fragments, and concentration of HA. The aim of this study was to evaluate the effect of HA sources and concentrations on dry matter, accumulation and uptake efficiency (UpE) of nutrients for eucalyptus seedlings cultivated in nutrient solution. A randomized block experimental design was adopted, with four replications, in a 3x4+1 factorial scheme, in which three sources of HA (HA-L1 and HA-L2, derived from different commercial HA extracted from leonardite with 0.5 mol L-1 NaOH and 0.5 mol L-1 KOH, respectively, and HA-Com, which was extracted from compost with 0.5 mol L-1 KOH) were combined with four concentrations of C-HA (5, 10, 25, and 100 mg L-1) as well as a control treatment in which eucalyptus was cultivated without HA. Eucalyptus seedlings were grown for 20 days, maintaining three plants per pot. At the end of the experiment, production of shoot dry matter (SDM), root dry matter (RDM), and total dry matter (TDM); the Root: Shoot dry matter ratio; and the accumulations of N, P, K, Ca, B, Cu, Fe, and Zn in the shoot and root were evaluated as well as the UpE of these nutrients by the seedlings. Maximum production of SDM was reached at the concentrations of 14 and 13.5 mg L-1 of C-HA, for HA-L1 and HA-Com, respectively. The RDM increased with the use of HA-Com, with optimal concentration of 55 mg L-1 of C-HA. Accumulation of Fe in the eucalyptus shoot decreased in HA-treated pots, compromising Fe translocation from the root to the shoot. Regardless of HA source used, Cu accumulation by plants diminishes as C-HA concentration increases. A greater growth of eucalyptus at concentrations lower than 15 mg L-1 of C-HA occurs more through the stimulus to shoot and root growth than due to greater acquisition and UpE of nutrients by plants. Use of high concentrations of C-HA requires greater concentration of some micronutrients in the nutrient solution tested.