Direct and indirect effects of humic substances on soybean, bean and maize growth

Abstract

Humic substances (HS) play an important role in the chemical, physical and biological soil quality. They increase nutrient availability, act as bioactivators, regulating crop growth and several physiological and biochemical processes of plants. Factors such as HS source and concentration, plant species, and soil type are determining factors that modulate the plant response to HS application. The aim of this study was to evaluate the direct and indirect effects of different HS sources and concentrations on growth and nutrition of soybean, maize and bean. The soil P availability and soybean growth were evaluated in samples of Red Latosol (LV) and Red-Yellow Latosol (LVA) treated with five concentrations of C-HA: 0; 5; 10; 50 or 100 mg kg -1 , combined with two sources of P: Araxá phosphate rock (APR) and single superphosphate (SS). The treatments were incubated during 15 days and, then, soil solution P, C, pH and electrical conductivity (EC), and soil P availability were determined. After that, the soybean was sown, and 41 days later the root dry matter (RDM), shoot dry matter (SDM), total (TDM), residual P were determined. Using a modified Hoagland & Arnon (1950) nutrient solution, one experiment with maize and another with bean were carried out. It was tested the effects of the following HS sources: HA Acros Organics® (AHA), HA Leonardite (HAL), and WEHSwater extractable humic substances at the concentrations of 2, 5, 15, 40 and 75 mg L -1 C-HS, besides the control. It was evaluated in both experiments nutrient contents, EC and pH in the initial nutrient solution, SPAD index, SDM, RDM, TDM, and nutrient accumulation in shoot. The organic acids were determined in the maize root exudates of plants treated with WEHS and in the bean plants under the effect of HAL. In the LV fertilized with SS, the addition of C-HA concentrations increased the soil solution P up to 17%, residual P resin by 42%, SDM, and P and N accumulation in shoot. In the LVA samples fertilized with APR, residual P increased 18% over control. The addition of HS increased the P, K, Zn and Fe in nutrient solution and the maize SPAD index. Among the sources studied, WEHS increased maize SDM by 29% and root organic acid exudation. In nutrient solution grown with bean, the addition of HS increased the concentrations of P, K, Ca, Mg, S, Fe, and Mn. Only HAL increased the bean leaf SPAD index and TDM up to 29%. The increase in P availability and soybean growth with the application of HA depends on the type of soil, and the bioactivity of HS depend on the plant species, the HS source and concentration used. In general, maize is more responsive than soybean and bean to HS addition