Physical and Chemical Changes in Silty-Clay Soils in Response to Humic Acid and Sulfuric Acid, and Its Effects on Avocado Plant Water Status

In response to low soil oxygenation, avocado plants reduce stomatal conductance, CO₂ assimilation and consequently plant biomass. In avocado orchards low soil oxygenation can affect fruit production and tree survival. Today in Chile some avocado orchards established in clay soils are managed including the application of high charges of sulfuric acid to the soil, as a tool for increasing soil macroporosity. However, the effect of this management has not been evaluated regarding to plant physiology and it environment. On the other hand, there are organic products offered as alternatives to improve soil physical and chemical features. The objective of this study was to test the effects of sulfuric acid application to a silty-clay soil, and to compare soil and plant responses respect to control and humic acid application. The experiment was conducted during 11 months with 2-year-old ‘Hass’ avocado trees, grafted on clonal ‘Duke 7’ rootstock. The plants were established in 20-L pots containing silty-clay soil. In the experiment, trees were subjected to 4 treatments, T0: only water; T1: H₂SO₄ (0.3cc/plant once a month); T2: humic acid (9 cc/plant every 15 days); T3: H₂SO₄ + humic acid. Some of the variables measured were: soil electrical conductivity (EC), soil pH, soil bulk density, soil Oxygen diffusion rate (ODR), soil saturated hydraulic conductivity (Ks), stem water potential (SWP) and leaf stomatal conductance (gs). The main results of this experiment showed that there was a significant effect of T1 on soil pH compared to control (T0) and humic acid (T2) applications. T2 had a significant effect in increasing soil ODR compared with T0 at one evaluation date. Soil EC was signifficantly higher in T3 at the end of the experiment. T1 had Ks differences within the pot, with very low Ks in the soil located at the upper portion of the pot, but a very high Ks at the bottom of the pot. Respect to plant water status, avocado trees didn´t show differences in gs, but T1 showed a better SWP in one of the evaluation dates. Soil bulk density and others variables did not show differences during the evaluation period. As conclusion the evaluated treatments showed different but temporal effects in some soil physical features and soil pH, however it was not possible to see an effect in plant water status, indicating that probably longer time studies are needed to elucidate risks and advantages of each soil treatment on avocado plant physiology.