Integrative Soil Application of Humic Acid and Foliar Plant Growth Stimulants Improves Soil Properties and Wheat Yield and Quality in Nutrient-Poor Sandy Soil of a Semiarid Region

Sandy soils (containing > 50% sand) are widely distributed worldwide and are characterized by their poor structure, low organic matter, weak hydraulic and nutritional properties, and low crop productivity. Using a 2-year pot experiment, in this study, we investigated the effects of humic acid (HA) as a soil amendment and study two plant growth stimulants (PGSs), zinc oxide nanoparticles (ZnONPs), and L-tryptophan (L-TRP), as a foliar application on wheat grown in nutrient-poor sandy soil. Three HA rates (0 (HA₀), 0.2 (HA_0.2), and 0.4 (HA_0.4) g kg^-1 soil) and five PGS levels [control, 50 mg l^-1 (ZnONPs₅₀), 100 mg l^-1 (ZnONPs₁₀₀), 0.25 mmol l^-1 (L-TRP_0.25), and 0.5 mmol l^-1 (L-TRP_0.5)] were used. The soil hydro-physico-chemical properties, morpho-physiological responses, yield, and quality were measured. HA addition amended the soil structure by allowing rapid macroaggregate formation, decreasing bulk density and pH, and increasing porosity and electrical conductivity, thereby improving soil hydraulic properties. HA_0.2 and HA_0.4 additions improved growth, yield components, and grain minerals, resulting in higher grain yield by 28.3-54.4%, grain protein by 10.2-13.4%, wet gluten by 18.2-23.3%, and dry gluten by 23.5-29.5%, respectively, than HA₀. Foliar application of ZnONPs or L-TRP, especially at higher concentrations compared to the control, noticeably recorded the same positive results as HA treatments. The best results were achieved through the integration of HA_0.4 + ZnONPs₁₀₀ or L-TRP_0.5 to the tested nutrient-poor sandy soil. The interactive application of HA_0.4 + ZnONPs₁₀₀ or L-TRP_0.5 and the use of mineral fertilizer, which is considered a surplus point in permaculture, can be recommended for sustainable wheat production in nutrient-poor sandy soil.