A field study was carried out to evaluate the effect of humic acid application under water stress on chlorophyll, starch contents and mineral nutrients of potato plants grown on sandy soils. The fertigation trial was conducted on sand Entisol soil in El-Nubaria region, Egypt using Spunta, the most popular potato variety in the research area. The used experimental design was split plot design with three replicates. Main plots were assigned to the two water stress levels. i.e., water unstressed plants (irrigation at 25% of available soil moisture depletion (ASMD)) and water stressed plants (irrigation at 45% of available soil moisture depletion (ASMD)). While, subplots were Humic acid treatments that applied through fertigation programme at rates of 0, 60 and 120 kg ha -1. Results indicated that increasing humic acid application rates up to 120 kg ha -1 enhanced plant growth prameters and tuber pruduction, biochemicl indicators i.e., chlorophyll, ascorbic acid, nitrate, starch, total soluble solids and protien contents. The increase of humic acid application rates was associated with the decrease of nutrients leaching, which was reflected on increasing macro- and micronutrient concentrations in potato leaf tissues. Moreover, water unstressed plants were found to be more efficient than water stressed plants on improving plant growth parameters and tuber production, biochemical indicators and leaf mineral nutrient contents. Generally, leaf chlorophyll contents were positively influenced more by micronutrient than macronutrient elements under the combined effect of water stress and humic acid. Nevertheless, stepwise regression analysis exerted that all the nutrient elements except for P element clearly close to the equality line and was higher than it under water stress treatments.
