Effect of Humic Acid on Morphology, Fluorescence, and Nutrient Uptake of Spring-Sown Potato Crop Under Saline Sandy Loam Soil

Soil salinity is one of the most important abiotic stresses that significantly limits agricultural productivity, particularly in hot arid and semiarid areas. In such areas, crops are also subjected to high irradiance, which may exacerbate the physiological stress. The conventional chemical and cultural practices have proved ineffective in achieving the sustainable production of crops under these combined stress conditions. Humic acid has been reported to enhance tolerance of plants to salinity; most of the studies emphasize the generalized physiological responses but with the least information on photosynthetic and nutrient uptake efficiency of potato plants under field conditions characterized by both salinity and high irradiance. The present two-year investigation aimed to assess the effects of varying humic acid application rates on the growth, fluorescence, yield, and nutrient uptake of potato cv. Santana. The trial was carried out using a randomised complete block design (RCBD) with a factorial arrangement of treatments. Humic acid was soil-dressed at three rates (1000, 1500, and 2000 kg ha⁻¹) and assessed at two different periods (65 and 85 days after sowing), with an untreated control for comparison. The significant (p ≤ 0.01) improvements in plant growth, fluorescence, yield, and nutrient uptake were observed with increasing humic acid application rates. However, the highest increases in plant height (89%), number of stems plant⁻¹ (95%), number of branches plant⁻¹ (49%), number of leaves plant⁻¹ (75%), leaf area index (220%), quantum yield of photosystem II (Φ_II; 130%), chlorophyll content (65%), number of tubers plant⁻¹ (115%), average tuber weight (34%), total tuber yield (60%), marketable yield (47%), plant N uptake (36.7%), plant P uptake (73%), N uptake efficiency (50.5%), P uptake efficiency (182% times), and decreased nonphotochemical quenching (Φ_NPQ; 75%) and nonregulatory energy dissipation (Φ_NO; 39%) were achieved with the highest application rates of humic acid (2000 kg·ha⁻¹) compared to the control. In conclusion, the use of humic acid at 2000 kg·ha⁻¹ substantially improved potato growth, yield, photosynthetic efficiency, and nutrient uptake, proving it a promising strategy for sustainable cultivation.