Document Type : scientific research article
Authors
1
PhD student in Agrotechnology, Faculty of Agriculture, Tabriz University, Tabriz, Iran
2
Associate Professor, Department of Plant Ecophysiology, Department of Crop Ecology, Faculty of Agriculture, Tabriz University, Tabriz, Iran.
3
Assistant Professor, Soil and Water Research Institute, Agricultural Research, Education and Extension Organization
Abstract
Background and Objective: Understanding the physiological and ecological processes underlying crop yield formation is critical for optimizing agricultural management practices. Soil structure and water retention capacity play pivotal roles in regulating stomatal conductance and enhancing photosynthesis, ultimately influencing crop productivity. In many agroecosystems, water scarcity and limited availability of immobile nutrients, particularly phosphorus, significantly constrain yield potential. Considering the ongoing challenges of climate change, adopting effective strategies to enhance soil water storage is a key approach to improving water use efficiency and sustaining crop production. This study aimed to evaluate the potential of Leonardite—a rich source of humic acid—as a partial substitute for conventional phosphorus fertilizer (triple superphosphate) to promote sustainable agriculture, reduce input costs, enhance phosphorus bioavailability, and improve the yield of medicinal black seed (Nigella sativa L.).
Materials and Methods: A field experiment was conducted during the 2021–2022 growing season using a randomized complete block design (RCBD) with three replications on sandy loam soil in Parchin, Ardabil Province, Iran. Nine treatments were applied: control, 100 kg ha⁻¹ triple superphosphate (P100), 50 kg ha⁻¹ triple superphosphate (P50), 200 kg ha⁻¹ Leonardite (H200), 100 kg ha⁻¹ Leonardite (H100), 50 kg ha⁻¹ triple superphosphate + 100 kg ha⁻¹ Leonardite (P50H100), 25 kg ha⁻¹ triple superphosphate + 150 kg ha⁻¹ Leonardite (P25H150), 75 kg ha⁻¹ triple superphosphate + 50 kg ha⁻¹ Leonardite (P75H50), and 25 kg ha⁻¹ triple superphosphate + 50 kg ha⁻¹ Leonardite (P25H50). Measured parameters included soil moisture indices, bulk density, mean weight diameter (MWD) of soil aggregates under dry and wet conditions, stomatal conductance, photosynthesis rate, plant height, and grain yield.
Findings: Treatments P25H150 and P50H100 increased grain yield by 120% and 110%, respectively, compared to the control. Plant height also increased by 61% and 59% under the same treatments. Stomatal conductance improved by 33% and 32% for P25H150 and P50H100, respectively. The H200 treatment achieved the highest soil moisture indices and dry aggregate stability (MWD). Grain yield and plant height exhibited strong positive correlations (P < 0.01) with stomatal conductance, as well as with gravimetric and volumetric soil moisture and dry aggregate MWD. A positive association was also observed between stomatal opening and soil moisture indicators.
Conclusion: Leonardite, as a humic acid-rich amendment, effectively reduced the requirement for triple superphosphate fertilizer, enhanced soil water retention, and improved aggregate stability. The findings suggest that applying H200 or the combined treatment P25H150 can promote sustainable soil management while enhancing black seed yield.
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