Synergistic impacts of organic modifications and foliar phytostimulants on soil properties, growth, yield, and chemical constituents of borage (borago officinalis L.)

The use of organic waste provides a promising long-term amendment strategy for restoring low soil fertility. The present study aimed to evaluate the impact of organic materials, phytostimulant extracts, and their combined application on soil characteristics, vegetative growth, yield parameters, and chemical composition of borage plants. The investigation was performed using a split-plot design with three replicates. The main factor (organic materials) included three treatments: control, compost, and spent mushroom substrate (SMS). The secondary factor comprised five treatments: control (tap water), seaweed extract (SWE) at 2 and 4 g L−1, and spirulina extract (SE) at 2 and 4 g L−1, applied as foliar treatments. All possible combinations of organic amendments and foliar treatments were applied to evaluate their individual and interactive effects on the studied parameters. The results demonstrated that foliar application of phytostimulant extracts at different concentrations significantly enhanced plant growth parameters, including height, number of branches, herb biomass, flowering, and yield, while having no observable effect on soil properties. Spent mushroom substrate was more effective than compost in enhancing plant growth, yield, and chemical properties. Across two growing seasons, spent mushroom substrate in combination with spirulina algae extract (4 g L−1) increased plant height by 20%, branch number by 28%, and herb wet weight by 46% compared to the control on an average basis of two seasons. Consequently, herb dry weight and seed yield increased by 46% and 52%, respectively, based on the average of both seasons. The principal component analysis revealed that the combination of compost and spent mushroom substrate with phytostimulants improved soil properties, vegetative growth, flowering, yield, nutrient content, and chemical composition. Organic matter was identified as a key factor influencing the performance of borage plants. Combining organic materials with phytostimulants can significantly improve borage yield in soils with limited organic matter.