Can hydrochar mitigate the effects of irrigation with brackish water to improve cowpea growth?

Hydrochar derived from cashew bagasse has potential as a soil conditioner for agricultural use; however, its effectiveness in mitigating soil salinity and promoting plant growth remains uncertain. In this context, we hypothesized that hydrochar application would mitigate the effects of salinity by improving the chemical properties of surface soil layers and enhancing cowpea growth. Therefore, we evaluated the effectiveness of hydrochar from cashew bagasse in alleviating soil salinity caused by brackish water irrigation and investigated its influence on cowpea (Vigna unguiculata (L.) Walp.) growth and productivity under saline conditions. To this end, a greenhouse experiment was conducted using soil columns arranged in a completely randomized design with a 5 × 2 × 4 factorial scheme, consisting of five hydrochar doses, two irrigation salinity levels, and four soil layers, with four replicates. Soil properties, including pH, electrical conductivity, cation exchange capacity, and exchangeable sodium percentage, were evaluated alongside cowpea biomass production and nutrient accumulation. Hydrochar application increased soil nutrient availability regardless of irrigation with brackish water. It also influenced pH and EC, suggesting enhanced salt leaching. Cowpea biomass production, growth, and nutrient accumulation improved significantly with hydrochar application. Hydrochar from cashew bagasse effectively improved soil chemical attributes and plant performance even under saline irrigation, with the strongest responses at 27.6–35.1 Mg ha⁻¹. However, higher doses (≥ 40 Mg ha⁻¹) may result in diminishing returns or adverse effects.