Effects of Biochar and Super Absorbent Polymer on Substrate Properties and Water Spinach Growth

Most previous researches have focused on biochar application in agricultural soils; however, limited information is available concerning the effects of biochar amendment on greenhouse substrate properties. A greenhouse experiment was conducted to investigate effects of wheat straw biochar (0–160 mL L−1) and super absorbent polymer (SAP, 0.8 g L−1) on physical and chemical properties of a substrate based on spent pig litter compost and the growth of water spinach (Ipomoea aquatica Forsk). Total porosity, water-holding capacity, pH and electrical conductivity (EC) of the substrate significantly increased with increasing biochar rates, especially in the substrate without SAP. The values of pH and EC were significantly lower in the substrate with SAP than those without SAP at the high biochar application rates (100–160 mL L−1). The germination rates of water spinach decreased with increasing biochar rates when biochar was added alone (76.9%–83.7%), whereas the rates increased to 83.6%–85.8% when biochar was added in combination with SAP. Growth parameters of water spinach and nutrient uptake by shoots and roots increased with increasing biochar rates and reached the maximum values at the biochar rate of 100 mL L−1. There were significant cubic relationships between the uptake of nutrients (N, P, and K) and biochar rates, both with and without SAP addition. In order to avoid negative effects on plant growth, the biochar application rate should be controlled at an optimal level (100 mL L−1). The SAP addition not only enhanced the positive effects of biochar application on the properties of the substrate, but also inhibited the excessive rise of pH and EC following biochar additions, which led to better plant growth and enhanced nutrient uptakes by water spinach.