Effects of biochar addition on plant available water of a loamy sandy soil and consequences on cowpea growth

Addition of highly porous biochar to agricultural soil may improve the availability of water to plants. A fully randomised greenhouse experiment was conducted to test the effect of adding biochar (at a rate of 0, 1 and 2% w/w; equivalent to 0, 10 and 25 t ha-1 to 10 cm depth) to a loamy sand on plant available water content and plant growth (cowpea, Vigna unguiculata (L.) Walp.) at three moisture levels, 20, 40 and 70% field capacity, by weighing and watering pots daily. A soil moisture release curve was constructed using a WP4C dew point potentiometer. Addition of biochar significantly increased water content at field capacity (-0.01 MPa) as compared to the control, and consequently plant available water was increased significantly, from 0.22 to 0.26 v/v at the highest rate of biochar application. Plant biomass was significantly increased with biochar application, but only in the no water stress (70% field capacity) treatment. The lack of response to biochar application under water stressed conditions was attributed to the lack of an effect of biochar application on soil moisture at permanent wilting point.