Objectives Soilless culture substrates based on wastes of intensive feeding are known for low porosity and water holding capacity, and high electric conductivity, etc. Exploration to improve the properties of them by addition of biochar and super absorbent polymer (SAP) was tried in this paper.

Methods A pot experiment was conducted using water spinach (Ipomoea aquatica Forsk) as tested materials. The used substrate was prepared by mixing fermented straw bedding compost, vermulite, pearlite and peat in ratio of 3∶2∶3∶2 (v/v), in which the total N, P and K contents were 24.2, 8.62, 10.1 g/kg, available N, P and K contents were 1.94, 0.83 and 3.72 g/kg. Biochar derived from wheat straw (600℃) was added to the substate in ratio of 0, 2%, 4%, 6%, 8%, 10%, 12%, 14% and 16%, and the above substrates were divided into two parts, one part was added with 0.8 g/L of SAP and the other was not added, 18 treatments were setup in total. The bulk density, porosity, water holding capacity, pH and electric conductivity (EC) of the treatments were determined. In situ water distribution within the substrates inside the growth pots was determined during growth of water spinach. The emergence rate and some growth parameters of water spinach were measured.

Results The total porosity, water holding capacity, pH and EC of the substrates were significantly increased with the increase of biochar rate, and those in the substrates without SAP addition increased even faster. As a result, the pH and EC in substrates added with SAP were significantly lower than those without SAP addition when the biochar adding rates were higher than 10% (P < 0.05). In the biochar treated substrates added with SAP, the water holding capacity, total porosity and air porosity rate, growth parameters (plant height, stem diameter, aboveground and belowground fresh/dry weights), root indexes (total length, superficial area, volume, root number) and nutrients (N, P and K) uptakes by water spinach were increased first with the increasing of biochar addition, and reached the maximum at addition rate of 10%, and then decreased when the biochar addition rates were continuously increased. The water spinach growth parameters in the treatments with both biochar and SAP were higher than in those without SAP.

Conclusions Biochar addition at rate of 10% could effectively improve physical and chemical properties of the substrates derived from agricultural wastes. Combined SAP addition (0.8 g/L) could enhance the positive effects of biochar, and inhibit excessive rise of pH and EC caused by high biochar addition rates, which is helpful to make better growth substrates and achieve higher yield of water spinach.