Characterization of soilless growth media of agricultural wastes and biochars based on integral and hydraulic‐energy indices

Abstract

Conventional volumetric indices, such as easily available water (EAW), quantify the amount of water retained in growth media but do not account for the energy required by plants to extract that water. Therefore, this study evaluated integral and hydraulic‐energy indices, obtained from the integration of water retention curve, in several inorganic and organic substrates, and their biochars. Biochars of agricultural wastes were produced by slow pyrolysis at 300°C (B‐300) and 500°C (B‐500). Organic substrates and biochars of sawdust, wheat straw (WS), rice hull, date palm bunches, and sugarcane bagasse (SC), and inorganic substrates of vermiculite, pumice, perlite, rockwool, sand, and zeolite were used. Integral energy ( E _I ), integral water storage ( W _I ), absolute aeration energy ( A _a ), absolute water retention energy (WR _a ), relative aeration energy ( A _r ), relative water retention energy (WR _r ), relative air‐water energy (AW _r ) and Dexter's index of physical quality ( S ) were calculated. The E _I values in the growth media were lower than their values in soils due to the larger pores of the media. Growth media with the same EAW had different E _I values. Highest and lowest E _I values in the EAW range were observed in vermiculite and zeolite 1–2 mm, respectively. The WR _a of biochars produced at 500°C was higher than those produced at 300°C. The lowest and highest WR _a values were obtained for the zeolite of 1–2 mm and SC‐B‐500, respectively. The substrates with higher porosity and smaller pores had higher saturated water content ( θ _s ) and A _a . The S index had a negative relation with E _I and W _I , implying more water availability for plants. Based on the W _I , E _I , and W _I / E _I values, the water availability of WS is the highest among the investigated substrates. Overall, integral and hydraulic‐energy indices offer a more sensitive and mechanistic assessment of soilless substrates than conventional volumetric metrics, providing improved criteria for evaluating substrate physical quality and optimizing irrigation management in controlled environments. These findings may support the screening and formulation of greenhouse growing substrates with improved water availability and reduced energy demand for root water uptake.