Flexible polyurethane foam with sodium bentonite: improving the properties of foams for use as a synthetic growing media

Common substrates used in soilless cultivation of vegetables have been shown to have several drawbacks including variability of organic substrates and disposal of some inorganic substrates. Polyurethane foams (PUF) meet several of the requirements to be a synthetic growing medium. However commercial formulations are not optimised for hydroponics and the addition of a functional filler could improve these properties. Sodium bentonite was added to foam formulations due to its use as a soil amendment which improves the water holding and cation exchange capacity (CEC) of soils. Sodium bentonite was added at loadings varying between 0-20 PPHP. The addition of bentonite increased PUF density from 41.1 to 48 kg m^‑3, increased water holding content from 464 to 767 g dm^‑3, increased CEC from 0 to 4.65 cmol_c kg^‑1 and increased the number of open cells visible in SEM images. The water drop penetration time decreased from 132 min to 78 min and the compression force deflection decreased from 10.6 to 6.2 kPa. These physical and chemical changes improved tomato variety (Sub arctic plenty) vegetative growth, dry shoot mass increasing from 7.2 to 12.9 g at a loading of 10 PPHP sodium bentonite.