Experimental design as a framework for optimising polyurethane foam as a soilless growing media

The increasing use of soilless cultivation for the production of fruit and vegetable crops allows for constant innovation and development of new soilless growing media. Flexible polyurethane foams (fPUF) have many of the properties required for this purpose. They have the mechanical strength to anchor plants and their highly porous nature allows for liquid and gas transfer to occur in the polymer foam cells. However, the optimum PUF physical properties required for efficient plant growth have not been well explored. Design of experiment (DoE) techniques were used to generate 15 fPUF's with a large range of select properties by varying catalyst and surfactant ratios. Density, cell size, water holding capacity (WHC), water drop penetration time (WDPT) and airflow were selected as foam properties likely to influence plant growth. Airflow ranged between 0.970-108 L min-1, foam cell size ranged between 0.966-1.98 mm, density of foams ranged between 30.4-41.9 kg m-3, WDPT ranged between 0.500-6050 s and WHC ranged between 204-966 gH2O Lfoam-1. Spring onions (Allium cepa) were grown hydroponically in the fPUF and plant height, dry shoot mass, nitrogen content and phosphorus content were measured. These response variables were modeled using the foam physical properties as factors. Water holding as well as the foam cell size were the only significant factors in determining the plant height, nitrogen content and phosphorus content. Water holding, cell size as well as airflow through the foam were significant factors in predicting the dry plant mass. All responses were successfully modeled using a DoE framework, indicating the strength of such a methodology in optimising fPUF as a soilless growing media.