However, re-use of liquid wastes also represent unique problems when re-used in soil-less applications. Physical issues include continual monitoring of the growing environment , provision of fertilizer sources, appropriate media for plant positioning and aeration, refilling of water due to evapotranspirational losses, threat of apparatus failure, high infrastructure costs, poor climatic conditions or inapproptriate timing during the year. Keeping in this context, non-physical issues include minimal buffer action , pH fluctuation, nutrient imbalance and fluctuating levels, form of nitrogen, soluble salts and excessive electrical conductivity level, presence of phytotoxic or degradatory compounds (eg root exudation compounds) and heavy metals, residual dissolved organic carbon, low disease immunity, lack of soil micro-organisms and potential heavy metal partitioning to toxic levels. The overall interactive effect of these issues often leads to differences in biomass production between tops and roots of plants, or further symptoms of stress.

Research in this area is ongoing, but generally lacking or preliminary. For example, the use of zeolites, struvite or effective micro-organisms are unique and recent initiatives. Research comparing growth response and mineral nutrient status of plant growth in hydroponic culture will be detailed to primarily discuss the aforementioned ‘problematic’ issues. Recent research findings will be discussed to better evaluate the role of these issues in soil-less environments.