Review of pH recommendations for soilless cultivation systems

Sammanfattning: Our human population is growing, and with that, so is our demand for food, food that today still is not a guarantee for all of us. To help increase global food security, the diversification of our food production through the implementation of soilless, hydroponic cultivation systems has been suggested. In soil-based cultivation pH is a crucial parameter for the success of the system. Hydroponic cultivation, being soilless, however, raises the question if the effect of pH still applies. To increase the complexity, multiple variants of hydroponic cultivation exist. The use of a growing media, such as sand, peat moss, or perlite, is optional and liquid based hydroponic systems are commonly used for commercial applications. These liquid based, media less, hydroponic systems can potentially diverge even more regarding the effects of pH on the growth of cultivated plants. Liquid based hydroponics can be further categorized into three primary production forms, 1) deep water culture, 2) nutrient film technique, and 3) aeroponics. To shed light, in hopes of bring clarity to this matter, this study aims to summarize and evaluate how pH effects biomass production, focusing on aeroponic cultivation. The subject was approached trough collection and review of available literature, both experimental studies and ecological theory. In combination, the content of a commercially applied aeroponic nutrient solution was also estimated. Only four experimental studies were found, however, multiple reports referred to a general pH recommendation for optimal nutrient availability in the pH range 5.5-6.5. It is however unclear as to how accurate this applies for the different types of hydroponic systems. The theoretical effect found states that pH can have a wide range of influence. This includes factors such as hydrogen ion toxicity and pH interactions with the microbial community. The pH also affects the availability of elements through changes in solubility, formation or dissolution of precipitates, sorption to particulates, or changes of the speciation of elements to different forms or into formation of soluble complexes. Since liquid based hydroponic systems can be generally believed to lack particulates, such as solid organics and mineral particles, sorption might not be of importance in this context. This, however, suggests a highly complex interaction of pH. A complexity also indicated by the experimental trials and illustrated by the estimated nutrient solution composition. Due to this complex nature of pH, the suggested increased availability, in pH range 5.5-6.5, will not necessarily correlate to biomass production, unless availability is the limiting factor. With that said, if optimization is desired, the pH in which plant nutrients are the most available could be preferred if all other parameters are optimal, something which might be hard to achieve in practice.