Oxygen!
© a.k.a. MBR
Conversely, in compacted soils or stagnant pooled nutrient solutions, oxygen levels can be depleted to a point where cellular damage occurs in the roots which subsequently leads to shoot growth retardation. This depletion is caused both by the plants themselves, as well as naturally occurring bacteria using the available oxygen faster than it can be replenished through diffusion from the atmosphere above the soil or substrate surface.
Low-oxygen (anaerobic) soils also allow for the growth of detrimental bacteria which can deplete mineral nutrients such as nitrogen an sulfur, which can ultimately lead to deficiencies and further extend crop losses.
Oxygen concentration maximization is the basis for the Nutrient Film Technique (NFT) which uses a thin film of nutrient solution to feed the plants and the nutrient film must be thin enough to allow for easy oxygenation. The use of perlite (pearlite) and other substrates also allow for increased oxygenation, but this gain in production can many times be counteracted by practices such as wrapping blocks of media in plastic liner, or placing plastic "mulch" over the substrate in order to reduce evapo- transpiration. Oxygen levels can be increased in the nutrient solution be using aeration devices like "air-stones" and pumps commonly used in fish tanks.
Water and oxygen management is a discrete science with a balance being achievable between crop production and water usage. Where water consumption is not an issue, oxygenation should be increased through the use of porous, aerating substrates. When evapo-transpiration reduction is necessary, plastic mulch may be used more efficiently by cutting holes in the material around plant stems to allow oxygen to feed the roots, and transpiration-reducing wax emulsions may also be applied to the plants' leaves and stems in moderation.
It has been scientifically proven that oxygen reduction in the shoot zone can actually increase dry mass production, but due to associated root zone depletion in normal practice, this is not a viable practice.
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