Organic fertilizers influence the composition of micro-organisms in the hydroponic cultivation of lettuce

Reclaiming nutrient sources

Reclaiming nutrient sources from otherwise wasted resource streams is critical for establishing sustainable urban environments. A vast source of potential plant nutrients is urine, which contains phosphorous and nitrogen-based compounds that are common plant fertilizers.

Different methods of refining these nutrients from waste water have been explored by researchers at Ghent University.

The best performing of these waste water derivative nutrient preparations were added to rain water and used to grow hydroponic greenhouse lettuce. Two of the treatments, one a solid preparation, and the other a liquid preparation, performed well, one being comparable to the performance of a standard hydroponic nutrient solution.

DNA-based microbial analysis

Hydroponics lettuce

DNA-based microbial analysis was also performed across the treatment groups of lettuce.

Samples were taken of both the nutrient solutions and the mature lettuce roots for DNA extraction and analysis. The rain water samples were taken immediately upon preparation.

The rain water based nutrient solution preparations had significantly higher microbial diversity, with lower overall microbial activity, when compared to samples taken from the roots of mature lettuce plants.

The lower diversity and higher activity of the root-based samples are suggestive of the microbial enrichment and selection activity of the lettuce plants, even in a hydroponic context.


Plant-microbe relations

The field of plant-microbe relations has been explored primarily in soil contexts and is understudied in hydroponic contexts.

Some of the microbial genera that were found to be prevalent in some of the waste water derivative nutrient solution preparations have many plant growth promoting microbes amongst their ranks.

However, further investigation will need to be conducted in order to determine if certain waste water derivative preparations can affect plant growth promoting microbes or otherwise beneficial microbes.

Interestingly, the waste water derivative preparation that produced the best lettuce crop, comparable to the control, also has a very comparable microbial community to the control. Microbes are well understood to have intimate metabolic relationships with plants.

Further understanding of these relationships may allow growers to make nutrient management and other operational decisions that improve overall plant health and growth by promoting certain microbial communities that provide symbiotic functions while preventing the growth of undesired microbes that cause stress and disease. 

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