Utilization of compost tea produced from greenhouse wastewater

Article by Robert Nicol

Numerous challenges exist in organic greenhouse production, one of the largest being a lack of amendments that can effectively serve as plant nutrients or pest controls and still be considered organic. Compost is a recognized organic input, and compost can be used in the production of compost tea, useful in fertility and biocontrol applications.  Compost tea is produced by mixing compost, with or without additional nutrients, and water together and then allowing naturally occurring microbial metabolism to take place over one to several days.  The resulting liquid has been shown to contain appreciable amounts of plant nutrients as well as to potentially control microbial plant pathogens.  In partnership with an organic greenhouse vegetable producer, we evaluated the use of compost tea created from greenhouse wastewater as a fertility and biocontrol treatment in greenhouse grown tomatoes.  In addition to direct applications in greenhouse production, recycling greenhouse wastewater through the compost tea process would allow for recycling of nutrients and reduce the environmental impact of greenhouse production.

Greenhouse wastewater (i.e. spent fertigation solution) was collected from an organic greenhouse vegetable producer and mixed with poultry compost, kelp extract and potassium humate for 72 hours to produce compost tea.  The resulting tea was analyzed for nutrient content and used as fertilizer treatments for greenhouse tomato production experiments as well as used as a treatment for controlling fungal pathogens.

Electrical conductivity, a measure of the salt content, was found to be high in compost tea and tea may need to be diluted with water before being used in greenhouse production.  After diluting the compost tea, nutrient analysis revealed that this liquid treatment could provide 40 % of the phosphorus, 23 % of the potassium, 47 % of the calcium and 21 % of the magnesium requirements of a standard greenhouse nutrient solution.  Nitrate was lacking in the tea, but another useful nitrogen source, ammonium was present.   

For greenhouse fertility experiments, additional synthetic nitrogen phosphorus and potassium was added to the compost tea in order to match the concentrations found in the synthetic fertilizer used as a control.  These experiments revealed that amended compost tea was a useful fertility treatment as tomatoes grew well and did not exhibit signs of nutrient deficiency (Figure 1).  Dry weight of the tomato plant, tomato yield and tomato Brix in plants fertilized with compost tea was not significantly different than those plants fertilized with a synthetic control.

The effectiveness of compost tea used “as is” as a biocontrol treatment was also evaluated by conducting in vitro and whole plant experiments.  Fungal plant pathogens were cultured on potato dextrose agar with or without the application of compost tea by drenching the fungal colony.  Results demonstrated that compost tea inhibited the growth of the plant pathogens Phytophthora capsici, Rhizoctonia solani and Fusarium graminearum by 27 % to 52 %.  These experiments were expanded upon by evaluating the effectiveness of compost tea in controlling plant disease in the greenhouse setting.

Tomatoes were grown under standard greenhouse environmental conditions at the University of Guelph Ridgetown Campus and purposely infected with Phytophthora capsici, causative agent of Phytophthora blight, by adding wheat kernels infested with the pathogen to the tomato potting substrate.  Plant disease progress and plant mortality were then recorded daily for thirty days after inoculation.  Compost tea reduced disease progress, measured as Area Under the Disease Progress Curve (AUDPC), by 56.2 % and reduced plant mortality by 53 %.      

Compost tea made from poultry compost, kelp extract, potassium humate and greenhouse wastewater provided nutrients to greenhouse grown tomatoes, and these tomatoes were indistinguishable from those grown with synthetic fertilizer as the sole nutrient source.  The tea was also effective in inhibiting the growth of plant pathogens as well as protecting plants from Phytophthora blight.  In order to improve biocontrol efficacy, more work needs to be done on application methods and identification of the antifungal component(s) of the tea.  Use of compost tea within an integrated pest management strategy may also allow for greater overall plant protection.      

Funding for this research was provided by Ontario Ministry of Agriculture, Food and Rural Affairs.

For more information, please contact Robert Nicol at rnicol@uoguelph.ca

Posted September 2016