Liquid Fertilizer Needed to Sustain Organic Tomato Production

Organic Agriculture Centre of Canada

Tomatoes are the top greenhouse vegetable crop in Canada, with a value of nearly $372 million in 2008. The size of the market makes tomatoes an attractive choice for organic producers. Yet there are differences between organic and conventional greenhouse tomato production – particularly in the use of fertilizers – which commercial producers need to know.

This is where the work of researchers in Canada and China can help. David L. Ehret of the Pacific Agri-Food Research Centre in Agassiz, BC, is the lead author of the paper, “Organic fertilizers for greenhouse tomatoes: productivity and substrate microbiology”. One of the main purposes of the research was to find out if organic tomatoes could be grown using only “dry” ingredients such as compost and gypsum. If so, then production would be simplified.

But the researchers discovered the process for growing organic tomatoes is more like making a cake. “You have to have your dry ingredients and your wet ingredients to make it work,” says Ehret.

The “wet” ingredients in this case are organic liquid fertilizers used to provide a nutritional supplement. Disadvantages of organic liquid feed can include a higher cost than conventional fertilizers and potential complications such as the plugging of drippers.

Yet with a typical North American greenhouse tomato crop in production for 10 months, producers need to know how long compost alone can sustain consistent growth.

Compost derived from either yard waste or swine manure, along with supplemental organic calcium (Ca), potassium (K), magnesium (Mg) and sulphate (SO4), in a peat-based organic mix, wouldn’t maintain a greenhouse tomato crop for more than one month without showing tissue nutrient deficiencies and reduced yield, researchers found.

A supplemental fish-based or plant-based liquid feed containing nitrogen (N) was needed for adequate growth of a long-term crop. Researchers found the best organic combination for maximum yield was 50% compost from either yard waste or mushroom substrate, combined with a low concentration of liquid feed derived from plant sources. When this was done, yields were as high as those obtained from a conventional hydroponic system.

The value of this research to commercial organic tomato producers is that they now know they will need liquid organic supplements. Ehret doesn’t think organic producers will be deterred by these findings.

“I think people who go into organic often do it for philosophical reasons and I think they understand there are going to be challenges” he said. “They know it’s not going to be simple.” If they have to apply liquid feed, then they’ll figure out how we’re going to do this.”

Though the experiments were done using tomatoes, some of the general findings could also apply to other greenhouse crops, such as peppers or cucumbers. In 2008, the value of the greenhouse pepper crop was $212 million and that of cucumbers was $187 million, reports Statistics Canada. The value of organic greenhouse vegetable production wasn’t found, but in 2006 2% of Canada’s farms were organic.

The tomato research had an international flavour, with one of the researchers coming from China, and others from Quebec, Ontario and British Columbia.

Other key findings involve microbiological activity. One of the underlying philosophies of organic growing stipulates that a biologically-active substrate must be present, states the report. Researchers found the types of microorganisms in organic systems were similar to those in hydroponic systems, but they were more active.
Interestingly, a higher concentration of either fish-based or plant-based liquid organic feed caused proliferation of Fusarium crown and root rot, which severely reduced yield.

The results have been presented to greenhouse growers and suppliers and researchers, and interest has been high, said Ehret.

He said that based on his knowledge of the greenhouse industry in Canada, conventional producers are already showing good leadership in many of their practices.  “For example they practice very safe pest control,” he said. “They use biocontrol, which is good insects fighting bad insects. They’ve been doing that for years and years.”
Based on these observations, the commercial production of hothouse tomatoes would seem to be a good candidate for conventional producers to become organic.

This article was written by Steve Harder on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).  The Organic Science Cluster is a collaborative effort led jointly by the OACC, the Organic Federation of Canada and industry partners
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Posted June 2010