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To the Editor, NZ Gardener

I wish to comment on an item in the latest edition of your magazine (Nov 2005). In the section entitled "Tasks for November" it was suggested that liquid fertiliser could be made by mixing cow, horse or chicken manure with water, diluting this mixture to the colour of weak tea and applying it with a watering can to lettuces and other salad vegetables.

Unfortunately, in following this advice your readers will be exposing themselves to the risk of contracting foodborne disease, resulting in diarrhoea, vomiting and possibly even worse results. This is because studies carried out in New Zealand and internationally show the occasional presence of pathogenic bacteria such as Campylobacter, Salmonella and pathogenic E. coli in manure. These bacteria can remain alive for a considerable time. For example, pathogenic E.coli can remain viable in cow faeces for up to 60 days at summer temperatures and salad vegetables treated with contaminated manure were contaminated with Salmonella for more than 200 days.

The result of this is not some theoretical risk. Many outbreaks of gastrointestinal infection have been associated with lettuces and other salad vegetables that had been fertilised with animal manure.

However, there are safe methods of producing liquid fertilisers from manure. Fresh cow, horse or poultry manure can be included in the production of compost. The composting process generates high temperatures that inactivate pathogenic bacteria and so this properly composted material can be soaked in water and used to make a liquid fertiliser. Alternatively, you can mix manure with water, as suggested in your article, but it should be held for at least a month before application to the plants. This allows for fermentation to occur resulting in the destruction of any pathogens present.

I would ask you to bring this important information to the attention of your readers.