Is vermicompost really odourless and safe?

Two claims sit on most vermicompost bags: that it is odourless, and that it is safe for the garden. Both are close to true when the product is well made, and both need a more careful wording than the labels give.

Smell

The Fertiliser (Control) Order says an absence of foul odour. It does not say no odour at all, and earthy is normal. A sharp ammonia smell or a rotten one means the material is immature or has gone anaerobic. The University of California notes that a foul smell in compost extracts signals anaerobic bacteria.

Pathogens

Vermicomposting is a cool process, not a hot one, so it does not rely on heat to kill pathogens. The USDA organic guidance says pathogens are eliminated in 7 to 60 days depending on the technology, and asks producers to keep records of how long a batch was processed. Composting before the worms, then vermicomposting, met a pathogen-reduction requirement for sewage biosolids in one study. A 2020 Czech study found that a related worm, Eisenia andrei, eliminated or sharply reduced E. coli, enterococci and Salmonella in cow, pig and dog waste over six months.

Weed seeds

We found no authoritative figure for how many weed seeds survive. Oregon State notes that finished worm compost can still contain viable seeds. It is one more reason to cure and sieve carefully, and not to promise a bed free of weeds.

Heavy metals

The Order sets maximums of 5 mg per kg for cadmium, 50 for chromium, 50 for nickel and 100 for lead. Heavy metals are not broken down by worms, and what ends up in the castings depends on what went in. Cow dung and plant waste are a very different starting point from sewage sludge or mixed municipal waste.

Sieving and curing

The Order asks for at least 90 percent of the pack to pass a 4 mm sieve. Curing, the rest after harvest, lets the material finish stabilising. Immature material can harm seedlings even when its carbon-to-nitrogen ratio and pH look right, which is why a germination test is worth doing.

Vermiwash and compost tea

Vermiwash is the liquid that drains from worm beds. Kerala Agricultural University analysed it as honey-brown, with a pH of about 8.5 and very little nitrogen, about 0.02 percent. The University of California does not recommend teas or extracts on any edible crop and says research has not shown that foliar sprays prevent disease. We do not sell vermiwash.