Japanese police investigating a so-called "Black Widow" who could be behind the deaths of six partners have found traces of cyanide in soil from her pot plants, reports said Friday.

Chisako Kakehi, 68, who was arrested last month on suspicion of murdering her 75-year-old spouse, received one billion yen ($10.15 million) in insurance and other payouts over 10 years following the deaths of husbands and lovers, media have said.

Chisako Kakehi speaks to the media following her arrest. Credit:AP/Kyodo News

Police believe she killed Isao - her fourth and most recent husband - with cyanide in December last year.

Investigators who tested plant pots she had taken to a recycling centre about six months after her husband died discovered a bag at the bottom of a planter. Lab results showed the presence of cyanide compounds, Kyodo News and other media reported.