Net profit for BHP Billiton fell by 30% in the 2012/13 financial year to $US10.9 billion.
AAP reports weaker commodity prices were the main cause, and BHP is slashing costs and capital expenditure in response, cutting spending to $US16 billion in 2013/14 from about $US22 billion in 2012/13.
Iron ore was the biggest earner with $US11.12 billion, but that was down 22% on the previous year.
Underlying earnings in all of BHP's businesses fell in the 2012/13 financial year.
But chief executive Andrew Mackenzie has backed the future demand for potash.
Mr Mackenzie said he was not concerned about China's economy, saying its urbanisation required 250 million people moving from the countryside to the cities in the next decade, and annual growth of 7% - 8%.
"That will only support the commodities we make and underpins our desire to be diversified," he said.
Excluding one-off financial items, BHP's profit dropped by 31% to $US11.8 billion.
The year was characterised by slowing global growth and weaker commodity markets, Mr Mackenzie said.