That's an increase of more than $US100 a ton over the previous week.

With prices up more than $US200/t over the past month, farmers seem in the United States seem assured of having to pay $US1000/t or more to obtain supplies for next season.

Dealers in the US Midwest reportedly were quoting above $1000/t for immediate deliveries to on-farm storage tanks, with pre-paid quotes hitting $US1300/t.

Traders were watching to see if there was resistance to the sky-high market, especially with grain prices down sharply during the same time period.

While ammonia prices continue to soar, values for natural gas, the primary feedstock for nitrogen based fertilisers, are dropping sharply along with the rest of the energy complex.

Natural gas futures in the US broke through support at $US9 this week, and appear headed to a test of $US8 if they can't find footing soon.

This week's storage report showed stocks increasing another 56 billion cubic feet, a little below trade estimates.

Inventories are tighter than last year at this time, but in the middle of the range for the build season over the past five years.

The tropical storm striking the Texas coast this week didn't do serious harm to platforms in the Gulf, and forecasts for mild temperatures for much of the eastern US over the next two weeks should keep demand for air conditioning in check, limiting the need to start up excess power generation that's typically fired with natural gas.