The USDA's revisions reflect the impact of dry weather in South America and floods in Australia, which have compounded supply constraints that first started to emerge in the middle of last year, when a drought in Russia ravaged that country's wheat fields.

WSJ: Grain Prices Soar as U.S. Slashes Outlook

The clay has the effect of storing rainwater, reducing the impact of flooding, while during dry spells, the water is gradually drained back into the rivers and streams.

BBC: Making a home for rare wildlife in north Devon