Wal-Mart Stores Inc said it purchased more than £12 million of cotton from farmers who are changing over from conventional to organic farming, to help boost the supply of certified organic cotton in the marketplace.

"We heard from our supplier and other partners... that this was necessary in the market," said Tara Raddohl of Wal-Mart.

Wal-Mart has been increasing the number of organic products it offers in its stores. But because of its size as the world's largest retailer, it needs a large and steady supply of these goods to stock its more than 4,100 US stores.

When farmers commit to switching from producing conventional cotton to organic cotton, Ms Raddohl said it can take roughly three years before the crops can be certified as organic.

In the interim, she said farmers face increased production costs but cannot sell their cotton at a premium because it is not yet certified as organic.

"They're undertaking the financial burdens of organic cotton farming, but they aren't able to sell it (their crop) at a price of certified organic," she said, which has hindered some farmers' decision to produce organic cotton.

To encourage farmers to switch to organic cotton farming, Wal-Mart said it bought more than £12 million of transitional cotton from approximately 1,000 farmers at the same premium cost of certified organic cotton.

Wal-Mart said that this month, it will sell Faded Glory brand T-shirts made of transitional cotton for $3.50 to $6.

The efforts are part of Wal-Mart's larger Earth Month campaign, which kicked off this month and is being backed by a national advertising campaign.

In addition to selling the special Faded Glory T-shirts, Wal-Mart is also selling T-shirts co-branded with Coca-Cola Co that are made with RPET, a material manufactured with recycled plastic bottles.

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