BUENOS AIRES (Reuters) - Argentina's central bank purchased $451 million of foreign currency on Wednesday to bolster its dwindling hard currency reserves, after daily sales from farm exports topped $1 billion, providing some relief for the country's hard-hit finances.

The currency move on Wednesday marks the biggest such purchase since late December.

In May, the central bank bought a total of $855 million, according to traders consulted by Reuters, the largest monthly purchase of greenbacks since last September.

The government has incentivized grains exports - Argentina's main source of dollars - in the last two months with a preferential exchange rate, helping bring in over $5 billion in total. That measure formally ended on Wednesday.

The South American country's prolonged economic slump has taken a toll on the bank's foreign currency reserves, which are needed to pay down debt as well as to finance many imports.