GRAINS-Wheat extends drop to fresh contract lows on technical sell-off

* Corn falls to nearly 1-year low on wheat declines

* Soybeans firm on short-covering, export demand

* U.S. crop tour finds smaller soybean pod counts (Updates prices, adds comments; changes byline, dateline, previously HAMBURG)

CHICAGO, Aug 22 (Reuters) - U.S. wheat futures sank to contract lows on Tuesday, weighed down by abundant global supplies and technical selling as prices extended a fall that started in early July.

Chicago Board of Trade corn futures weakened slightly to the lowest levels in about a year, following the declines in wheat, while soybean prices were narrowly higher.

Traders were tracking the annual Farm Journal Midwest Crop Tour, which found below-average corn yield potential in Indiana and above-average in Nebraska while soybean pod counts were average to below-average in the states.

But wheat futures notched the largest moves of each of the commodities. Benchmark CBOT December wheat fell 6-1/4 cents to $4.30-3/4 per bushel, a decline of 1.5 percent, as of 1 p.m. CDT (1800 GMT). MGEX September spring wheat eased 2 percent, or 13-1/2 cents to $6.41-3/4 per bushel.

"There's not enough reason for wheat to rally," said MaxYield Cooperative analyst Karl Setzer, adding that higher wheat prices earlier this year due to drought in the northern Plains was likely to boost spring wheat plantings next year.

A record grain harvest in Russia was expected to test infrastructure to export wheat and extend its shipping season, traders told Reuters.

Meanwhile, officials in top world wheat importer Egypt were looking to reject a cargo of Romanian wheat due to the presence of poppy seeds.

CBOT December corn was down 2-1/2 cents to $3.60-1/2 per bushel, holding just above its lifetime low of $3.58-1/2 reached on Aug. 31, 2016.

CBOT November soybeans climbed 2-1/4 cents to $9.38-1/2 per bushel, underpinned by recent purchases of U.S. soy supplies by top global buyer China.

(Additional reporting by Michael Hogan in Hamburg and Colin Packham in Sydney; Editing by Lisa Shumaker)