Why did the soviet union start the berlin blockade apex?

After the defeat in WWII, Germany's capital, Berlin was divided into four zones which were separately governed by France,Britain,US and the USSR. In 1946,the three nations merge the zone of France, Britain and US to be governed by one government, USSR strongly protested against it as the merged zones account for 75% of Germany's original population. In reaction,USSR cut off all land routes linking Berlin and the Western zone,claiming that it was to protect USSR's economy. The event was known as Berlin Blockade.