I'll copy from a previous answer I've given.

The Soviet Famine of 1932-33 affected the major wheat production areas of the Ukraine, North Caucuses, and Kazakhstan. This area was known as the Wheat Belt. These areas were hugely instrumental in feeding the Soviet population, and the collectivization policies meant that all grain which was produced by the farmers in those areas was divided and rationed out. Farmers kept enough to feed themselves, and the remaining grain was distributed to feed the rest of the country. The soil in these areas was worked very hard in order to feed the population, and soil samples were often taken. The first major cause for the famine was these soil samples. Fungal rust, plant disease, and pest infestation was either overlooked or minimized by party officials who chauvinistically mistrusted the peasant reports on soil conditions. The faulty reporting was made worse by draught in 1931 and an extremely heavy rainfall in 1932. The volatile rain patterns and the faulty reporting lead to massive crop failure in the 1932 harvest. Mark Tauger estimates that the weather and soil conditions alone accounted for at least a 20% decline in harvests.

The crop failures in 1932 are the basis of the famine in 1933. Grain procurement by the state continuously rose from 1928-1932, where it maxed out in the North Caucuses at roughly 50%. There was no real concern from 28-31, since the output by the collective farms was rising by 4-8 million centners a year. (1 centner = 100 kilograms) However, the grain harvest in the North Caucuses dropped by 50% from 1931 to 1932. The other grain centers experienced similar drops. The Soviet government was stuck. They had a vastly depleted grain total, and millions of people to feed. The decision was made to continue the grain requisitions from the affected areas, so that the entire soviet population was not subjected to famine. Additionally, the affected regions were essentially penned off from the rest of the country in an attempt to control the famine and keep people from fleeing. If people fled, there wouldn't be enough people to continue farming and the situation would remain dire. The decision to continue the rationing meant that roughly 50 million people remained fed during the famine.

The Wheat Belt was the hit the hardest, but famine deaths were reported across the country. The number of famine related deaths is somewhat contested. However most historians place the total count at somewhere between 4-7 million people.