In the space of little more than a decade the Soviet Union managed to inflict two devastating famines on its people and provide a blueprint for how governments could and would transform limited natural disasters into full blown starvation.
The famine of 1921 began with a drought that caused massive crop failures, including total crop failure on about 20 percent of Soviet farmland (Pipes 1994, p.411). Although certainly a disaster of large proportions, such periodic drastic crop failures were not unknown in Russia. A similar drought struck in 1892, for example, which led to the worst crop failure of late tsarist Russia (Pipes 1994, p.412).
The comparisons between the droughts ends, and the tragedy begins, when the Bolsheviks reacted markedly different to the natural disaster. Tsarist officials arranged for the delivery of food supplies to the affected regions which, in combination with private relief efforts, kept deaths down to 375,000 to 400,000 (Pipes 1994, p.413).
The Bolsheviks, by contrast, simply ignored the famine until it was largely too late. Unable or unwilling to admit natural disasters could strike in the worker's paradise, Lenin took actions to protect himself politically but did nothing to prevent the starvation. In May and June 1921, Lenin ordered food purchases abroad, but earmarked them for the politically important cities rather than for starving peasants. Bolshevik leaders avoided visiting the areas suffering from famine. (Pipes 1994, pp.413-6).
Even when finally requesting famine aid, the Bolsheviks relied on the nominally private All-Russian Public Committee to Aid the Hungry (Pomgol). Pomgol requested the assistance of the American Relief Association founded by Herbert Hoover, then-U.S. Secretary of Commerce. The ARA responded by spending over $61.6 million to relieve the Russian famine. The ARA fed up to 11 million people a day at the height of relief efforts. The ARA suspended relief operations in June 1923 when it was revealed the Soviet Union was offering foodstuffs for sale abroad -- specifically millions of tons of cereals which it preferred to sell for hard currency rather than feed its starving people (Pipes 1994, pp.415-9.)
With the worst of the famine over, though, this posed little political risk. For helping relieve the famine, Pomgol's members were liquidated; all but two of its members were arrested by the Soviet secret police and imprisoned (Pipes 1994, pp.416-419).
Although exact casualty figures don't exist, a Soviet estimate put the death toll at 5.1 million (Pipes 1994, p.419).

The massive famine which struck the Soviet Union in 1932-33 was, like the 1921 famine, caused by government actions but unlike that famine appears to have been at least partially intentional as part of Stalin's efforts to further his political goals.
Following the disastrous famine of 1921 and similar failures of central planning, the New Economic Policy liberalized agricultural policy in the Soviet Union and the country experienced a recovery. That all changed in 1928 when uncertainty caused by Soviet central planning schemes created disequilibrium in agricultural products (farmers had grain which they held in reserve due to artificially low prices created by the Soviet regime) (Conquest 1986, p.87-9).
Rather than rectify those problems, the Bolsheviks exacerbated the problem by ordering the seizing of grain from peasants. This soon gave way to dekulakizaton -- the liquidating of "rich" peasants -- and collectivization of agriculture. Combined with agricultural quotas that left peasants with almost nothing to eat, the results were predictably tragic. So predictable in fact that historians such as Robert Conquest believe Stalin intentionally inflicted the 1932-3 famine as part of a general assault on the Ukraine.
Conquest notes, for example, that in an unprecedented move in the autumn of 1932, seed grain was removed from the Ukraine and put in storage in cities -- a move which Conquest suggests shows authorities were concerned at protecting seed grain from hungry peasants who surely would have eaten it had they access to it at the height of the famine (Conquest 1986, p.326). More ominously, Conquest reports that beyond merely withholding food aid from the Ukraine, the Soviets stationed troops on the Ukrainian-Russian border to ensure neither food nor people went in or out of the Ukraine during the famine (Russia was spared the worst of the famine). As Conquest writes,
The essential point is that, in fact, clear orders existed to stop Ukrainian peasants entering Russia where food was available and, when they had succeeded in evading these blocks, to confiscate any food they were carrying when intercepted on their return. This can only have been a decree from the highest level and it can only have had one motive (Conquest 1986, p.327-8).
Regardless of the motives, the death toll was staggering. Conquest estimated 7 million people died from famine in 1932-3, with 5 million of those being Ukrainian victims. An additional 7.5 million died from dekulakization and other state violence from 1930-7 (Conquest 1986, p.306).