Explain why by 1928, the Soviet leadership had decided on collectivisation of agriculture?

Stalin's economic aims where to improve efficiency and production of farms. He achieved this by merging small scale peasant farms with those of the larger mechanised socialist farms (Kolkhoz) and the gargantuan state farms (Sovkhoz). He also wanted to maintain a higher provision of seeds tractors and machinery. Stalin made sure that all produce was sold to the state at a fixed price further enforcing communist ideals. Problems then incurred with this during the 'grain procurement' crisis of 1928-1929.

Stalin also wished to band all individuals together into one class, he therefore set out to disband and  destroy the 'Kulaks' the Soviet middle class (Anyone who can afford to employ someone for more than six months a year.) who where generally wealthy landowners who where benefiting from Lenins N.E.P. He organised 250,000 party workers from the city together to suppress the opposition  he received from the effects of collectivisation. Collectivisation eventually further extended Stalin's political grasp of the countryside.

Ultimately Stalin managed some progression eg: Motor Tractor Stations, education for the majority of towns, solved the land crisis and solved the needed shift of urban to rural dwellers changing from an agricultural society to an industrial one. However he left the peasants feeling uprooted and bewildered, not only this but by 1939 Soviet agriculture productivity had barely reached its level in 1913. However Stalin's aims where understandable yet perhaps not justifiable (Murder of the Kulaks).