Salt March became an effective tool of resistance against colonialism because: 
(i) Mahatma Gandhi found in salt a powerful symbol 
(ii) Gandhiji sent a letter to Viceroy Irwin stating eleven demands. The most stirring of all was the demand to abolish the salt tax. 
(iii) Salt was the most essential item of foods and was consumed by rich and poor alike.

(iv) Irwin was unwilling to negotiate, so Gandhiji started Salt march with 78 volunteers. (On 6th April) he reached Dandi, violated law and made salt.

This March developed the feeling of nationalism, people in different parts of the country broke the salt law and manufactured salt and demonstrated in front of government salt factories.

Salt is the basic requirement for every person.the british government took the monopoly of making salt and passed a law,banning manufacture of salt.on march 12,1930,mahatma gandhi began his epic dandi march from sabarmati ashram to dandi.that is how the march came to be known as dandi march.he was accompained by 78 supporters.they made salt from sea water and broke the law.that is how the ''salt march''become an effective tool of resistance aganist colonialism.