Why did gandhi choose salt as the subject of a massive protest march?

Reasons for choosing salt as the symbol of protest during dandi satyagrah-
1. at that time, making salt was state monopoly. since it was necessity of the household, they have to purchase it on the higher prices from the shops.
2. the monopoly over the salt making further deprived the people of the valuable village industry.
3. it also led to the national destruction as the British used to destruct salt collected by villagers from the sea which led to the increase in the national expenditure.
4. this wisdom of gandhiji also mobilised wider discontent beyond the hindus, muslims and backward classes.
the impact of dandi satyagrah was massive. salt satyagrah was organised at many places all over the india. different other approaches were also adopted like the breaking of forest laws. strikes and lock outs by the miners and industry workers, no rent no revenue campaign at many places by tenants and zamindars.
this proved to the second true mass movement of the country after the non cooperation one.
for the first time, the workers and labors indulged in the movement in such mass scale.
this also saw the involvement of the business class and zamindars as they also realized that congress and gandhiji would be the future of the country. the success of the satyagrah and following movement was massive. many people were imprisoned and still the movement did not stop like the dharsara instance which ultimately led to the gandhi irwin pact and second round table conference. people also got psychological boost from this satyagrah and movement as for the first time their leaders were treated equal to the Britisher.

The constant presence of diseases or infectious agents within a given geographic area or population group. it may also refer to the usual prevalence of a given disease with such area or group. it includes holoendemic and hyperendemic diseases. a holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). a hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. so probably family history.