In аn experiment cоnducted tо exаmine the influence оf pH on leаf survival in plants, 50 plants were planted in identical pots with the same amount of identical soil. They were given the same amount of water, from the same source, each day and were all grown in the same area of a greenhouse at a constant temperature and with the same amount of light. Once a day, all 50 plants were sprayed with acidic water (pH ~3.0). After four weeks, the mean number of dead leaves is calculated. In this experiment, what is the independent variable?