Why is purification of water and nutrient salts so important in studies involving minerals nutrition using hydroponics?

1 ‘All elements that are present in a plant need not be essential to its survival’ comment. 3 Explain with examples: macro nutrients, micro nutrients, beneficial nutrients, toxic elements and essential elements. 4 Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency. 5 If a plant shows a symptoms which could develop due to deficiency of more than one nutrient, how would you find out experimentally the real deficient mineral element?