If a plant shows a symptom which could develop due to the deficiency of more than one nutrient how would you find out experimentally, the real deficit mineral?

To find out if the deficiency symptom is due to the deficiency of which nutrient , in the case when more than one nutrient can cause the symptom, a number of wide mouthed bottles is taken. Each is fitted with split cork.

One of them has the normal culture and is called control. While the other bottles have deficiency of one mineral each. For example necrosis is caused by the deficiency of Ca, Mg or K. 
Thus three bottles having defieciency of Ca, Mg and K respectively is taken and the plant is grown in each.

The plants in the three bottles are checked if they have developed chlorosis. If the symptom is corrected by the supply of the particular mineral. Then it is concluded that the deficiency was because of that mineral.