Plants, like people, require a balanced diet of appropriate amounts of carbohydrates, proteins, vitamins, minerals, fats and water.
Plants take up most of their nutrients from the soil solution (dissolved in water). Chemically, nutrients in solution are ions with a positive or negative charge. The balance of these charges determine which nutrients are taken up by the plant.
An excess of one nutrient can antagonise the uptake or function of others.
As your doctor would order a blood test to determine whether you have a vitamin deficiency, a comprehensive soil test is the only way to determine the nutrients in your soil and their availability to your plants.