The correct answer is Both A and B. The concept of Criteria of Essentiality of Nutrients was put forward by Arnon and Stout in the year 1939. Key Points They considered 16 elements essential for plant nutrition. For an element be considered as an essential nutrient, the element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element, the plants do not complete their life cycle. The requirement of the element must be specific and it should not be replaceable by any other element. The element must be directly involved in the nutrition and metabolism of the plant and must have a direct influence on plants. Additional Information Essential Nutrients for Plant Nutrients- Macro Nutrients Micro Nutrients Nitrogen is necessary for the formation of amino acids, proteins, DNA and RNA. It is essential for plant cell division and vital for plant growth. Boron is an essential component of cell wall formation and is key for the germination of pollen grains and the growth of pollen tubes. Phosphorus promotes early root formation and growth and is involved in photosynthesis, respiration, energy storage and transfer, cell division and enlargement. zinc aids plant growth hormones and enzyme system and is necessary for chlorophyll production and carbohydrate formation. Potassium is involved in carbohydrate metabolism and the breakdown and translocation of starch. Potassium also enhances disease resistance and improves winter hardiness. Copper plays a major role in photosynthesis. This element improves the flavour of fruits and vegetables and can help prevent ergot in cereals. Calcium increases fruit set and quality and is important for continuous cell division and formation (regulates hormonal activity). Manganese aids in chlorophyll synthesis and increases the availability of phosphorus and calcium. Magnesium the centre molecule of chlorophyll improves the utilization and mobility of phosphorus. Iron promotes the formation of chlorophyll and acts as an oxygen carrier. Sulfur is an integral part of amino acids. It helps develop enzymes, vitamins and oil contents, and aids in seed formation. Chloride promotes crop health and enhances the maturity of small grains on some soils. Molybdenum is needed to convert inorganic phosphates to organic forms in the plant and aids in the modulation of legumes, especially in acidic soils.