Molybdenum is required last and in the least amount, BUT that does not mean by any means that it is not completely and absolutely essential. It is actually essential for enzymes that convert nitrate to nitrite, then to ammonia, then amino acids. It is even required to fix nitrogen. It is used to convert inorganic phosphorus to organic, plant available phosphorus. Deficiency appears in older leaves and causes paleness, then spreads to new growth. May also cause leaves to “shrink” in size, especially in severe cases.