In early growth stages of a plant, phosphorous is especially important to root growth. Phosphorous deficiency results in retarded growth throughout the plant, but is especially noted in the roots. Potassium is involved in cell metabolism, protein synthesis, and water use. Both phosphorous and potassium are key to inducing flowering. Phosphorous, is especially important in the development of the reproductive organs.
Potassium contributes to vigor and color, and seed formation.
Micronutrients Micronutrients, or trace elements, are used in very small quantities. Generally, enough of these minerals already exist in our soils, or come to our gardens through organic materials added as mulches. They include iron, manganese, boron, zinc, copper, molybdenum and chlorine. The more likely cause of a deficiency is due to an element being bound up in the soil by a wayward pH. Phosphorous and iron are notorious for getting locked up by soil particles so the roots can t get to them. Many commercial fertilizers will not have trace elements in the formula.