They use nitrogen in large amounts. Tomatoes, peppers, and other plants that produce flowers and fruit can get too much nitrogen. When this happens, they grow into huge plants with few flowers and fruit. Phosphorus provides energy for plants and promotes root development, growth, and fruiting. A phosphorus-deficient plant is slow growing, stunted, and its leaves may appear dark green, dark blue or reddish.
Plants require potassium for vigor and strength.
It encourages early root development and increases resistance in the plants to certain diseases. It also helps plants tolerate stress from heat and drought. Plants that are deficient in potassium are spindly and poorly developed. They tend to fall over, or lodge easily, and are susceptible to leaf and stem diseases. Severe potassium deficiencies result in leaves curling and browning around the edges starting at the tips, beginning with the leaves lowest on the plant stem and moving up the plant.