Nitrogen, Phosphorus and Potassium are the three primary nutrients in any fertilizer. The three numbers on your fertilizer container represent Nitrogen (N), Phosphorus (P) and Potassium (K) in that order. The number is the % by weight of the nutrient contained.
Nitrogen is the primary building block for plant protoplasm. Protoplasm is the translucent substance that is the living matter in cells. It is needed for flower differentiation, speedy shoot growth, the health of flower buds and the quality of fruit set. It also acts as a catalyst for the other minerals.
If you have too little nitrogen leaves become yellow-green, the oldest showing yellowing first. You will see the tips of leaves turn brown and the brown will be surrounded by yellow than green. The brown and yellow strips are V-Shaped. Since the plant can move nitrogen, when it is low it takes it from older growth and gives it to newer growth. Growth is reduced, there will be less and small fruits.
If the nitrogen is too high then fruits take longer to ripen, will be soft and have short storage life. Too much nitrogen also hurts root growth and water efficiency of plants. It also will make your plants less tolerant of our cold spells.
When buying nitrogen for your plants spend the money for the slow release formula. Soluble can cause leaf burn and rapid growth for a week then nothing. Soluble must be reapplied frequently and in the end costs more because you need to apply it more often. Except of course in your vegetable bed where you want rapid growth for the short growth season.
Phosphorus is needed for energy transfer and storage in plants. It is needed for the formation of nucleic acids ( RNA and DNA ). It helps plants to mature and promotes root, flower and seed development.
Phosphorus is also mobile in the plant showing in older leaves first. Too little phosphorus causes leaf die back, older leaves stems and veins may appear purple. The plant will be slow reaching maturity with poor seed and fruit quality. Often fruits and seeds will have a bitter taste when there is too little phosphorus. Symptoms can be similar to those of a virus and are often mistaken for a virus.
Potassium is needed to activate enzymes, form sugars and oils. It improves cold weather tolerance. Fruits will be a better quality with better color, less acidity and more oil and thicker skins.
Potassium is also mobile, too little potassium causes leaves to curl and distort, oldest leaves first. Older leaves may get brown edges and tips and drop. Stalks may be weak or short. Root systems will not develop properly.
If potassium is too high then there might not be enough nitrogen to keep up and a nitrogen deficiency can develop.
It is important to keep nitrogen and potassium in balance. Higher nitrogen and less potassium is good for leaf growth, your lawn and other foliage plants prefer this. High potassium lower nitrogen is better for flower, fruit and vegetable growth.