Fertilizer use is often misunderstood and abused. Fertilizers are erroneously called plant food. They are not plant food because plants manufacture their food, but they are essential nutrients to facilitate growth and development. Plants need 17 nutrients. Carbon, hydrogen and oxygen are needed in the largest supply, but we don't worry about adding these because they come from air and water.
Fertilizers refer to soil-supplied nutrients. A basic discussion of soil-supplied nutrients helps us to know how and when to apply different kinds of fertilizer.Nitrogen is needed in the largest amount. It helps form amino acids and proteins in the plants. Lack of nitrogen reduces shoot and root growth. In severe cases, a general yellowing or lighter color develops in the plant. Excessive nitrogen produces abundant, succulent, tender growth and may reduce flower or fruit development. Typical nitrogen fertilizers include ammonium sulfate (21-0-0), ammonium nitrate (34-0-0), and urea (45-0-0). Nitrogen is always the first number on a bag of fertilizer. Nitrogen leaches readily and must be resupplied to annual plants and turf grass every 4-6 weeks. Phosphorus, by contrast, does not move readily in the soil. Phosphorus is always the second number on the fertilizer bag. Deficiency symptoms of phosphorus include poor growth. Plants often develop a purple or olive green color and root growth is reduced.
Phosphorus uptake is limited in cool soils. It is added to give annual plants a good start in the early spring. Spring leaves on roses and turfgrass often have a purplish cast but develop normal green color as the soils warm. Excessive phosphorus is dangerous as it ties up iron, zinc or other micronutrients that affect the growth of the plant.
Potassium is the third element listed on a bag of fertilizer. Potassium deficiencies are difficult to recognize, but an overall lack of vigor shows in the plants. Fruits are small and ripening is delayed. Too much potassium increases salt problems in the soil.
Nitrogen, phosphorus and potassium are collectively referred to as primary nutrients. Secondary nutrients include calcium, magnesium and sulfur. Calcium deficiency is never a problem in Utah soils. In acidic soils, lime is added to supply additional calcium as well as adjust the pH. Since Utah mountains are limestone, our soils contain excessive amounts of calcium and can develop problems because of these excesses. Adding calcium to garden soils is never recommended in Utah.
Magnesium is seldom lacking under Utah conditions. Magnesium deficiencies occur more readily in acid soils than in alkaline soils, so it is not a problem here. Sulfur is essential to the plant, but it seldom limits growth in Utah. Sulphur is sometimes applied to give a temporary reduction in soils pH and make other nutrients more available.
Other nutrients, essential to plant growth, are referred to as micronutrients. They are all necessary but are needed in very small amounts. Iron is the most serious micronutrient deficiency that we have in Utah. Deficiency symptoms include interveinal chlorisis, which is yellowing between the veins of the leaf. The veins remain as thin green lines, while the leaves turn yellow or white. Excessive iron is toxic as well as ties up other micronutrients.
Manganese deficiency is occasionally seen in Utah, and the symptoms are similar to iron. Manganese deficiency is not a frequent problem and should not be treated unless the deficiency is correctly identified. Zinc deficiency is sometimes seen in Utah. It causes tiny leaves, rosettes (clusters of leaves on branch ends) and chlorosis. Excessive zinc ties up other micronutrients.
Boron is not a problem in our area. Most water in Utah has adequate amounts of boron to supply plant needs. Copper likewise is not a problem in our area. Kennecott's copper mines indicate that we need not add copper to our soils. Chlorine is needed by plants, but deficiencies of chlorine cannot be shown except under laboratory conditions. Cobalt is thought to be necessary for the growth of some plants but never needs to be added as a fertilizer element.
Micronutrients are only used by plants in very small amounts. While minute amounts are necessary, elevated levels can be toxic to the plants. Never add micronutrients unless there is a definite need because the balance of these nutrients is very important. It is virtually impossible to remove these micronutrients from the soil once they have built up to toxic proportions.
Next week I'll share some ideas on soil testing and how to fertilize your fruit trees and other plants.