The major elements that plants need to manufacture their food are present in various combinations in commercial fertilizers. They are represented by numbers on the bag - 10-10-10 for example - in the order of N, P and K.
The first number represents the percentage of nitrogen (N). Plants use nitrogen to maintain their green color and for good leaf and stem growth. The middle number represents the percentage of phosphorus (P), important for the root development of the plant. The last number represents the percentage of potassium (K) in the fertilizer. Potassium is necessary for plant growth and resistance to disease.
Melvin Wong, a county extension agent with the University of Hawaii's Institute of Tropical Agriculture and Human Resources, explains it all.
"All three elements are mobile, he said, meaning that they move through the plant, and deficiency symptoms develop on the older leaves first. Nitrogen deficiency is recognized by the leaves turning from light green to yellow to necrotic (brown and dead). The leaves drop off, the plant is stunted and secondary shoot development is poor.
Phosphorus deficiency symptoms include stunted growth, and the leaves initially turn a dark green. As they age, they may turn purple. Potassium deficiency shows up as yellow and then brown edges on the leaves. Scattered yellow spots may also show up on the leaves.
Wong's design for the ideal fertilizer would be a formula of 10-2-10 because the nitrogen leaches out of the soil more rapidly than does phosphorus. Good luck finding this combination in a garden shop, however. It doesn't exist.
"If you use a balanced fertilizer like 10-10-10, you are replacing the phosphorus at the same rate as the nitrogen, and the soil doesn't need it," he said. "High phosphorus fertilization can lead to phosphorus-induced iron deficiency." This deficiency can be recognized by the yellowing and bleaching of new leaves. Plants vulnerable to phosphorus-induced iron deficiency include citrus, allamanda, hibiscus, pikake, gardenia and azalea.
Secondary, macronutrients - sulfur, calcium and magnesium - and micronutrients (even smaller amounts) - such as manganese, zinc, copper, boron, molybdenum and iron - usually can be found in the soil, Wong said. They used to be included in commercial fertilizers, but no more. Now you must buy additives when needed.
A general nutrition deficiency, Wong pointed out, will result when the top growth grows beyond the capability of the root system to absorb sufficient nutrients.
Said Wong, "Think of a tree that grows OK to about 10 feet, then its leaves lose their gloss, the lower leaves fall off, and the growth is straggly. In this case, what you don't need is more fertilizer. You need to prune or thin out the extra growth and to improve the soil conditions.
"You need to develop an extensive root system from the beginning by holding back water so that the roots go deep. Water when the plant wilts in the early morning."
Then there is the pH level of your soil. This refers to its acidity or alkalinity, on a scale of 1 to 14. As the numbers get higher, the alkalinity increases. Neutral is 7.
"Alkaline soils may induce minor element deficiencies," he said. "Some plants vulnerable to this problem are ixora, hibiscus, bauhinia, parrot heliconia, carissa and cape honeysuckle."
The county extension offices of the University of Hawaii will do soil tests for a charge, depending upon the information wanted. For information on these tests call the Agricultural Diagnostic Service Center at 956-6706 or 956-7980.
