Spring rains mean it’s time to fertilize

By Norman Bezona

University of Hawaii at Manoa
College of Tropical Agriculture
and Human Resources
Agricultural Extension Service

Are your plants perky or pooped? Spring showers and warming temperatures cause active growth of coffee, ornamentals, macadamia and most other plants. Active growth requires a good supply of nutrients to assure abundant crops and healthy plants. So if you have not applied fertilizer recently, now is an important time.

As a general rule, established plantings should receive fertilizer every three to four months. However, it is helpful to have a soil analysis done to give a more accurate picture of what the plants need. For a soil analysis, bring about one pound of soil, without rocks, to the University of Hawaii Cooperative Extension Service office nearest you and we will send it to Honolulu for testing. It takes about three to four weeks to get the results and recommendations. Presently there is a nominal charge.

There are also private labs that can analyze soil in a shorter time. The basic soil analysis will test for calcium, magnesium, potassium, phosphorus and pH.

The tests may show your soil is too acid and needs lime. When soil is too acid, plants do not respond to fertilizers as well. There may even be some dieback if conditions are severe. Some soils require as much as two tons of liming material per acre if conditions are very acid.

For folks that are “backyard” farmers — here are some additional fertilizer tips. Be sure not to over fertilize, nor wait too long between applications. Of course, the correct amount to use depends on the formula — the higher the formula, the less should be used unless it is a slow release type. A formula which contains the three major fertilizer elements — nitrogen, phosphorus and potash in a 1-1-1 ratio is a common one that is sufficient for many uses. For example, you might use a 16-16-16 or 14-14-14 or 8-8-8 for shrubs and other ornamentals. Use according to directions on the label.

For the lawn, the turf specialists usually suggest enough fertilizer to give one pound of actual nitrogen per 1,000 square feet. The formulation used for grass is usually high in nitrogen, such as a 21-7-14, 16-6-8, and 28-3-5. The first number in the formula represents nitrogen. Number of applications per year depends on type and grass and soil. Centipede Grass does well with two or three applications, but hybrid Bermuda may need six to 12 or more if you want a golf course quality lawn.

“Weekend farmers” are sometimes confused by the vast array of fertilizer brands and formulas available. However, most plants are not so specific in their nutritional needs that they can’t use and thrive on the same or similar fertilizer mixtures. The numbers represent the percent of nutrients in a bag.

A mixture with a 1-1-1 ratio is very satisfactory for the majority of plants including potted house plants. Some folks are upset when their garden supply dealers suggest a 10-30-10, 18-5-12, 20-10-10 or some other formula than a 1-1-1 ratio fertilizer. Plants will respond about the same for 18-6-12 as they will for 16-16-16. However, the middle number, phosphorus, is sometimes locked up in certain types of soils and is not available to plants.

Phosphorus is the element that encourages strong roots and cell development. Homeowners who use lots of fertilizer containing phosphorus may over a long period of time build up too much in the soil. They would do better to use a low phosphorus fertilizer, if it has been supplied year after year in high amounts. Certain plants like macadamia and protea may suffer severely if too much phosphorus is applied.

A formulation high in phosphorus and potassium like 2-8-10 has less nitrogen than most other formulations and has a tendency to stimulate flowering and fruiting of many plants. This type is commonly referred to as “bloom aid” or “fruit trees special.” A 21-7-14 or 28-3-5 is quite high in nitrogen and has a tendency to stimulate leaf development. This type is often used on ornamental shrubs, trees and grasses. Other elements like magnesium, zinc, and iron are also important and should be included in some fertilization program.

Chemical fertilizers are the most readily available and often the least expensive, but if you don’t mind the cost, organic fertilizers are also available at many garden shops and nurseries. Organic and other slow release sources of nutrients seem to have added benefits, since they last longer and do not over stimulate growth that may be more susceptible to insects and diseases. Organics are also beneficial to the soil micro flora and fauna.

The soil should be moist when fertilizer is applied, and the fertilizer should be watered in immediately after application. Also, care should be taken to insure that the fertilizer material is applied over the entire root zone of the plant. Allowing clumps of fertilizer to stand in spots under the plants or against the stems may cause excessive burning.

During times of heavy rainfall, an extra application of fertilizer may be necessary, especially in rocky or cinder soils. Organic and slow release fertilizers generally do not leach out as readily. On the other hand, unless you’re equipped for irrigation, fertilization should be withheld during periods of drought.

Many ornamentals need extra applications of the minor elements, especially ixora, gardenia, citrus and palms. Royal palms, queen palms, arecas and pygmy date palms in particular need applications of magnesium, manganese, zinc and other minor elements each year. Without it, bleached pale green or distorted leaves may occur. This is an unnatural condition of the leaves.

Remember, the gardener with a real green thumb is the one who knows how to fertilize properly.

This information is supplied by the University of Hawaii College of Tropical Agriculture and Human Resources. For further information, contact the office near you.