Rains leach nutrients from Hawaiian soils

  • The contrast between dark green coconut palms in the distance and the orangeish green of the palms in the foreground show symptoms of mineral deficiencies, mostly magnesium. (Voltaire Moise/Special to West Hawaii Today)

Although heavy periods of rainfall in some parts of our island have caused the loss of essential nutrients, other areas have experienced dry conditions. This makes it difficult to make general recommendations that apply to all. Parts of the windward side received enough precipitation to remove nutrients like nitrogen and actual top soil as well. West Hawaii received much less but with our excessively porous rocky areas, even 5 or 6 inches of rain can leach important elements essential to plant growth.

Some areas were even suffering from drought. We have had several calls asking why some palms appear to be dying in makai landscapes. The answer seems fairly obvious if one notices the brown grass and wilted shrubs nearby. As long as we have sufficient summer and fall rains, our landscapes stay green. When irrigation has been restricted, and once the rains stop, the entire region experiences drought conditions. Thus hundreds of shrubs and trees could be on the verge of dying unless we get a drenching storm. If one considers the cost of purchasing mature palms and other trees, this is a waste of valuable landscape material. Our landscapes, when added all together, are worth millions of dollars.


Where rains in some locations leached nutrients from the soil, farms, lawns and gardens may need fertilizer now. These rains 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. If you have not applied fertilizer recently, now is an important time. As a general rule, new plantings should receive fertilizer every three to four months. Established landscapes may need less depending on the condition of the soil. Where rains and irrigation are sufficient, fertilizer applied now will perk up your garden. However, where there is insufficient moisture, fertilizing would aggravate the water stress condition so hold off unless you can irrigate.

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. For example, a 20-20-20 is much more concentrated than an 8-8-8. Another thing to note is that the fertilizer may be a slow-release or a quick-release type. A formula that contains the three major fertilizer elements of nitrogen, phosphorus and potassium 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 1 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. This nutrient is very likely to be deficient after heavy rains. Number of applications per year depends on type and grass and soil. Centipede Grass does well with several applications, but hybrid Bermuda may need six to 12, if you want a golf course-quality lawn.

Don’t be confused by the vast array of fertilizer brands and formulas available. 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 houseplants. 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. Plants like macadamia trees and their relatives the Proteas are particularly sensitive to too much phosphorus.

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.” When citrus fruit are puffy and dry, it is usually a sign of too much nitrogen and water. 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. The minor elements, magnesium, zinc, and iron are also important and should be included in a good fertilization program.

Chemical fertilizers are the most readily available and are the least expensive, but if you don’t mind the cost, organic fertilizers are often a better choice. 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 insect and disease. 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.

Many ornamentals need extra applications of the minor elements, especially acid soil loving plants like ixora, gardenia, citrus and many palms. Royal palms, queen palms, arecas and pygmy date palms in particular need applications of magnesium, manganese, zinc and other minor elements like boron each year. Without it, bleached pale green leaves may occur. However, these minor or trace elements can be toxic if applied too heavily, so be careful

Remember, take care of your garden like you would your pets, family and neighbors. Plants have feelings too!

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