Torrential rains along the east side of our island as well as parts of Ka‘u have leached nutrients from the soil so farms, lawns and gardens may need fertilizer now. Even parts of West Hawaii that had abundant rain earlier this year may need additional nutritional supplements. These rains 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. 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—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 one thousand 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 2 or 3 applications, but hybrid Bermuda may need 6 to 12 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. Plants like macadamia trees and 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”. 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.
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, 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.
Remember, the farmer and gardener with a real green thumb is the one who knows how to fertilize properly.
This weekly column is provided by the University of Hawaii at Manoa College of Tropical Agriculture and Human Resources Agricultural Extension Service.