Tropical Gardening: Composting and mulching to build healthy soils from scratch

  • Photo courtesy of KYLE MCWHIRTER

    The eruption in Leilani Estates and lava river has destroyed thousands of acres of farms, homes and forests, but once the lava cools, life returns with mosses, ferns and ohia. The rebirth can be hastened with compost, mulches and fertilizer.

With thousands of acres covered with lava in just a few weeks, it is hard to imagine they will ever support vegetation again — at least in our lifetime.

In heavy rainfall regions such as Kapoho it only takes a few decades once the lava stops flowing. The process of healing can be more rapid with a little help from humans.


Where rainfall is scant, it takes more effort.

Where weather conditions are dry, it is a good to explore ways to conserve water as well. Organic material is essential to healthy growing conditions.

Decomposed organic matter helps increase water- and nutrient-holding capacity of the soil. Rotted material such as leaves and clippings used as surface mulch can help conserve moisture and keep weeds under control.

Nematodes, those little microscopic worms that feed on plant roots, will do less damage in a high-organic soil.

Organic matter also can increase the minor element and microbiological activity of a new planting medium. Technically, what you are building on a young lava flow is not soil but a growing medium to bring back life more quickly.

For simplicity, let’s just call it soil.

Be sure to save your grass clippings and leaves. They are like money in the bank. You can store these materials in a corner of the garden.

Decay of plant material deposited in a compost pile can be hastened through the use of fertilizer and manures. For each bushel of leaves, grass clippings or other green waste, add 2 cups of balanced fertilizer such as organic 8-8-8 and 1 cup crushed coral, dolomitic or hydrated lime.

The compost is ready to use in about three months. It is an excellent material to mix with soil for vegetable gardens and new plantings.

Anthuriums especially thrive on compost. They love that high-organic mixed with good water retention capability and yet good drainage. A good mix needs to be able to anchor the roots and stem so the plant will not topple over as it grows upward yet provide sufficient moisture, nutrients, and aeration to the plant.

Cinder or crushed rock added to composted wood shavings, sugar cane bagasse, macadamia nut shells, peat or tree bark will serve to better anchor the roots.

Even with composting and mulching, you will still need to fertilize your garden. Some Hawaiian soils are very young and low in nutrients. Larger amounts of fertilizer are needed for growing plants and lawn grasses in these areas than where soils are older and better developed.

The young soil is not only lacking in the primary elements, such as nitrogen, phosphorus and potassium, but it might be deficient in the minor elements such as manganese, copper, zinc, and boron. When plants are grown in these mineral deficient soils and fertilized with ordinary plant foods, they often develop various diseases.

Several years ago, plant doctors studied these deficiencies and learned not only how to recognize the affected plants, but also that they could be corrected by spraying them with the mineral in which the plant was deficient. But what average gardener has the training that enables him or her to recognize deficiency symptoms in plants?

To overcome this problem, the nutritional spray was developed. It is a mixture that contains about all of the minerals in which a plant can be deficient. Some plants are more subject to mineral deficiencies than others.

Especially vulnerable to mineral deficiencies such as dieback, mottled leaf, small or deformed leaves and yellow leaves are hibiscus, gardenia, mock orange, ixoras, mangoes, avocados, macadamia, coffee and citrus.

In new gardens, it might be necessary to apply a nutritional spray about every three months for the first year in order to keep ahead of deficiencies.

Along with the nutritional spray, it is a good idea to use a soil application of other elements. Magnesium and sulfur are the most important, but occasionally we find plants with boron, manganese, copper and other trace element deficiencies.

There are several combinations available at your garden supply store. Certain plants require larger amounts of the trace elements than other plants.

You will find, for example, that iron is especially important on ixoras, hibiscus, azaleas and gardenias, or that magnesium keeps leaves of coconut and areca palms from getting orange colored and dying prematurely. Zinc is the vital element in growing queen palms, royal palms and palms in the date group.


Increasing your soil’s organic matter and using minor element treatment as a spray or soil application or both will keep your plants from having these deficiencies under most conditions.

And remember to follow directions on the label. Too much of the important plant nutrient materials can be as bad as than too little.

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