Tropical Gardening: Composting and mulching to build healthy soils

  • Courtesy of VOLTAIRE MOISE Many tropical trees growing naturally in very shallow soils have developed roots that run along the surface, such as this blue marble tree. The tree is growing on a 400-year-old lava flow from Hualalai. Forests growing in these conditions are terrestrial and at the same time lithophytic, growing on rock. This makes them susceptible to drought in that they have no tap roots to reach down to moisture.

The beginning of a new year and the death and destruction that occurred in 2020 teaches us that life in general goes in cycles.

Madam Pele manifests this phenomenon as Kilauea erupts again. With thousands of acres covered with lava in the past few years, 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. In these locations, it is a good to explore ways to conserve water as well.

Organic material also is essential to healthy growing conditions. Decomposed organic matter helps increase the soil’s water and nutrient holding capacity. Rotted material such as leaves and clippings used as surface mulch can help conserve moisture and keep unwanted plants under control.

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, we think of it as soil. However, if you wander the upslopes Hualalai or Kilauea, where lush forests of ohia and hapuu are the common vegetation you will notice the forest is actually growing on a shallow layer of organic matter.

We can achieve the same effect using the resources at our disposal. For example, 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.

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 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.

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 are others. 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 others.


Remember to follow directions on the label. Too much of the important materials can be as bad as too little!

Next week we will focus in more detail on plant nutrients and how to use them.

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