Composting and mulching to build healthy soils from scratch

  • The eruption at 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. (Photo courtesy / Kyle Mcwhirter)

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 like 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 like 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 may also 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 two cups of balanced fertilizer like organic 8-8-8 and one 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, mix with good water retention capability and yet good drainage. A good mix needs to be able to anchor the roots and stem so that 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 may be deficient in the minor elements like 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 learned that they could be corrected by spraying 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 are 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 may 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. As you get acquainted with our tropicals, 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 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. 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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