Compost, mulch, and other sources of fertility
Healthy, mulched soil is dark, has good structure, and holds moisture (left), whereas bare soil has poor structure and loses moisture faster.Decomposing organic materials are the primary source of fertility in tropical organic gardens. The richest source of organic materials is plant matter such as tree and garden trimmings, grass clippings, weeds, and kitchen scraps. These can be used directly in the garden as mulch or indirectly after composting. Mulch is a layer of decomposing organic matter on the soil surface. Mulching improves nutrient and water retention in the soil, encourages favorable soil microbial activity and worms, and suppresses weed growth. When properly done, mulching can significantly improve the well being of plants and reduce maintenance as compared to bare soil culture.
Composting is a controlled, intensive decomposition of organic matter into a form that is rich in available nutrients. The composting process involves a minimum volume of organic material with a certain ratio of nitrogen-rich (green leaves, manure) and carbon-rich (woody) materials at the appropriate moisture and air content. When done correctly, compost generates heat from intensive microbial activity until it decomposes into a dark, rich, finished humus. Compost can either be mixed into garden soil, or used as mulch, preferably underneath other mulch materials so that it does not dry out.
Composting can turn yard trimmings into nutrient-rich humus.Both mulching and composting are easy to carry out, but there are some principles to learn. See the resources below for further information.
Other sources of fertility include vermicompost, animal manure, and ground covers. Vermicompost is an ambient temperature compost where worms do much of the decomposition of the organic matter. Animal manures from poultry, rabbits, goats, and other livestock are rich sources of nutrients. Ground covers, especially leguminous plants, build fertility as they grow.
Finally, sometimes it is necessary to supplement soil nutrients that are lacking with mineral amendments that are brought in from elsewhere. These include crushed coral (calcium), rock phosphate and bone meal (phosphorous sources), green sand and wood ash (potassium source) and basalt rock powder (wide range of minerals). The need for these amendments can be determined by nutrient deficiency symptoms in your plants, a plant tissue analysis, or a soil test.
Books
How to Have a Green Thumb Without an Aching Back: A new method of mulch gardening by Ruth Stout and Leta Macleod Brunckhorst. 1990. Fireside. Describes gardening with mulch in a fun, narrative style.
The Rodale Book of Composting: Easy methods for every gardener edited by Grace Gershuny and Deborah L. Martin. 1992 (revised edition). Rodale Books. An updated version of Rodale’s Complete Book of Composting (1960), this book covers all aspects of composting.
Teaming with Microbes: A Gardener’s Guide to the Soil Food Web by Jeff Lowenfels and Wayne Lewis. 2006. Timber Press. An excellent guide to soil health from a biological perspective.
Worms Eat My Garbage: How to set up & maintain a worm composting system by Mary Appelhof. 1997 (revised edition). Flower Press. A seminal work in worm husbandry at home.
Craig Elevitch is director of Hawai'i Homegrown Food Network and an educator in agroforestry. His books include Agroforestry Guides for Pacific Islands (2000), The Overstory Book: Cultivating Connections with Trees (2004), Traditional Trees of Pacific Islands: Their Culture, Environment, and Use (2006), and Specialty Crops of Pacific Islands (2011) all of which promote diverse agricultural systems that produce abundant food.