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How to Handle Biomass in Dry Tropical Systems: Mulch Pit Gardens

The dry tropics cover a significant land area of the planet, particularly around the regions of the tropics of Cancer and Capricorn. Characterized by a majority of the year when evaporation potential is greater than rainfall, they also support rapid biomass growth during and following the rainy season. Legume species normally form a significant portion of the species present, and provide for rapid biomass production.

Management of this biomass can be tricky, particularly when left above ground in dry mulch piles, as it normally stays dry, inhibiting both fungal and bacterial breakdown. On the flip side, dry tropics soils, whether sandy or clay-based are in need of organic matter to balance structure, enhance water retention or drainage and build humus. One approach for creating such conditions are mulch pit gardens.

Papaya, banana, and coconut circles are developed by digging pits up to two meters in diameter (for papaya or banana – up to three meters for coconuts) and about 1 meter deep. These are then filled with dampened, compacted organic material to a height of 1 meter above ground. Up to seven plants of the appropriate type are then planted in the rim of the pit. Taro or other moisture loving plants may be planted on the inside edge, and sweet potato along the outside edge to provide a living mulch as well as extra production.

Double mulch pit greywater system being developed at Baja BioSana, Baja

The breakdown of the organic material in the pit provides nutrition for the plants growing on the perimeter. Waste grey water can be piped into such systems, where the abundance of free carbon and rich microbial activity contributes to the locking up of free phosphorus and other elements in the grey water.

The critical component to successful management of such systems is maintaining the biomass of mulch within the pit. Especially in the tropics, breakdown is rapid and fresh organic material will be required every few days. Some cultures are particularly fond of sweeping around houses and yards, and such materials can easily be placed inside mulch pits.

Mulch pit gardens allow for the stockpiling of a large amount of otherwise difficult to manage biomass in a convenient location. The moisture conservation nature of a pit and stacked layers of biomass promote conditions for fungal, bacterial and insect breakdown of materials, as well as compost worms. This speeds up by many times the above ground breakdown cycle, producing humus in a timeframe of months rather than years. Large and difficult to manage biomass, as well as thorny biomass can be included in the mulch pit garden.

The photo below is of a double mulch pit garden (also called an ‘infiniti’ garden) on Molokai that was established in November of 2009, at which time bananas and coconuts were planted, along with croatalaria and pigeon pea. The system processes household grey water.

Despite nearly constant, punishing trade-winds, and heavy deer browsing pressure, the bananas are throwing out their first bunch of fruit, and withstanding conditions in which they would normally not thrive. All appear healthy and are producing suckers despite wind-shredded leaves.

Double mulch-pit garden at Nash residence, Moloka’i

As mentioned, species typically used in mulch-pit gardens include banana, papaya and coconut. Bananas are not normally mixed with other species, however, as their management requires that they ‘march’ around the pit over time. Additional experimentation is taking place in Baja with palms — both dates and native Washingtonia palms as the main species around the mulch pit.

Developing guilds around the major species involves thinking about the different functions that are required in the guild — aromatic or visual pest confusers, dynamic mineral accumulators, beneficial insect attractors, nitrogen fixers, and ground covers. In the Baja context, we are experimenting with Mexican marigolds as aromatic pest confusers (for both gophers and insects), croatalaria and pigeon pea for nitrogen fixation, sweet potato as ground cover (although it suffers if too exposed in summer heat) and Aloe Vera as a mineral accumulator/living mulch.

For those of you interested in taking a Permaculture Design Course in the dry tropics, we are offering a PDC to be held at Buena Fortuna Botanic Park in Baja California Sur, Mexico. This will kick off on December 6 – 19, 2010, and will be bi-lingual in both English & Spanish.

Recognized international teachers: Penny Livingston-Stark, Andrew Jones, Daniel Jaramillo, Bruce Horowitz, Gabriel Howearth, plus local guest presenters will be involved. Learn how to be part of the change the world needs so urgently, have fun networking and enjoying with like minded people from close and afar, discover the magic of the surrounding Sea of Cortez and the Sierra de La Laguna Mountains and feast on delicious local organic food.

Check out the website for more information and registration details.


  1. I’ve always read that you shouldn’t mix grey water systems with root crops to minimize possibilities of food contamination.

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