Stone Mulching

This is the text of an article which first appeared in Clean Slate magazine # 14 (the journal of the Alternative Technology Association)

David Riebold uncovers a layer of traditional dry climate farming techniques, which may hold water for many arid areas.

What is Mulch ?

A mulch in gardening or farming practice is any kind of material laid on the surface of the ground. Conventional agriculture and horticulture have always had a penchant for ‘clean, bare ground’, but this is extremely rare in nature, where soil is nearly always covered by vegetation and/or a layer of decaying plant matter. The various schools of ‘organic’ practice have all emphasized the desirability of keeping the~ soil covered as much as possible.

The type of material used varies according to availability and the specific functions required, The most common reasons for applying mulch are: suppressing temperatures; protecting soil from physical damage by the elements; reducing nutrient loss through leaching; stimulating the activity of soil organism; preparing ground for sowing or planting; and to improve appearance, Nutrient-rich mulches can also serve as slow release fertilizers:.

The normal response of governments and aid agencies to the problem of limited agricultural water in hot arid zones is to exploit new sources; a dam or borehole feeding a new irrigation scheme. This has frequently resulted in disaster; large dams have triggered earthquakes while boreholes have caused subsidence and contamination of aquifers with sea water. Diseases such as malaria and schistosomiasis often follows irrigation canals. High evaporation rates (75% is considered normal) cause a fatal build up of salt. Countless acres of once fertile land have been ruined in this way.

There are, of course, alternatives to this, which make better use of existing supplies. One such technology, developed over past centuries by the farmers of Lanzarote, deserves to be more widely known.

Despite a climate comparable to the nearby Sahara desert, Lanzarote farmers produce a wide variety of food (fruit, maize, wheat, onions, potatoes, goat fodder etc.) using only a meager average annual rainfall of 150mm. This is made possible by the use of stone mulch, a protective soil covering at least 1.0cm deep. Most commonly this consists of lapilli’s. This is lightweight volcanic gravel known locally as picon. In one region a layer of coarse sand is exploited with similar results.

Preventing evaporation

Canary Island geographies and tourist guides often suggest that picon works by somehow extracting dew from the air and adding it to the soil. This is unlikely, as the theoretical maximum dewfall is far less than the minimum daily evaporation. What I believe to be the true explanation of picon’s remarkable effects is a little more complex (but at least conforms to the laws of thermodynamics).

Picon aids infiltration of a heavy rain but afterwards forms a capillary break which almost eliminates subsequent evaporation from the soil. This is so effective that a worthwhile crop can be grown in a prepared (i.e. weeded and leveled) stone mulched field even if the rains fail, using water stored in the soil from the year before.

In Lanzarote’s climate, no other type of mulching material compares in terms of preventing evaporation, high soil temperatures and erosion. Although picon adds no new minerals directly to the soil, it improves soil conditions, preserving soil life and so conserving existing nutrients.

Picon also forms a barrier against weed germination. Any weeds that do emerge can be removed by a very light hoeing of the picon; zero till cultivation is possible without herbicides. A hand plough, or one drawn by a single donkey or camel, is sufficient to make furrows for planting as these only penetrate the mulch layer.

In non-irrigated fields, there is no advantage to having a closed crop canopy to shade out weeds, so there is typically wide spacing of individual plants (appropriate to the amount of soil moisture). This in turn enables moist air around the sides of a plant to be constantly replaced; giving side (in addition to top) leaves the opportunity to directly absorb dew and mist, useful but overlooked sources of water. The importance local farmers place on these has been confirmed by experiments in Israel showing 100% improvement in crop yield attributable to foliar absorption of dew.

The wider potential

In terms of evaporation suppression, stone mulch techniques are probably of most relevance to hot regions where rainfalls low (500mm p.a.) and seasonal; picon can be a disadvantage with frequent light rain.

Picon and pumice, being very light, are Ideal materials, but supplies are limited to areas with recent volcanism. Any other available stone of particular Size, from coarse sand up to about 2cm diameter, may be worth trying.

Stone mulches may have a role in wetter climates for tree planting, soil temperature reduction and prevention of erosion, the latter benefit demonstrated by the endurance of picon fields with slopes of over 20 degrees, common on Lanzarote despite constant high winds and fierce winter storms.

The future for Lanzarote farmers

Cheap EC Imports have dealt a body blow to local farming. Unfortunately there is another problem ahead; when used for trees or grapevines picon seems to have an unlimited life, but when harvesting annual crops the picon becomes mixed with the underlying clay. This eventually spoils the capillary break on which the suppression of evaporation depends.

Depending on the type of crop and the amount of care taken, a layer of picon has a useful life of between 20-100 years. At present rates of use, picon supplies will run out m about twenty years. Any ideas for an economical method of renovating old picon would be timely. Both winnowing and washing are effective but obviously not practical on a large scale.

Whatever the future of picon cultivation, it’s a good reminder that alternatives to environmentally disastrous irrigation schemes are not only possible but may already be perfected by Ignored ‘peasants’. The world’s water crisis IS urgent. We cannot continue to ignore this wealth of indigenous technical knowledge. I have accumulated a sad little pile of scientific papers representing many years of research on mulches and evaporation, almost all of which merely duplicates information known on Lanzarote for centuries.

Several other fascinating local solutions to water shortages are described in a series of articles in New Scientist by Fred Pearce (25 Mar, I June, 7 Dec 1991); to borrow that author’s conclusion: in hydrology we may have to look backwards to make progress.

This letter was published in New Scientist, 14 May 2005, p. 29:

Harvesting fog

From David Riebold

Farmers here on Lanzarote value dew, but do not agree that the stone mulch they use works by feeding dew directly to crop roots (16 April, p. 52). Quick examination confirms that even the heaviest dew only moistens the surface of the mulch and that this rapidly evaporates during the day.

Deposition of dew (unlike fog or mist) on a surface is greatly limited by the need for the surface to lose the heat gained as water condenses, normally by radiation into a clear sky. Plants, especially in drought conditions, can take in water directly through their leaves, but the quantities available are of limited significance to a crop with a closed canopy. A stone mulch can, however, be so effective at preventing weed growth that it becomes practical to space the crop plants widely. Not only is this appropriate to Lanzarote’s rainfall conditions (equal to the nearby Sahara) but it also allows other parts of the plant to absorb dew directly through the leaves, measurably improving growth.

Lanzarote, Canary Islands

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