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.
Stone
Mulching
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).
Pic
on
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.
For further details of this deceptively subtle pre-scientific technology see http://harialanzarote.com/stone_mulching.htm.
Lanzarote, Canary Islands
After his visit to Haria Fred Pearce wrote this excellent article for New Scientist:
