Fire - Agroecology: The Ecology of Sustainable Food Systems - page 138

FIGURE 10.6 Burned slash piles in Chiapas, Mexico. When biomass production is limited by climate or short fallow, slash can
be piled for burning and the ash spread.

the replanting of new cacao seedlings. Bananas are
harvested up to the time the new cacao plants come
into production (5 to 7 years after planting), at which
point the renovation cycle is complete. Local farmers
claim that without the use of fire, it would be at least
10 years before cacao could begin to be replanted on
such a site — a long time to wait for this valuable cash
crop. Research is needed to tell us exactly how fire
benefits this agroecosystem.

amount to between 0.4 and 0.67 kg/m 2 , it has significant
potential as a nutrient input to agroecosystems (Ewel
et al., 1981; Seubert et al., 1977).
Of course, being so soluble, these nutrients can
easily be washed out of the system, so effective plant
cover and good root development should accompany
the addition of nutrients from ash. Timing of ash appli-
cation is very important. There must be active plant
roots in the soil to rapidly take up the highly soluble
nutrients. And knowledge of rainfall patterns is needed
to avoid having heavy rains follow burning or ash appli-
cation, so that nutrients are not leached below the root
zone or washed off the surface. Research is needed that
determines which crop systems or combinations can
best take advantage of fire-released plant nutrients.

Nutrient Additions to the Soil

In many cropping systems in the world, the ash left after
burning crop residues, noncrop slash, and even wood
for cooking or heating is seen as a valuable nutrient
source that should be returned to the soil. Ash is quickly
carried into the soil with rainfall and the nutrients it
contains are readily available as part of the soil solution.
The loss of nitrogen and sulfur to volatilization during
burning is more than offset by a gain in all other nutri-
ents and by an increase in their availability to plants.
Ash has been shown to contain as much as 2.6% potas-
sium, and appreciable amounts of phosphorus, calcium,
magnesium, and other mineral elements. Since ash can

Crop Residue Management

Fire is often used as a tool for crop residue management.
One of its main benefits is to make nitrogen from the
residue more easily available to the following crop. When
the residue is very high in carbon as compared to nitrogen
(C/N 25 to 100), the nitrogen in the residue can be immo-
bilized by incorporation into microbial biomass (and then