K(X)I,O~;Y oF P r m IN QHAHYLANDH
227
the noii owing txj t,hc a h n d a n c e of alkaline earth metals (Ca, Mg, K)
in plant mh, and this in turn undoubtedly alters the solubility relation6
of Roil constituentu, needed nutrients and toxic substances alike, and
alters the activity of Nitrogen fixers. However, the degree of change
within the pH range characteristic of grasslands is pmbably of little
significance, and usually persists only a year or two.
No consistent change of pH could be detected in the top 25 mm of
soil following burning of Boutelmu steppe in southeastern Arizona
(Reynolds and Bohning, 1956). Neither could a change in pH be detected in burned drtemisia tridentata steppe in eastern Idaho, at least
when measured 2 year after the burn (Blaisdell, 1963).
I n lime-deficient British grass-heaths where Agrostia, Festuca, Holcus
and Deschamnpsia are conspicuous dominants, the soil of recently burned
areas was rendered very acid, then became less acid aa vegetational
readjustments proceeded (Eden, 1924). This anomaly was paralleled by
a temporary increase in humus content of the burned land.
3. Macronutrievts other than Nitrogen
Except for volatilizing N and S, fire results in no direct loss of
nutrients from an eoosystem, but rather effects an abrupt release of
elements that normally would have become available gradually in consequence of the slow decay of plant litter. However, all this fertility is
made available at once at the soil surface, where it is subject to horizontal displacement by wind or water, or perhaps to loss by leaching
through the soil profile too quickly for i t to be adsorbed on colloids or
taken up by soil organisms.
In Nigerian savanna, mild fires coming at the start of the dry season
increase cation exchange capacity, available P, exchangeable Ca, Mg,
and K, and the per cent base saturation (Moore, 1960). But hot fires
coming late in the dry season reduce the cation exchange capacity,
exchangeable Ca and K, with available P and the exchange capacity
remaining unaltered and the per cent base saturation slightly increased.
In Kenya the base exchange capacit.y of regularly burned grassland
was found rcdiiced, presumably in consequence of a reduction in the
humus content of the Roil (Edwards, 1942). Coutt,s (1945) found that
humus colloids sttsrt to be destroyed by heat in the range 1O0-25O0C,
with destruction of clay colloids startring between 260-450°C, the critical
values varying with soil type. However, even at 500OC base exchange
capacity is reduced by only about 20%, and since even the surface of the
soil is seldom hcated to this level, he concluded that the effect of veld
burning on httnr: exchange: ciipacitg is very small and probably transitory.
On the CoaRtal Plain of southeastern North America the frequent
burning of gram beneath opcn stand of pine tends to raise the
227
the noii owing txj t,hc a h n d a n c e of alkaline earth metals (Ca, Mg, K)
in plant mh, and this in turn undoubtedly alters the solubility relation6
of Roil constituentu, needed nutrients and toxic substances alike, and
alters the activity of Nitrogen fixers. However, the degree of change
within the pH range characteristic of grasslands is pmbably of little
significance, and usually persists only a year or two.
No consistent change of pH could be detected in the top 25 mm of
soil following burning of Boutelmu steppe in southeastern Arizona
(Reynolds and Bohning, 1956). Neither could a change in pH be detected in burned drtemisia tridentata steppe in eastern Idaho, at least
when measured 2 year after the burn (Blaisdell, 1963).
I n lime-deficient British grass-heaths where Agrostia, Festuca, Holcus
and Deschamnpsia are conspicuous dominants, the soil of recently burned
areas was rendered very acid, then became less acid aa vegetational
readjustments proceeded (Eden, 1924). This anomaly was paralleled by
a temporary increase in humus content of the burned land.
3. Macronutrievts other than Nitrogen
Except for volatilizing N and S, fire results in no direct loss of
nutrients from an eoosystem, but rather effects an abrupt release of
elements that normally would have become available gradually in consequence of the slow decay of plant litter. However, all this fertility is
made available at once at the soil surface, where it is subject to horizontal displacement by wind or water, or perhaps to loss by leaching
through the soil profile too quickly for i t to be adsorbed on colloids or
taken up by soil organisms.
In Nigerian savanna, mild fires coming at the start of the dry season
increase cation exchange capacity, available P, exchangeable Ca, Mg,
and K, and the per cent base saturation (Moore, 1960). But hot fires
coming late in the dry season reduce the cation exchange capacity,
exchangeable Ca and K, with available P and the exchange capacity
remaining unaltered and the per cent base saturation slightly increased.
In Kenya the base exchange capacit.y of regularly burned grassland
was found rcdiiced, presumably in consequence of a reduction in the
humus content of the Roil (Edwards, 1942). Coutt,s (1945) found that
humus colloids sttsrt to be destroyed by heat in the range 1O0-25O0C,
with destruction of clay colloids startring between 260-450°C, the critical
values varying with soil type. However, even at 500OC base exchange
capacity is reduced by only about 20%, and since even the surface of the
soil is seldom hcated to this level, he concluded that the effect of veld
burning on httnr: exchange: ciipacitg is very small and probably transitory.
On the CoaRtal Plain of southeastern North America the frequent
burning of gram beneath opcn stand of pine tends to raise the