Page 139 - Soil Degradation, Conservation and Remediation

128 5 Chemical Soil Degradation

of evapotranspiration for the effective downward movement. So, leaching is an
important process of nutrient removal in the humid regions. Leaching occurs even in
natural vegetation, but agricultural activities can greatly increase leaching losses
(Havlin et al. 1999). Soils with high water infiltration rates and low nutrient retention
capacity such as sandy soils and well-structured ferralitic soils (Oxisols, Ultisols)
with low activity clays and low organic matter contents are particularly susceptible
to nutrient leaching. Nutrients differ in their mobility in soil; nitrates are more mobile
than other anions. Nitrate ions are not attracted by the negatively charged matrix of
the top soil. Nitrates are continuously produced in soil by nitrification of ammonia
obtained from fertilizers and soil organic matter. Nitrate leaching is a serious problem
in sandy and low retentive soils (Robertson 1989). It often pollutes the groundwater
and water of open reservoirs around agricultural landscapes. Sulfates are also leached
easily from agricultural soils, but phosphates are relatively immobile. Soil colloids
and calcium, aluminum, iron, and manganese immobilizes phosphates. The soil
solution that percolates through the soil profile is essentially electrically neutral; it
contains anions and equivalent amounts of cations. So, cations are also leached, and
the more susceptible cations are calcium and magnesium than potassium (Pieri 1989).
In sandy soils, considerable amounts of magnesium can be leached after applications
of potassium chloride or potassium sulfate fertilizers (Havlin et al. 1999). Potassium
is leached in much lower amount than calcium and magnesium.

5.2.2 Nutrient Loss due to Residue Burning

When biomass residues, crop residues or litter, are burned, most nitrogen (N) and
sulfur (S) in the residue are lost, while mineral nutrients, such as phosphorus (P)
and potassium (K), are retained. A Manitoba laboratory studied wheat, oat, and flax
residue burned in an uncovered container (Heard et al. 2001). After the burn, the
remaining ash was collected and weighed. Much of the N and S was oxidized
and lost as volatile gases, while the mineral elements (P and K) remained in the ash.
The study showed N losses of 98–100 %, S losses of 75 %, P losses of 21 %, and K
losses of 35 %. It was concluded that the P and K loss resulted from smoke and ash
that escaped from the burn container. In a field burn, some of this ash may be
redeposited onto the field, depending on wind and other environmental factors.
Other mineral nutrient loss could be assumed to be similar to P and K. The weather
conditions after the fire often play an important role in nutrient loss from the field.
For example, high winds can blow ash from the field or can pile the ash into drifts.
Shifting cultivators burn their biomass residues to supply nutrients to their crops.
Such nutrients have actually temporary benefit because heavy rains following land
preparation often remove the bases released due to burning. In addition, soil
erosion from both wind and water can occur more readily when the residue cover is
destroyed and the soil is left exposed. A western Canada study where the cereal
residue was burned annually for 19 years showed average annual soil organic matter
declines of 0.03 and 0.07 % (Biederbeck et al. 1980).