Agriculture Reference
In-Depth Information
mAnAgIng soIl stRuctuRe And enhAncIng AggRegAtIon
There exists a strong and positive correlation between SOM pool and soil aggrega-
tion. Formation of stable micro- and macroaggregates is one of the several positive
impacts of enhancing and managing the SOM in soils of agricultural and other man-
aged ecosystems. Improvement in soil aggregation is essential to improving edaphic
environments critical to plant growth and productivity. Increase in amount and sta-
bility of aggregates enhances aeration, improves water retention and transmission,
increases CH
4
oxidation and soil sink capacity for absorbing CH
4
, reduces emis-
sion of N
2
O, strengthens the mechanism of elemental cycling, and enhances overall
soil resilience (Figure 18.5). The attendant improvement in soil tilth enhances root
growth and proliferation, increases use efficiency of input (water, fertilizer, energy),
and accentuates net ecosystem productivity (Figure 18.5).
Soil structure and tilth are intimately linked to the choice of tillage methods, resi-
due management, use of cover crops in the rotation cycle, frequency and weight of
vehicular traffic, and application of biosolids (e.g., manure, crop residue mulch, sew-
age sludge). Conversion of plow tillage to no-till farming, use of crop residue mulch,
and application of manure improve soil aggregation, total and macroporosity, and
water transmission (infiltration) and retention (available water) capacity. Soils with
favorable structure are less prone to crusting, surface sealing, compaction, and hard
setting. Removal of crop residues for other uses (e.g., fodder, fuel, and construction/
industrial material), excessive tillage, use of heavy machinery, and excessive and
uncontrolled grazing cause a decline in aggregation and susceptibility to physical
degradation. Maintenance of soil structure is important to sustainable management
of agricultural soils.
nutRIent mAnAgement
Soil fertility depletion and chemical degradation are associated with the use of
extractive farming practices. Soils of sub-Saharan Africa and those of South Asia
under dry farming are severely depleted of their inherent fertility because plant
nutrients harvested in crops and animal products are not replaced through appli-
cation of chemical fertilizers or organic amendments. It is widely recognized that
soils of sub-Saharan Africa have been subjected to negative nutrient balance at the
rate of 30 to 40 kg of nitrogen/phosphorus/potassium hectare per year for decades
because of the extractive farming practices. The data in Table 18.11 show that fertil-
izer consumption over the 40-year period (1961-2000) increased by a factor of 4.4 in
the world (from 31.2 million tons in 1960-1961 to 136.4 million tons in 2000-2001),
43.2 in South Asia (from 0.5 to 21.6 million tons), 14.7 in East Asia (from 3.1 to 45.5
million tons), and 13.2 in Latin America (from 1.0 to 13.2 million tons). In contrast,
however, fertilizer consumption in sub-Saharan Africa increased from 0.16 million
tons in 1960-1961 to 1.2 million tons in 2000-2001 (by a factor of 7). In addition to
the low rate of fertilizer use, the nutrient use efficiency is also low because of poor
soil structure, susceptibility to crusting and compaction, and high losses by runoff,
erosion, volatilization, and leaching.
