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Problem soils of the Asia Pacific Region
When considering the natural constraints to agricultural production the soils of the Asia Pacific Region can be subdivided into 12 categories as follows (Dent 1990):
A. Too Cold
land areas with a 24-hr mean temperature of less than 5oC during the growing period*. Within the FAO Asia Pacific Region land areas identified as too cold are also too steep (see category C).
*Growing period is defined as the duration in days when both soil moisture and temperature conditions permit crop growth.
B. Too Dry
desert and semi-desert (excluding cold deserts which are included under category A) which are either rainless/dry or with growing periods of between 1 and 74 days duration.
C. Too Steep
land areas neither cold nor dry; but which are dissected and with slopes in excess of 30%.
D. Too Shallow
land areas which are not too cold, dry or steep but where rooting depth is limited within 50cms of the surface by the presence of coherent and hard rock or hard-pans.
E. Too Wet
land areas which are not cold, dry, steep or shallow; but which are waterlogged and/or flood for a significant part of the year. Poorly drained saline/sodic, acid sulphate and peat soils are excluded from this category because of the special nature of their constraints, and are considered under categories I, J and K respectively.
F. Too Coarse
land areas which are not cold, dry, steep, shallow or poorly drained; but which are coarse textured (less than 18% clay and more than 65% sand) or have gravel, stones, boulders or rock outcrops in surface layers or at the surface.
G. Vertic (Heavy Cracking Clay)
land areas which are not cold, dry, steep, shallow, poorly drained or coarse textured; but which have 30% or more clay to at least 50cm from the surface (after the upper 20cm of soil are mixed), with cracks at least 1cm wide at 50cm depth at some period in most years (unless irrigated), and high bulk density between the cracks.
H. Infertile
land areas which are not cold, dry, steep, shallow, poorly drained, coarse textured or heavy cracking clays; but which, to a greater or lesser degree, exhibit deficiencies in major, secondary and minor plant nutrients when cultivated.
I. Too Salty (Saline/sodic limitations
land areas comprised of soils with a high salt content or exchangeable sodium saturation within 100cm of the surface.
J. Acid Sulphate
land areas comprised of soils in which sulphidic materials have accumulated under permanently saturated, generally brackish water conditions. Upon drainage the sulphides oxidise to form sulphuric acid; and the pH, which is around neutral prior to drainage drops below 3.5.
K. Peat
land areas in which more than half of the upper 80cms is composed of organic materials saturated with water for long periods of time or artificially drained.
L. No Constraints
land areas with no physical constraints to sustained agricultural production.
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