Classification system for problem soils of the Asian Network on Problem Soils

The expert consultation of the Asian Network on Problem Soils at Bangkok, Thailand, 29 August to 1 September 1989, agreed upon the following definitions of problem soil categories:

A - Cold: land areas with a 24-hr mean temperature of less than 5°C during the growing period.

B - Dry: desert and semi-desert areas (excluding cold deserts which were included under category A) with growing periods which are either rainless-dry or with a length of between 1 and 74 days.

C – Steep: land areas neither cold nor dry; but which are steeply dissected with slopes in excess of 30%.

D – Shallow:  land areas which are not cold, dry or steep; but which have depth limitations within 50 cm of the surface caused by the presence of coherent and hard rock or hard-pans.

E – Poor drainage: land areas which are not cold, dry, steep or shallow, but which are waterlogged and/or flooded 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 – Coarse texture: land areas which are not too cold, dry, steep, shallow or poorly drained; but which have coarse texture with 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 – Heavy cracking clays: land areas which are not too cold, dry, steep, shallow, poorly drained or coarse textured; but which have 30 % or more clay to at least 50 cm from the surface after the upper 20 cm of soil are mixed, cracks at least 1 cm wide at 50 cm depth at some period in most years (unless irrigated), and high bulk density between the cracks.

H – Severe fertility limitations: land areas which are not cold, dry, steep, shallow, poorly drained, coarse textured or composed of heavy cracking clays; but which, to a greater or lesser degree, exhibit deficiencies in major, secondary and minor plant nutrients when cultivated.

I – Saline/sodic limitation: land areas comprised of soils with a high salt content or exchangeable sodium saturation within 100 cm of the surface.

J – Acid sulphate limitations: land areas comprised of soils in which sulphidic materials have accumulated under permanently saturated, generally brackish water conditions. Upon drainage the sulphides oxidize 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 80 cm 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.