Soil Science: Formation, Classification and Pakistani Soils

Soil is the thin, weathered surface layer of the Earth — a complex mix of mineral particles, organic matter, water, air and living organisms — that supports terrestrial plant life. Soil science (pedology and edaphology) is foundational to agriculture, forestry and land planning.

Soil-forming factors (Jenny, 1941)

Hans Jenny summarised soil formation as S = f (Cl, O, R, P, T):

• Climate — precipitation, temperature; the dominant factor over long timescales.
• Organisms — vegetation, micro-organisms, fauna; humus formation.
• Relief / Topography — slope, drainage, aspect.
• Parent material — rock or sediment from which soil develops.
• Time — duration of pedogenesis.

Soil profile

A vertical section of soil shows horizons:

Horizon	Common name	Description
O	Organic	Litter, partially decomposed organic matter
A	Topsoil	Mineral with organic matter, biologically active
E	Eluviated	Leached, light-coloured (in forest soils)
B	Subsoil	Illuviated clays, oxides; structural development
C	Parent material	Weathered rock fragments
R	Bedrock	Unweathered rock

Texture and structure

Texture is the proportion of sand (2.0–0.05 mm), silt (0.05–0.002 mm) and clay (< 0.002 mm) — USDA classes. The textural triangle plots these.

Structure is the arrangement of particles into aggregates: granular, blocky, prismatic, columnar, platy, massive, single-grained.

Key chemical properties

• pH — Pakistani soils are largely alkaline (pH 7.5–8.5), calcareous.
• CEC (cation exchange capacity) — clay and humus content drive nutrient holding.
• Organic matter — typically very low in Pakistan (< 1%), against an ideal of > 2%.
• EC (electrical conductivity) — measures soluble salts.
• ESP (exchangeable sodium percentage) — sodicity indicator.
• Macronutrients: N, P, K, Ca, Mg, S.
• Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.

Salinity and sodicity

A defining issue in Pakistan:

• Saline soil — high soluble salts; EC > 4 dS/m; structure intact.
• Sodic (alkali) soil — ESP > 15; pH > 8.5; structure destroyed.
• Saline-sodic — both problems together.
• About 6.3 million ha in Pakistan are salt-affected (SCARP surveys).
• Reclamation: gypsum (for sodic), leaching with good-quality water, drainage (SCARP), salt-tolerant crops (Kallar grass, Leptochloa fusca).

Major soils of Pakistan

Region	Soil type	Key features
Indus plains (Punjab, Sindh)	Alluvial — bangar and khaddar	Recent and old alluvium; fertile, calcareous
Pothwar Plateau	Loess / loessial	Wind-deposited, prone to erosion
Thal, Cholistan, Thar	Sandy desert soils (Aridisols)	Low OM, low water-holding capacity
Western dry mountains	Skeletal lithosols	Thin, stony
Northern mountains	Mountain soils (Inceptisols, Entisols)	Young, slope-bound
Indus delta	Saline marine deltaic	High salt, near sea

USDA classification of Pakistani soils (1968 survey, revised):

• Aridisols — most of arid Pakistan.
• Entisols — recent alluvium, no profile development.
• Inceptisols — weakly developed.
• Aridisols with calcic horizons — Indus terraces.
• Mollisols — limited, in some uplands.

Soil fertility management

• Crop rotation with legumes (gram, lentil, berseem) to fix nitrogen.
• Green manuring (dhaincha — Sesbania; cowpea).
• Farmyard manure and compost.
• Balanced fertiliser — Pakistani N:P:K is heavily N-skewed; target ~2:1:0.5.
• Foliar micronutrient sprays (Zn for rice, B for cotton).
• Conservation tillage and mulching to retain moisture and organic matter.

Soil degradation in Pakistan

1. Water erosion — Pothwar barani belt.
2. Wind erosion — Cholistan, Thar.
3. Salinity/sodicity — irrigated plains.
4. Waterlogging — areas with poor drainage and shallow water table.
5. Loss of organic matter — residue burning and over-tillage.
6. Urban encroachment — Lahore-Karachi peri-urban farmland.

Soil survey and mapping

• Soil Survey of Pakistan (SSP) — established 1953; produced reconnaissance maps in the 1960s.
• National Reference Soil Information Centre (NRSIC) at NARC.
• Modern remote-sensing combined with field sampling is now standard.