Factors in Soil Formation

Soil is defined as that part of the unconsolidated material covering the surface of the earth which supports plant growth. It has three major constituents: solid particles (salts, minerals and organic matter), air and water. The type of soil formed is a function of topography, climate, vegetation, and the parent rocks from which the soil material is derived. Soil material transported and deposited by running water is termed alluvium, while that transported and deposited by winds forms aeolian soil. Soils formed in situ are termed residual.

Soil texture varies with the size of the soil particles. Coarse textured soils are sandy, fine textured soils are clayey, and a mixture of sand and clay is called loam. The organic content of the soil also varies, being largely dependent of the extent and type of the vegetative cover. Soils of high organic content are darker in colour, and have more nutrients for plant growth than those of low organic content. Since most of Pakistan is arid or semi-arid, the soils contain little organic matter.

Soil-forming processes are complex and continuous. As a result, soils vary in their chemical composition, colour, texture, and organic content from place to place, even within small areas. The ensuing discussion describes only the major soil-groupings of Pakistan.

Soils of Pakistan

Indus Basin Soils

The Indus Basin comprises a vast area of alluvial plains deposited by the Indus and its tributaries, and a small area of loess plains. Most of the material is sub-recent or recent in origin, calcareous, and low in organic content. The soils cab en divided into three major categories: Bangar Soils (old alluvium); Khaddar Soils (new alluvium); and Indus Delta Soils.

Bangar Soils cover a vast area in the Indus Plain, including most of the Punjab, Pewahwar, Mardan, Bannu and Kachhi plains, and the greater part of the Sind Plain. These soils are deep, calcareous, of medium to fine texture, low in organic matter, but very productive when irrigated and fertilized. In some ill-drained areas, these soils have become waterlogged, and capillary action has carried salts to the surface. Some areas show a puffy salt layer at the surface, but these can be reclaimed by simple leaching, if supplied with plenty of irrigation water. Over very small areas, strongly alkaline soil of patches have developed, and these, being non-porous are difficult to reclaim.

In the Upper Chaj and Rechna Doabs, the submontane area bordering the Peshawar-Mardan Plain, and in the eastern Potwar, the Bangar soils have developed under sub-humid conditions. Because of the higher rainfall, they have been leached of lime and are non-calcareous, medium to fine textured, and have a slightly higher organic content. These soils are also fertile when supplied with plenty of water and manure.

Khaddar soils are formed from recent and present-day deposits along the rivers. Part of these soils are flooded each year, adding depositional layers of silt loam and silty clay loam. The organic content of these soils is low, but they are usually free of salts.

Indus Delta Soils are formed of sub-recent alluvium and estuarine deposits. They cover the entire area of the Indus Delta from south of Hyderabad to the coat. Clayey soils, developed under flood water conditions, cover about one-third of the area. With irrigation, these soils are used for rice cultivation. Saline loamy soils cover most of the delta. Some with salt crust at the surface, have been reclaimed by simple leaching and better drainage. Extremely saline patches can be used only for poor grazing.

Coastal estuarine deposits form the lower part of the Delta, which is a maze of tidal flats, basins, and sea-water creeks. The soils are extremely saline and barren, except for a weedy vegetation.

Mountain Soils

Mountain soils occur in the highland areas of the north and west, and are residual as well as transported. Along the steep crests and slopes, and in the broken hill country, shallow residual soils have developed. Under arid and semi-arid conditions, these soils are usually strongly calcareous, with low organic content. Further north, under sub-humid conditions, there is more leaching, and a higher organic content.

In the mountain valleys, soils are formed from the alluvial infills of the streams. These soils are calcareous silt loams and sandy loams of low organic content. They are cultivated in patches only.

In the sub-montane area of the Potwar Plateau, shallow residual soils and silty eroded loess have been formed. In places these soils are massive, susceptible to erosion, and strongly gullied, producing a dissected landscape. Lime content is high, and organic content low, but, with plenty of water, these soils are relatively productive.

In the lowest parts of the inter-montane valleys and interior basins of the arid and semi-arid regions, strongly saline soils develop. Excess of evaporation over precipitation leaves a thick crust of salts at the surface of the intermittent lakes. For the most part, these soils are barren. The margins carry low shrubs and salt bush, used for poor grazing.

Sandy Desert Soils

The soils extend over some parts of western Baluchistan, and the Cholistan and Thar Deserts. Thal desert soils occur in large sections of the Sind Sagar Doab. Desert soils include rolling to hilly sandy soils, and clayey flood plain soils. Where the soils are formed of deep sand, as in much of Baluchistan, they are moderately calcareous, and largely aeolian. In places, the windblown material is mixed with old alluvium. The arid and semi-arid desert sand areas have few possibilities for improvement, beyond very poor grazing.