Soil and Land Development
The soil is the uppermost crust of the Earth’s surface which supports plant life. It is a product of rock weathering and biological processes. It is composed of solid particles and pore spaces filled with air and soil water between the particles. The constituents are mineral matter, water, air and organic matter both living and dead. The quantity of these constituents varies with the locality. Climate, vegetation and man have influenced the soil variations that are found. It gives anchorage to plants and is a store-house for plant nutrients and water.
- 1 Soils of Sri Lanka
- 1.1 Soil Classification
- 2 Maps
- 3 Soil science
- 4 Soil erosion
- 5 Soil conservation
Soils of Sri Lanka
Sri Lanka has a wide diversity of soil types. This diversity is chiefly due to climate and topographic factors. The mean annual rainfall varies from 1250 – 5000 mm in the south western quarter of the island to less than 1250 mm in the northwest and southeastern parts of the country. Based on the rainfall patterns, three geographic zones, dry, wet and intermediate zones have been identified. The topographical boundaries are low country 0 – 300m, mid country, 300-900m and the upcountry > 900m. The parent material of most soils has been dislocated and is considered to be colluvial and colluvial residuum.
Classification of soils based on morphological and other characters began with the work of Dokuchaev etal in the 19th century. The most accepted types of classification are those developed by of the United States Department of Agriculture (USDA) (Soil Taxonomy) and the FAO-UNESCO. The former is used due to its simplicity. In the USDA system there are 11 orders and a large number of suborders, great groups, sub-groups, series and families. In this discussion classification is confined to orders, suborders and great groups, only.
Fertility Characteristics of Dry Zone Soils
- Reddish Brown Earths (RBE)
- Non-Calcic Brown Soils (NCB)
- Low Humic Gley soils (LHG)
- Red-Yellow Latosols (RYL)
Reddish Brown Earths (RBE)
These soils are confined to the dry zone and occur in a catenary sequence, occupy the crest, the welldrained upper and mid slopes of the undulating landscapes. The colour of the surface is reddish brown when dry, turns to dark reddish brown when moist. A subsoil horizon with a high propotion of quartz gravel is present, the depth to the gravel layer is variable. Soils are extremely hard when dry, friable to firm when moist. Soil reaction is slightly acid to neutral. Base saturation is 60-80%. Soils are low in P but are reasonably high in K. They have a low water holding capacity with a rapid release of soil moisture at low tensions. The organic carbon content of the soils is low, 1-2%.
Non-Calcic Brown Soils (NCB)
Occur in most parts of North Kurunegala, Anuradhapura, East Polonnaruwa and East Moneragala and Batticaloa Districts. Colour of the surface soil is dark brown to dark greyish to red yellowish brown. These soils are shallow. The surface layers are sandy loams and the water holding capacity is low with rapid infiltration. During rain upwelling of water is frequent. Soil reaction is medium acid. These soils are poor in organic carbon and P content. Base saturation is 50-70% with high % of Ca 2+ in the bases. They are easily workable than the Reddish Brown Earths.
Low Humic Gley soils (LHG)
They are developed from colluvial deposits on the foot slopes of undulating landscapes. They are characterized by wetness or gleying throughout the profile. Soils are extremely hard when dry and sticky when wet, drainage is poor and base saturation of subsoil is 90-100%, soil reaction is moderately alkaline . They have a high CEC and are more suitable for rice but other crops can be grown with proper drainage.
Red-Yellow Latosols (RYL)
Occur in the North Western parts of the Puttalam and Jaffna peninsula and in few locations in the northern and south eastern parts of the country. These soils are deep with no clear horizon boundaries. The physical properties of these soils are good. Base saturation is 20-60% and soil reaction is medium acid. They have rapid infiltration and low water holding capacities. They are very low in plant nutrients as very little weatherable materials are found.
Alluvial soils (AL)
Are found on flat flood plains. The poorly drained alluvium shows a greyish colour; better-drained soils are brownish to yellowish brown. Soil reaction is slightly acidic to slightly alkaline. Base saturation is 60-90%. They have more organic carbon, P and K than RBE soils. Poorly drained soils are suitable for rice but in welldrained soils many arable crops can be grown.
Solodized -Solenetz (SS)
Their occurrence is related to the presence of sodium salts in the parent material. These soils have a columnar structured B horizon. They occur on estuaries and in inland valleys of the Mahaweli system B in the Kandakadu – Tirikonamadu area. These soils are poorly drained and are generally not suitable for profitable cropping, except with amendments.
They are sandy and are found along or near the coast line. Regosols found in areas of Kalpitiya, Nilavelly and South of Batticaloa district, have weakly humiferous horizons overlying yellowish or brown sand. Texture is fine sand to coarse sand. Infiltration is rapid ; underneath the sandy layer a film of fresh water often floats over saline water due to difference in densities. This enables growth of deep-rooted plants and irrigation of shallow-rooted crops. Soils are very low in soil organic matter content and plant nutrients. Generally, crops can be grown with high management.
These soils are formed from transported material mainly of alluvial nature. They are found in pockets of Murunkan, Hettipola (Matale District) and Ambewila ( Ratnapura District). Soils are black to very dark gray brown with a higher percentage of clay. The clay is of the expanding type and cracks during dry seasons. A low amount, of gilgai is formed due to shrink and shrivel characters. These soils have a high CEC and are very productive. Drainage is poor and soil reaction is neutral. i. Immature Brown Loams (IBL) Found along the western borders of the Ampara and north eastern borders of the Badulla District. Soils are relatively infertile, shallow and need good management for crop production. The soil reaction is slightly acid to neutral. They are relatively young soils.
Soil is one of the most important natural resources available to the people of Sri Lanka. A very large proportion of the labour force is dependent on soil/land to earn a living. Until very recently, it was the agricultural products that formed the main export income earner. In particular, the wet zone of Sri Lanka supports the export crops, tea, rubber, coconut and spices, which are responsible for about one third of the present foreign exchange earnings. Nearly 50% of the labour force in the country finds employment in the agricultural sector. In the rural sector, agriculture is the main means of sustenance of its population and soil utilization plays a major role in their socioeconomic development. The total land area of the country is limited and finite. It is only about 6.5 million hectares. Of the total land area about 65% is not available for agricultural purposes due to unsuitable terrain, rockiness, water bodies, national reserves, protected areas and other uses. Further, the adverse climatic conditions encountered in the dry zone of the country limit the full utilization of the fertile soils for agricultural production. Due to inappropriate soil management practices, the effective extent of productive soil has been diminishing over the years. On the other hand, the population dependent on land-based livelihoods has been rising. The total population has increased from 5 million in 1948 to about 19 million by 1998. The per capita land area has reduced to such a low level that it is nearly impossible to sustain the population on agriculture. It is therefore essential that the limited land area available is used and managed properly to enhance and sus¬tain the soil productivity. R.B.Mapa Department of Soil Science, Faculty of Agriculture, University of Peradeniya
Erosion is a gravity driven process that moves solids (sediment, soil, rock and other particles) in the natural environment or their source and deposits them elsewhere. It usually occurs due to transport by wind, water, or ice; by down-slope creep of soil and other material under the force of gravity; or by living organisms, such as burrowing animals, in the case of bioerosion. Erosion is a natural process, but it has been increased dramatically by human land use, especially industrial agriculture, deforestation, and urban sprawl. Land that is used for industrial agriculture generally experiences a significant greater rate of erosion than that of land under natural vegetation, or land used for sustainable agricultural practices. This is particularly true if tillage is used, which reduces vegetation cover on the surface of the soil and disturbs both soil structure and plant roots that would otherwise hold the soil in place. However, improved land use practices can limit erosion, using techniques such as terrace-building, conservation tillage practices, and tree planting. A certain amount of erosion is natural and, in fact, healthy for the ecosystem. For example, gravels continuously move downstream in watercourses. Excessive erosion, however, causes serious problems, such as receiving water sedimentation, ecosystem damage and outright loss of soil. Erosion is distinguished from weathering, which is the process of chemical or physical breakdown of the minerals in the rocks, although the two processes may occur concurrently.
Soil conservation is a set of management strategies for prevention of soil being eroded from the earth’s surface or becoming chemically altered by overuse, salinization, acidification, or other chemical soil contamination. Decisions regarding appropriate crop rotation, cover crops, and planted windbreaks are central to the ability of surface soils to retain their integrity, both with respect to erosive forces and chemical change from nutrient depletion. Crop rotation is simply the conventional alternation of crops on a given field, so that nutrient depletion is avoided from repetitive chemical uptake/deposition of single crop growth.
The rows formed slow water run-off during rainstorms to prevent soil erosion and allows the water time to settle into the soil. When surface planting is not feasible, there are a variety of mechanical management tactics to protect surface soils from water and wind erosion. Need for these tools arises on construction sites and other situations of transition, where bare soils are exposed. The primary tactics applied are mulching of soil surfaces and use of surface runoff barriers
Cover crops serve the function of protecting the soil from erosion, weed establishment or excess evapotranspiration; however, they may also serve vital soil chemistry functions. For example, legumes can be ploughed under to augment soil nitrates, and other plants have the ability to metabolize soil contaminants or alter adverse pH. The cover crop Mucuna pruriens (velvet bean) has been used in Nigeria to increase phosphorus availability after application of rock phosphate. Some of these same precepts are applicable to urban landscaping, especially with respect to ground-cover selection for erosion control and weed suppression.
Windbreaks are created by planting sufficiently dense rows or stands of trees at the windward exposure of an agricultural field subject to wind erosion. Evergreen species are preferred to achieve year-round protection; however, as long as foliage is present in the seasons of bare soil surfaces, the effect of deciduous trees may also be adequate. Trees, shrubs and groundcovers are also effective perimeter treatment for soil erosion prevention, by insuring any surface flows are impeded. A special form of this perimeter or inter-row treatment is the use of a “grassway” that both channels and dissipates runoff through surface friction, impeding surface runoff, and encouraging infiltration of the slowed surface water.