Soil is the most basic natural resource that supports all terrestrial life on earth. It is finite in extent and non-renewable over human timescale. However, it is prone to degradation owing to land misuse, mismanagement and other natural and anthropogenic forces.
Soil degradation can be of different types:
- Physical: Erosion, decline in structure, water logging, crusting, compaction;
- Chemical: Salinisation, acidification, nutrient imbalance comprising of toxicity or deficiency;
- Biological: Depletion of soil organic carbon, reduction in soil biodiversity, decline in microbial biomass; and
- Ecological: Disruption in nutrient cycling and decline in carbon sink capacity.
Soil degradation leads to reduction in ecosystem functions and services to mankind and nature. Soil erosion is the most important causative factor that affects 56 per cent of the total area affected by human-induced soil degradation in the world. According to L.R. Oldeman (1992) nearly 2 billion hectares (ha) which is 22.5 per cent of the world’s agricultural lands, pastures, forest and woodlands are degraded.
Further the harmonized statistics of Indian Council of Agricultural Research (2010), point out that of 328.7 mha of India’s geographical area 142 mha is cultivated land. Of this, about 57 mha (40 per cent) is irrigated and the remaining 85 mha (60 per cent) is rain-fed. The total area of degraded and wasteland stands at 120.72 mha out of the total 328.7 mha geographical area. The extent of area affected by water erosion, wind erosion, chemical and physical degradations are given in Table 1.
Erosion by water is India’s prime most degradation problem, resulting in loss of topsoil and terrain deformation. Based on first approximation analysis of existing soil loss data, the average soil erosion rate was 16.4 tonnes/ha/year, resulting in an annual total soil loss of 5.3 billion tonnes throughout the country (Narayana & Babu, 1983). About 1 millimetre (mm) top soil is lost each year due to soil erosion. Nearly 29 per cent of total eroded soil is permanently lost to the sea, while 61 per cent is simply transferred from one place to another, with the remaining 10 per cent is deposited in reservoirs.
A study (Sharda et al., 2010) has revealed that soil erosion due to water resulted in annual crop production losses amounting to 13.4 MT in cereal, oil seeds and pulses, equivalent to 162 billion USD. A study by Mandal and Sharda, 2011 also showed that soil loss tolerance varies from 2.5 to 12.5 Megagram (Mg) tonnes/ha/year depending upon soil depth at a particular location. About 57 per cent of the country’s area has permissible soil loss of less than 10 Mg tonnes/ha/year, which needs to be treated with appropriate conservation measures. The highest priority needs to be accorded to about 7.5 per cent of the area where soil loss tolerance is only 2.5 tonnes/ha/year. Nearly 59 per cent of land in hilly regions is in need of soil conservation measures to arrest soil loss below the tolerance limit.
Process of soil erosion and its prevention
Soil erosion is a gradual process that occurs when the impact of water or wind detaches and removes soil particles, causing the soil to deteriorate. Soil deterioration and low water quality due to erosion and surface runoff are severe problems worldwide. Erosion is a serious problem for productive agricultural land and water quality concerns. The problem may become so severe that the land can no longer be cultivated and must be abandoned. Integrated watershed management, which involves soil and water conservation coupled with suitable crop management, is an excellent strategy for mitigating soil erosion.
The hydrologic processes of rainfall and runoff play an essential role in water erosion. The amount and rate of surface runoff can affect erosion and sediment transport. Controlling the sediment must be an integral part of any soil management system to improve water and soil quality.
Improving the soil infiltration rate, so that there is less surface runoff, can help reduce soil erosion. Agronomic, cultural or structural practices are available for controlling soil erosion. Plant residue management is another way of controlling soil erosion by intercepting raindrops, thereby reducing surface runoff and protecting soil surface particle detachment by raindrop impact. The adoption of a cropping system along with conservation based tillage practices such as no-tillage, strip-tillage and ridge-tillage is also equally vital.
Integrated Watershed Management Programme
The Indian Government implemented the Integrated Watershed Management Programme (IWMP) in 2009-10 in all states. The main objectives of the programme are to restore the ecological balance by harnessing, conserving and developing degraded natural resources such as soil, vegetative cover and water. This can reverse soil erosion, regenerate natural vegetation, help harvest rainwater and hence assist in recharging underground water.
The programme was revamped as the Pradhan Mantri Krishi Sinchayee Yojana (Watershed Component) (WDC-PMKSY) with a budget allocation of INR 1530 crore in 2015-16 by the Central Government. The World Bank assisted watershed management project ‘Neeranchal’—a new initiative in the form of a technical assistance programme, has been envisaged to be taken up at a cost of INR 2142 crore over a six-year period.
Soil degradation implies a decline in soil quality with an attendant reduction in ecosystem functions and services such as the production of food, feed, fibre and climate moderation through carbon cycling, as also waste disposal, water filtration and purification through elemental cycling.
Soil quality also has strong implications for human health. Accelerated erosion can deplete the soil organic carbon (SOC) pool and nutrient reserves. The SOC pool is a key indicator of soil quality, and an important driver of agricultural sustainability, and the most reliable indicator for monitoring soil degradation. Alongside depletion of the SOC pool, soil degradation reduces availability of nitrogen and other essential nutrients such as phosphorus (P) and sulphur (S).
The SOC pool contains twice the amount of the atmospheric carbon pool and is thus a potential driver of global climate change. Globally, SOC stock is the largest contributor to total global carbon stocks, contributing 1550 Pentagrams (Pg) (1 Pg = 1015 g) of carbon to 1 m depth, which is about three times that of biotic and twice that of the atmospheric pools (Lal, 2015). The stock in Indian soils has been estimated at 63 pg in the first 0–150 cm depth (Bhattacharyya et al., 2000). Depletion of SOC pool is currently a global issue and the principal cause of soil degradation. Strategies are being developed to ensure an increase in the SOC pool and preferably maintain an above threshold or at least the critical level of 10 to 15 g/kg (1.0 -1.5 per cent) which is essential for reducing soil degradation risks and reversing degradation.
SOC is of global importance because of its role in the global carbon cycle and the part it plays in the mitigation of atmosphere levels of greenhouse gases (GHGs), and especially carbon dioxide (CO2). To reduce the emission of CO2, carbon capture and storage (CCS) is an important method. Soil erosion is the most widespread form of soil degradation and signiﬁcant amounts of carbon are either relocated to lower situated soils, water bodies and sediments or degraded to CO2 during soil erosion. Erosion can be considered to decrease carbon stocks in the eroded soils and are thus a CO2 source to the atmosphere.
The SOC stock is one single parameter that can help effectively prioritise the restoration of soil health. Agricultural lands are believed to be a major potential sink and could absorb large quantities of SOC if trees are reintroduced to these systems and judiciously managed together with crops. Maintaining and enhancing SOC in soils may well prove the most effective method to maintain soil quality and the environment.
Bhattacharyya, T., Pal, D.K., Mandal, C., & Velayutham, M. (2000). Organic carbon stock in Indian soils and their geographical distribution.Current Science, 79(5), pp655-660.
ICAR. (2010). Degraded and Wastelands of India Status and Spatial Distribution. Directorate of Information and Publications of Agriculture, Indian Council of Agricultural Research.
Lal, R. (2015). Restoring Soil Quality to Mitigate Soil Degradation. Sustainability, 7, pp5875-5895.
Mandal, D., & Sharda, V. N. (2011). Assessment of permissible soil loss in India employing a quantitative bio-physical model, Current Science, 100(3), pp383-390.
Narayana, D.V.V. & Babu, R. (1983). Estimation of soil erosion in India. Journal of Irrigation and Drainage Engineering,109, pp419–434.
Oldeman, L.R. (1992). Global Extent of Soil Degradation. ISRIC Bi-Annual Report 1991-92, pp19-36.
Sharda, V.N., Dogra, P., & Prakash, C. (2010). Assessment of production losses due to water erosion in rainfed areas of India. Indian J. Soil Water Conserv., 65, pp79–91.