Land Degradation a decrease in exchangeable bases in a soil and destruction of layer-silicate clay as a result of leaching. In other words, the wearing down or away, and the general lowering and reduction of the earth's surface by the natural processes of weathering and erosion. Desertification is the spread of land degradation in arid, semi-arid, and dry sub-humid areas, leading to the outward spread of desert fringes, brought about by climatic variations and the activities of people and their livestock. Land degradation and desertification emerged as an issue of global concern over the last few decades and has been given special prominence since the United Nation Conference on 'Environment and Development' in 1992. Agenda 21 of the conference was a blueprint to address global environmental problems. Modern discussions on the ability of the land to support its people probably started in the late 18th century when Thomas Malthus published his essays on 'Principles of Population'.
Land degradation is one of the consequences of mismanagement of land and results frequently from a mismatch between land quality and land use. It is clearly a human induced social phenomenon and large areas with a considerable number of people are impacted by it. The linkage between land degradation and climate change is yet to be established but there is increasing evidence that land degradation is the driving force behind climatic change. The notion of desertification was probably first introduced by Aubreville who evaluated the alarming degradation of land through cross and other processes resulting from mismanagement of land by the resource poor farmers.
Causes of land degradation The main causes of land degradation in Bangladesh are: (i) Severe soil degradation by erosion, contamination, compaction, losses of organic matter through improper farming practices, salinisation and waterlogging; (ii) Soil degradation mainly through land transformation and deforestation; (iii) Deterioration of natural landscape by artificial replacement for cultivation, urbanisation, etc; (iv) Loss of bio-diffraction and fragmentation of ecosystems by intensive farming methods, and urbanisation, etc. The other causes of land degradation include drought, population pressure, poverty; constraints imposed by recent international trading agreements and local agricultural and landuse policies. Intensive agriculture with HYV (High Yield Variety) along with imbalance in fertilisation has degraded our lands seriously and this mechanism of degradation should be arrested or reversed through practices ensuring balanced fertilisation using an integrated soil and nutrient management approach.
'Land degradation and sustainability Intensive agriculture involving exhaustive HYV of cereals has led to heavy withdrawals of nutrients from the soils. The fertiliser (NPK, ie nitrogen - N, phosphorus - P and potassium - K) consumption remained much below the estimated removal. The gap between nutrient removal and supplies through fertilisers is likely to keep widening, eventually resulting in serious land degradation. Fertiliser use in Bangladesh is not only inadequate but also highly erratic because of its high price, irregular supply and lack of appropriate knowledge based on soil test values, about using recommended amounts and fertiliser types. The improper fertiliser use patterns along with almost no use of organic and bio-fertiliser is the major cause of land degradation in Bangladesh. An integrated nutrient management approach, therefore, aims at efficient and judicious use of all major sources of plant nutrients so as to get maximum economic benefit without any deleterious effect on physical, chemical and biological properties.
Desertification Degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities. Excluded in the definition are areas which have a hyper or humid climate. The definition of this climatic zone is related to the agroecological zones (AEZ) of FAO (1976) and because the maps depicting the affected region used on database which was not comprehensive they show significant variations. The World Atlas of Desertification (UNEP, 1993) provides perhaps the most authoritative estimate of the distribution. This report is probably based on climate analysis with minimum use of soil resource information. It, however, provides an estimate of the global area impacted and is thus very useful for global evaluation and for devising action plans. There are speculations on linkage between global climatic change and desertification. Recent reports of the Intergovernmental Panel on Climate Change have continuously emphasised the fact that there is a discernible human influence on global climate. Some facts that appear to have a consensus among scientists working in the field include: (a) the earth's average surface temperature is projected to increase by about 1.5C by the year 2100, a rate of warming greater than during any comparable time over the last 10,000 years; (b) sea level is projected to rise 10-30 cm by 2100; (c) increased evaporation in some areas and precipitation in others will lead to changing patterns with perhaps more floods and droughts.
Factors and processes The main factors and causes of desertification are based on biophysical, socio-economic and historical aspects. Geomorphology favouring soil erosion processes, alterations of the water balance, overexploitation of water resource, agricultural intensification, population pressure, urban and industrial expansion, etc are the main causes of land degradation. Desertification could be taken as the culmination of the process of land degradation steps are: (i) thinning of the vegetative cover of the soil; (ii) reduction of soil organic matter content and deterioration of its structure; (iii) dispersion of soil aggregates and surface sealing; (iv) runoff water and soil transport; and (v) terminal state of degradation-desertification. This description of land degradation shows that soil erosion could be taken as the dominant physical process of desertification.
A large part of current desertification processes are an inheritance from prior historical actions, immediate or remote. These factors and causes operate in different temporal and spatial contexts influencing the course of the natural processes involved in desertification. The temporal variability observed in the rates of desertification processes, is imposed by variations in the climate and mainly in the rainfall characteristics and bio-climatic parameters, and by variations in the intensity of human interference.
Landuse changes and degradation The per-capita availability of arable land is gradually declining. The present availability of net cultivable land is only 15 decimals per person in Bangladesh. Inelastic net cultivated area of about nine million hectares has a cropping intensity of 174.64%. Soil degradation occurs mainly due to erosion, nutrient depletion, imbalance in fertilisation, waterlogging, deforestation, salinity and alkalinity.
Management of the resource base ie soil and water, is vital for ensuring optimal production. So far, increase in crop production has largely been through mobilisation of production of high yielding varieties, fertiliser application, irrigation and use of agro-chemicals. No country in the world has been able to increase agricultural productivity without expanding the use of fertiliser and Bangladesh cannot be an exception. Unnecessary alarm created by environmentalists on the basis of the experience of some highly industrialised countries in temperate zones where excessive fertiliser was used should not discourage us from increasing a balanced use of fertilisers as it is a matter of national food security. Moreover, soils are of low fertility, our fertiliser input is very low and the climate is tropical and sub-tropical. However, we cannot overlook the ill-effects of increasing imbalance in fertiliser use, inadequate use of organic manure, crops residues, high losses and low use efficiency of N. There is, therefore, a strong case for balanced and integrated soil and nutrient management and regular monitoring of the changes in soil health.
Use of NPK fertilisers The effect of NPK fertilisers alone on crop productivity in different developing countries has been reported by FAO for some major crops by the production index ie the additional yield produced in kg by 1 kg NPK for wheat, rice, maize, sorghum and millets, and pulses being 7.5, 10.7, 11.3, 7.1 and 6.3, respectively. In Bangladesh, the average production index in kg by 1 kg NPK fertiliser is 10.5, 4.9 and 13.0 for rice, wheat and maize, respectively. The agricultural research institutes of Bangladesh have been carrying out research under controlled conditions and taking intensive care of their experiments. Therefore, no spectacular evidence of yield decline by land degradation was recorded in their experimental farms. Emphasis should, however, be given to research on development of agricultural technologies that integrate efficient management of land, water, nutrient, organic and other production inputs.
The consequences of different past land uses and their changes can help our understanding of the evolution of the process of desertification towards its present state and to formulate ways to arrest further land degradation and desertification. Agricultural intensification, agricultural abandonment, urban expansion, industrial increase, mining and landfill practices, infrastructure development and transport and tourism and recreation are today the main forces influencing desertification processes.
Climate change and bio-diversity Climate change and desertification are independent phenomena, but in constant interaction. Climate gives the boundary conditions for the desertification progress, and desertification changes the partitioning of energy and water fluxes that affect the atmospheric circulation. Both of them are influenced in their evolution by human action. The recurrent cycles of climate heating in history seem to have been altered in the last century mainly by the consequences of human development. However, the sinergical interaction between climate change, ie global warming, and droughts could favour desertification.
This situation is reflected clearly in the northwestern and western parts of Bangladesh, where the incidence of droughts in the last few decades and overexploitation of the aquifers and the apparition of soil salinisation processes are common. The exhaustion or eutrophication of groundwater and the degradation of the ecosystems have also accompanied it. The fragile equilibrium that sustains the ecosystems of the dry lands is affected by these variations in climate. Moreover, diversion of Ganges water at a crucial point of the upper riparian country has caused a depletion of soil water in the rhizosphere of the soil profile. This poses a dangerous threat to the survival of many species of flora and fauna common to these areas and contributing much to the richness of bio-diversity in the country.
Status and severity of land degradation The following four levels of land degradation are recognised: i) light: The terrain has somewhat reduced agricultural suitability, but is suitable for use in local farming systems. Restoration to full productivity is possible through modification of the management system. Original biotic functions are still largely intact; ii) moderate: The terrain has greatly reduced agricultural productivity, but is still suitable for use in local farming systems. Major improvements are required to restore productivity. Original biotic functions are partially destroyed; iii) strong: The terrain is non-reclaimable at farm level. Major engineering works are required for terrain restoration. Original biotic functions are largely destroyed; and iv) extreme: The terrain is unreclaimable and beyond restoration. Original biotic functions are fully destroyed.
Economic and social consequences of land degradation are undoubtedly a great concern for the country. One of the main social consequences is, in fact, an economic one, namely reduction of income for farmers. The milder forms of land degradation, for example soil nutrient depletion, can be reversed by changes in management; the resource is renewable and the degradation is reversible. However, in the case of two severe forms of degradation, land productivity in some cases can be restored by reclamation. In case of soil erosion, some of the effects may appear to be reversible, through checking further erosion by soil conservation measures and restoring lost nutrients and organic matter. But where gulling caused loss of land, or severe sheet erosion has removed the soil down to a gravelly residue, degradation is clearly irreversible.
Research Needs To avoid land degradation the thrust of research should be in the following areas: resource inventory - the objectives of resource inventory should include: i) refining AEZ based on climate, landscape and soil related criteria, ii) inventory of soil resources at thana level, iii) preparation of thematic maps for land use planning, iv) soil degradation assessment, v) developing a long-term climatic data base and vi) database for surface and ground water resources. In connection with the above objectives the achievement so far is the preparation of a map of Bangladesh delineating 30 AEZ and 88 sub-zones based on physiography, flood depth, climate and soils. So far, the soil resource maps of maximum thanas have been prepared. A GIS/AEZ data base updating has already been initiated by BARC with FAO assistance.
Cropping system research The objectives are to include: i) developing efficient and high-intensity cropping systems and compatible agro-techniques suited to different AEZ across the country, ii) understanding crop-weather relationships that serve as basis for preparing crop weather production models, iii) diversification of cropping systems by inclusion of high value crops for higher returns and iv) promoting FSR approach for enhancing biomass production. Achievements in cropping systems research have resulted in the development of several multiple cropping systems which need evaluation and field verification for use by the farmers.
Nutrient management Crops remove large quantities of nutrient elements from the soils. Unless these are replenished from time to time and in adequate quantities, crop yields would be adversely affected and the result would be land degradation. Nutrient management research thus advocates suitable remedies to keep the soil health intact. The objectives are: i) increasing the efficiency of fertilisers for higher crop response, ii) promoting the utilisation of indigenous resources and iii) evolving integrated nutrient management strategies for different cropping systems. Achievements in this area are enormous but very much scattered, unsystematic and under-utilised. It needs field verification in different AEZ.
Water management The key factor for improving crop production. The objectives are: i) improving use efficiency of available water resources, ii) integrated water management, iii) conjunctive utilisation of surface and ground water resources to increase command area and iv) putting to use poor quality groundwater, drainage and sewerage effluents for irrigation. Achievements in this area are limited and needs wider field use for different crops and cropping systems in different AEZ.
Waterlogging The lowering of land productivity through the rise in groundwater close to the soil surface. Also included under this heading is the severe form, termed ponding, where the water table rises above the surface. Waterlogging is also linked with salinisation, both being brought about by improper irrigation management. In Bangladesh, about 0.69 million hectare of land has been protected from tidal surges by constructing coastal embankment.
Salinisation In its broad sense, the term salinisation is to refer to all types of soil degradation brought about by the increase of salts in the soil. It thus covers both salinisation in its strict sense, the build-up of free salts, and sodification (also called alkalisation), the development of dominance of the exchange complex by sodium. As human-induced processes, these occur mainly through incorrect planning and management of irrigation schemes. Also covered is saline intrusion, the incursion of seawater into coastal soils arising from over-abstraction of groundwater. In Bangladesh, salinisation is one of the major natural hazards for land degradation. About thirty percent of the net cultivable area is in the coastal region of the country, and out of 2.85 million hectares of the coastal and offshore areas about 0.83 million hectares of arable land were affected by varying degrees of soil salinity. The factors which contribute significantly to the development of saline soils are: tidal flooding during the wet season (June-October), direct inundation by saline or brackish water and upward or lateral movement of saline ground water during the dry season (November-May). The severity of the salinity problem in Bangladesh increases with the desiccation of the soil. Recently, salinity both in term of severity and extent has greatly increased due to the intrusion of saline seawater because of the diversion of the Ganges water in the dry season. Soil Resource Development Institute (SRDI) showed that the salinity area has increased to 3.05 million hectares from 0.83 million hectares in 1975.
Soil Management Soil related constraints often limit the ability to attain desired crop productivity. The objectives are to include: i) diagnosing soil related problems and developing area specific technologies for amelioration. Some achievements in tillage practices, crop production under stress conditions, both drought and salinity, have been made under on-farm conditions but need verification in the farmers' fields.
Pan formation About 3-5 cm firm plough pan has been formed below the plough layer due to cultivation of transplanted rice year after year using the country plough. This firm pan restricts the proliferation of roots and utilisation of subsoil nutrients by growing crops. Studies showed that this pan formation was observed in about 2/3 million hectares of the Tista, Ganges, Brahmaputra, Meghna and Barind areas.
Acidification Most of the topsoils in the cultivated/deforested areas of the hills, terraces and other floodplains are acidified to a variable extent. Very severe forms of acidification have been observed in the coastal area in about 62,000 ha of active acid sulphate soils. As a consequence of drainage improvement extreme acidity (pH 2-4) has developed along with aluminum toxicity and phosphate fixation through microbial oxidation of pyritic sulphides to sulphates and sulphuric acid. Potential acid sulphate soils are reported to be occupying some 8,000 ha in the adjoining young tidal lands. Intensive acidification is also identified in the heavy clays in the Sylhet Basin areas, Lower Atrai Basin, and in some broad valleys within the Madhupur and Barind tracts. Northern and eastern hills have undergone strong acidification up to a considerable depth.
Deforestation and desertification Forest degradation is the reduction of standing biomass and in extreme cases, potential for re-growth of areas that still remain as forest or woodland. Forest degradation results from the cutting of woody formations in excess of their capacity for re-growth. Most involve cutting of natural forests but illegal clearance of forest plantations is also found in Bangladesh.
As mentioned earlier, human activities and climatic variations are the main causes of desertification, and with both the factors prevailing in Bangladesh, the problem is becoming more acute day by day. Diversion of water from the Ganges river at a point located in the upper riparian country is causing lowering of the groundwater table during the lean period, as a result of which soil water is depleted at higher rates, taking the whole of North-western Bangladesh towards desertification. Measures need to be taken inside the country to conserve water to avoid such a large calamity. The climatic variations are also responsible, as precipitation is extremely scarce or almost nil during the dry winter season. The agro-forestry systems, although old and traditional, have been renewed. The objectives are: i) identifying multipurpose tree species, ii) developing agro-forestry systems for different AEZ and iii) sustaining productivity of wastelands through agro-forestry intervention. The farmers have so far recorded no significant achievements for large-scale use. [SM Rahman]
Bibliography DL Deb, 'Excellence in research in the agricultural sciences in relation to food and environmental security', Journal of Higher Education, 17(1), 1994; SM Saheed, Country Report Bangladesh, Expert Consultation of the Asian Network on Problem Soil, FAO, Manila, 1995.