Rice has been the staple food in Sri Lanka since ancient times. Self-sufficiency has been achieved through centuries of experience from aswaddumised paddy lands occupying about 13 percent of the land mass of the country.
In this food production scenario, rice must compete with associated grasses, sedges, and broad-leaved plants to obtain soil resources for optimum growth, development, and production of economic grain yields. In contrast to most of these associated plants, rice roots under submerged conditions receive their oxygen supply from the areal parts of the plant. The roots excrete oxygen and form adjacent zones of oxidation within the reduction layer of the submerged soil.
Hence in rice culture, the land is developed with specific features for maintaining standing water on puddled soils for suppressing weeds. During the growing season, rice is transplanted or sown in a well-soaked fields with little standing water. Subsequently, the depth of water is increased a few centimeters as the plants grow, then at the flowering stage, water is gradually reduced until the field is almost dry at harvest. This land development adopted to facilitate water management is locally known as aswaddumisation.
Aswaddumisation is a word derived from the Sinhalese language native to Sri Lanka. It means development of lands that have been fallow for ages, for a particular type of paddy field which consist of plowed and puddled soils on bunded bench terraces which facilitates flooding to a required depth whenever necessary.
This article, written in consultation with a Senior Land Use Planner in Canada and a Senior Agronomist from Sri Lanka, is intended to make the public aware of the agricultural value of aswaddumised paddy lands for rice production and to encourage stakeholder engagement for protection, regeneration, and scientific management of these paddy lands.
Features of aswaddumised paddy lands
Aswaddumised paddy lands have a unique set of physical, chemical, and microbiological soil properties that promote optimum growth of the rice plant and resultant good harvests. During aswaddumisation, the land is terraced to form perfectly levelled cultivation plots. A permanent soil bund is established at the shoulder edge of each terrace to facilitate flooding to the desired depth whenever necessary.
The surface soil layer is seasonally plowed and puddled to a depth of about 22 centimetres to provide better physical conditions for flooding and to maintain standing water ensuring sustainable growth and development of the rice plant.
With continuation of seasonal puddling for several years, a relatively impermeable layer, a plow pan, forms immediately below the puddled horizon. This plow pan is attributed to compaction during the puddling process and illuviation of reduced iron and manganese compounds.
Although the plow pan reduces percolation losses and reduces supplementary water requirements, it does allow some leaching requirements for the rice crop. All soil horizons below the hardpan are kept undisturbed. This unique physical arrangement of soil layers itself stresses the need for protection of limitedly available aswaddumised paddy lands for rice production.
Soils
Major soils that comprise aswaddumised paddy lands include Low Humic Gley, Alluvial, Reddish Brown Earth, Red Yellow Podzolic, Reddish Brown Latosolic, Non-Calcic Brown, Yellow Latosol, Grumusol, Bog and Saline soils.
Low Humic Gley soils which dominantly support rice cultivation occur in the lowest slopes of landscapes throughout the country. The greater fraction of these soils is confined mainly to the flat lowlands in the districts of Anuradhapura, Polonnaruwa, Ampara, Hambantota and Kurunegala.
Alluvial soils occur in paddy lands in flood plains in flat terrain. Next to Low Humic Gley soils, they serve as a minor contributor for producing rice.
With the introduction of stream water diversion projects such as the ancient anicut schemes and modern irrigation systems such as the Mahaweli development project, uplands in the middle and upper slopes of landscapes have been brought into aswaddumisation with the objective of expanding rice cultivation. These lands have soils such as Reddish Brown Earth, Red Yellow Podzolic, Reddish Brown Latosolic, Non-Calcic Brown and Yellow Latosol soils.
In addition, minor extents of lands have been aswaddumised in the districts of Gampaha, Colombo, Kalutara, Galle and Matara on Bog soils. Grumusol soils in areas such as Tunnukaiin the West of the Mullaitivu district and Murunkanin the North of the Mannar district, are also used for rice production.
Some saline soils in scattered small extents in coastal and inland dry areas in North-Western, North-Central, Northern, Eastern, Uva and Southern provinces have also been aswaddumised and used for rice cultivation.
All these soils differently support domestic rice production. Hence, it is recommended to carefully consider the capabilities of these soils under aswaddumised conditions and to formulate land and crop management practices, accordingly. In this regard, practices such as plowing and other tillage operations, irrigationscheduling and fertiliser application could be considered as key factors in the development of soil specific crop production packages.
Plot consolidation and laser levelling
Modern farm implements such as combine harvesters require cultivation plots larger in size and rectangular in shape for performing at maximum efficiencies. Traditionally developed terraces in aswaddumised paddy lands are small and irregular in shape in most cases. It has been aggravated by social fragmentation of paddy lands.
This proposed land conversion could be implemented at two levels namely land consolidation and plot consolidation. However, tenure related issues restrict land consolidation. Therefore, plot consolidation would help address the issues and encourage modern implement use. In this proposed conversion, two or more cultivation plots within one land holding are consolidated. It could be implemented in flat to undulating terrain where most paddy lands occur in the country.
Cultivation plots of most traditionally aswaddumised paddy lands are not properly levelled and as a result, standing water is not evenly maintained. This may create some field problems such as increased weed infestation, salinity development, accelerated losses of surface applied urea and crop damage from animal pests such as rats. Plot consolidation coupled with laser levelling would help address these field problems. Modern laser levelers are available for levelling cultivation plots.
Mechanisation of field operations
All field operations in rice farming have not been fully mechanised yet and practices which require enhancing farm implement use include plot consolidation, laser levelling, bund making, transplanting or row seeding, post planting weed control, and power-driven drying machines for harvest during unexpected rains. It is recommended to focus more attention on farm mechanisation research for these field operations to facilitate exploiting the full potential of modern implement use in rice production.
Cultivation of upland crops
Upland crop cultivation in aswaddumised paddy lands has been a long-standing practice among farmers. It has evolved as a farming option in areas where water scarcity exists for growing rice particularly in the yala season.
More recently, commercial crops have been also grown on paddy lands because they are more profitable than rice. These crops include chilli, onion, maize, tomato, soybean in districts such as Anuradhapura, Matale, Kandy and Moneragala; sweet potato and Chinese potato in the Ratnapura district; and potato, bean, and tomato in the Nuwara Eliya, Badulla and Kandy districts. In areas such as Udawalawa and Rajanganaya, some paddy lands have been used for long periods for growing banana as a commercial venture.
However, the country needs a systematic crop diversification plan and implementation program to fully utilise seasonally abandoned paddy lands for growing upland crops without adversely affecting the domestic production of rice.
Land abandonment
Paddy land abandonment results from numerous factors such as sea water intrusion in coastal areas, long duration water logging in flood plains, aggravation of soil problems such as iron toxicity and soil salinity, and flash flooding hazards due to frequent occurrence of extreme events of rains.
Factors such as use of poor-quality water for irrigation, poor maintenance of irrigation and drainage networks, and water stagnation because of haphazard infrastructure development activities such as construction of new roads across paddy tracts are also responsible for this abandonment.
The socio-economic factors include labour shortages particularly in administrative districts such as Colombo and Gampaha, and the reluctance of the younger generation to get involved in this muddy work while obtaining only marginal profits from rice cultivation.
This land abandonment has been mainly confined to administrative districts such as Colombo, Gampaha, Kalutara, Galle, Matara, Ratnapura, Kegalle and Kandy. Adoption of case specific approaches to restore them is required.
Land filling
In-filling of paddy lands occurs for non-agricultural uses such as construction of houses and other buildings, and infrastructure development activities such as expansion of the highway network. In the recent past, this land filling has been considerable in districts such as Colombo, Gampaha, Kalutara, Galle, Matara, Kurunegala and Kandy. Large scale abandonment of paddy lands in these areas may have encouraged paddy land in-filling. Guidelines are available to regulate land filling projects.
Gem, sand and clay mining
Large scale gem mining occurs in districts such as Ratnapura, Matale and Moneragala. Sand and clay mining have been common in river levees and flood plains where productive paddy lands have been lost.
Guidelines are available to restore these mined lands for reintroducing rice cultivation. Permission for these mining industries could be granted based on land suitability assessments which could avoid more loss of productive paddy lands.
Lower Mahaweli development
The lower Mahaweli flood prone earth between the Manampitiya railway-bridge and the Mutur coast, the largest alluvial deposit in the country, has not been fully used for rice production due to crop damage from frequent flooding. On the other hand, satisfactory rice yields have not been achieved in the neighbouring Batticaloa district for decades due to lack of irrigation water.
Having considered the above situations in rice production in these three different districts, the writer recommends a diversion of excess water from the Mahaweli river at a suitable location above the Manampitiya railway-bridge to the Batticaloa district. This option could minimise frequent flood related hazards in the Polonnaruwa and Trincomalee districts and enhance rice production in the Batticaloa district through assured water supply.
Inadequate and delayed fertiliser application
In aswaddumised paddy lands, surface soil above the plow pan serves as a nutrient reserve for the rice plant. Usually, rice takes up nutrients from added fertiliser as well as soil reserves. Recently, due to financial constraints most farmers in this country, have been forced to practise inadequate and delayed fertiliser application. Under this limited fertiliser nutrient supply, rice crops depend much more on soil reserves. Continuation of this lack of fertiliser results in abrupt depletion of soil nutrients and thereby large-scale retardation of the crop growth.
This retardation appears in the field as stunted plant growth, reduced tillering, yellowing of leaves, increased damage from pest and diseases, emerging small panicles, increased partially filled grains in panicles and eventually reduced harvests. Before long, this would result in abrupt reduction in domestic rice production. Hence, it is important to ensure that rice fields receive adequate quantities of fertiliser to meet crop requirements as well as replenish depleted soil reserves.
Rice thrives over a wide range of geographic localities extending from flood plains in coastal areas to narrow valley bottoms in the central mountain massif, climatic conditions from wet to dry regions, and soil resource bases from young soils such as Alluvial soils to very old soils such as Red Yellow Podzolic soils.
This crop has a high resistance to droughts, floods, iron toxicity and soil salinity. However, an assured water supply to maintain rice crops in the field determines the extents of these paddy lands that could be aswaddumised in the country. Hence, it is the prime responsibility of all stakeholders involved in the industry to focus on protecting aswaddumised lands against loss and misuses. Adopting beneficial management practices and modern scientific technologies is required to ensure the production of rice to meet the daily dietary needs of the ever-increasing population in the country.
The writer is a freelance agriculture scientist in natural resource management.