Cultivating acid sulphate paddy tracts under Nilwala Flood Protection and Drainage Project

By Nanda Senanayake (Ph.D)

Additional Director (Retired)

Department of Agriculture

As a result of implementing a drainage and flood protection project for the Nilwala Ganga paddy tracts in the 1980s, some 8,000 acres of paddy lands have remained uncultivated up to date. I. Balasuriya, a researcher in the Department of Agriculture, studied the problem for several years and concluded that this was due to excessive drainage in the area leading to development of acid sulphate conditions and pH of paddy soils decreased. He also surmised that the increase in acidity was due to the oxidation of acid forming iron (Fe+2) containing compound namely Jarosite, which was found about 45-60 cms below the soil surface. Oxidation of this compound releases sulphur dioxide, which reacts with water and produces sulphuric acid and also releases hydrogen sulphide, polluting the environment. Rice plants cannot tolerate high acidity and die, whereas the only plant that survives such acidity is a plant called ‘Tikiya’ by the villagers. This sort of acid sulphate soils are found in countries like Thailand and Indonesia but in Sri Lanka this is the first report and the only area affected. This sort of land is either left fallow or used for purposes other than rice cultivation in other countries as well.

Since the 1980’s various researchers and research groups from universities and other organizations like the Department of Agriculture worked for the last 35 to 40 years to find a solution for this problem. Some of them concluded that there should be an irrigation source to flush out the acidity and to re-cultivate rice, but such a source is not available in the vicinity, while some others suggested modifying the land system to ‘sorjan System’, identified in global literature, and growing other crops such as coconut, cinnamon and yams, though these outcomes are not economical especially because the Nilwala basin is mostly rain fed and has comparatively poor rice yields. However, the most economical solution, which is the development of acid sulphate tolerant rice varieties by our breeders, was not successful up to date. It should be noted that all these research efforts were to overcome the effect of excessive drainage, which resulted in acid sulphate conditions.

Recently a group of scientists lead by well experienced rice researcher, Dr. Nanda Senanayake put forth a hypothesis to curb this situation in the Nilwala Project area, where they envisioned “by keeping the jarosite without oxidation by bringing up the water table in the rice fields above the jarosite layer (about 45 cms from the soil surface) can prevent the acid sulphate formation and therefore rice can be grown”. The research group consisted of Dr. Dayani Perera and Anura Weragoda, two eminent researchers in the Department of Agriculture. Their proposed motto was ‘Prevention is better than cure’, because all the efforts to ‘cure’ the situation have failed thus far, for the last three to four decades. Based on this they developed a project proposal and obtained funds from the National Science Foundation to conduct the study. They first studied the behaviour of the water table fluctuation in the rice field in relation to the water level in the drainage channel and the rainfall. By analyzing the data collected over a one year period they came across a very high correlation (r2 =0.98) between the water table in rice fields and the water level in the channel, indicating that natural hydraulic forces can bring about this all important correlation through equilibration. Under normal circumstances, when there is rainfall, the water level in the drainage channel increases and the water table in the field also increases through equilibration. But when the rain water inundating rice fields was pumped out to the sea (which is the normal practice by the project) the water level in the channel and the water table in rice fields decrease once again and this cycle continues throughout the year. This fluctuation will bring about jarosite oxidation during periods when the water table decreases below 45 cm.

Based on these observations a pot experiment was conducted in the greenhouse using acid sulphate soils removed from a soil pit from the project site in four layers; 0-12, 12-24, 24-36, 36-48 and 48-60 cms. The soil was repacked in the pots in the same sequence. The rainfall in the project site was simulated by providing measured quantities of water to the pots. Two treatments; water table maintained at 15 cm and 45 cm below the soil surface respectively, were used and maintained throughout the experiment period. A 3.5 month age group variety; Bg 357 was grown as the indicator crop. Results are shown in Figure. (a) and (b) proved the hypothesis put forward by the group. Results established very well that rice crops can be raised successfully when the water table is maintained at 15 cm below soil surface under pot conditions. Therefore, results further indicated and suggested that by increasing the water table in the channel (water table in the rice fields also increased simultaneously through natural hydraulic forces) by constructing water control structures, these rice fields can be easily cultivated.