NI NI TUN1, BURKHARD HEILIGTAG2, ANDREA KLEEBERG2, CHRISTIAN RICHTER2
1Myanma Agriculture Service (MAS), Land Use Division, Yangon, Myanmar
2University of Kassel, Institute of Crop Science, Germany
Rice (Oryza sativa) is the main staple food crop in Myanmar accounting for 97% of total food grain production. Changes in redox potential and pH are brought about by flooding the rice fields, with influences on nutrient availability, which was the objective of this study.
4 soils from Lower and 2 soils from Upper Myanmar were used. As in Lower Myanmar rainfall (>mma)is much higher than in Upper Myanmar (<mma), usually the pH of these soils is lower (around 5.5) compared to the upper part of the country (about 8). 20 seeds of Oryza sativa, variety Manawthukha, were sown in pots in 400 g of each soil and grown under submerged conditions in a climatic chamber at 25°C and 12 h day light, with 4 replicates in a randomised block design. Eh and pH were measured every 7 days. 24 and 50 DAS, plants were harvested and yield as well as mineral uptake and plant concentrations of P, K, Na, Ca, Mg, Fe, Mn, Cu and Zn were determined.
In all 6 soils tested, Eh declined from 400-500 to 100-200 mV in the 50 DAS and submergence. The pH in the alcaline soils declined from 7.3 to 6.9, probably due to CO2-pressure increase produced by organic matter decomposition, while in the acid soils pH increased from 4.5 to 5.5 due to water logging and reduction of Fe, Mn and organic acids. On the high pH soils, yield was a lot lower than on the lower pH soils. On the different soils, all examined nutrients could be taken up in normal amounts by the rice plants, but on the high pH soils, probably due to precipitation as hydroxide, Mn concentrations were very low in spite of water logging, and Cu concentrations were very high, probably toxic. This could be the reason for the low yield on the high pH soils. Application of acid fertilisers and/or introduction of legumes in the rotation, being able to reduce the soil pH in the rhizosphere, is recommended.
Keywords: Nutrient uptake, pH, redox potential, wetland rice