Alleviating soil acidity, aluminium and iron toxicity in an acid sulfate soil using lime and bio-organic fertilizer to increase rice yield

Rice (Oryza sativa) is a staple food for over half of the world's population. The production of rice should be increased because growth in rice production has been slower than the population growth. With no room for area expansion, improving the fertility of marginal soils (such as acid sulfate soils) is one of the ways to increase rice production and maintain food security in Malaysia. Acid sulfate soils are known to contain pyrite (FeS2) which upon oxidation results in the production of high amount of acidity (pH <3.5), aluminium (Al) and iron (Fe) which significantly affect rice growth. In order to increase the rice production, the infertility of acid sufate soils need to be alleviated first. A laboratory experiment was conducted to investigate the effects of pH, Al and/or Fe on rice root morphology and explain how rice growing under such conditions can withstand the stresses. Two rice varieties, MR219 and MR 253 were grown under various pH (3, 4, 5, 6 and 7), Al and/or Fe stress (0, 20, 40, 60, 80 and 100 μM) conditions. After 14 days, rice root length and surface area were determined using a root scanner while the organic acids released by the roots of rice were determined by high performance liquid chromatography (HPLC). Results showed that the root length decreased with increasing Al and/or Fe concentration. On the contrary, the root length increased as the pH of the solution increased. This phenomenon was in part related to the exudation of oxalic, citric and malic acids by the rice roots. It was observed that the amount of organic acids released increased with increasing Al and/or Fe concentrations in the solution culture. It is believed that these organic acids were responsible for chelating some of the Al and/or Fe in the solution, rendering them unavailable for their uptake by rice. Organic acids were also secreted at very low solution pH. With this tolerant mechanism, the rice planted on acid sulfate soil can continue to produce yield but less than 3t/ha/season compared to the average national rice production, 4.7 t/ha/season (DOA Paddy Statistic, 2012). Another study was conducted in a glasshouse to determine the effects of ground magnesium limestone in combination with bio-organic fertilizer application on the chemical properties of the soils and rice yield. Three rice seedlings were transplanted in pots which were previously amended with 0, 2, 4, 6 and 8 t/ha GML with or without bio-organic fertilizer. Rice varieties MR 219 and MR 253 were grown for two seasons in the same pots. Without applying the amendments, rice grown on the soils was affected severely by the high acidity, Fe and Al toxicity. Results showed that the critical pH for the two rice varieties was 6. The critical Al3+ activities for MR 219 and MR 253 were 4.23 μM and 5.53 μM, respectively. The infertility of acid sulfate soils in Malaysia can be ameliorated by applying 2 t GML/ha in combination with 0.25 t/ha of bio-organic fertilizer. At this rate of GML and bio-organic application, the soil pH increased to 5 and resulted in the concomitant reduction of Al3+ activity that would be translated into improved rice growth. The ameliorative effects of amendments had at least lasted for 2 seasons, indicating that this agronomic intervention is sustainable in the long run. The growth of rice was improved further by the presence of organic matter in the bio-organic fertilizer that inactivated Fe and/or Al present in the acidic water via chelation. The third study was a field trial. The field experiment was conducted in Kemasin- Semerak, Kelantan to determine the effects of applying ground magnesium limestone (GML) with or without bio-organic fertilizer on the properties of an acid sulfate soils and rice yield. In this study, the soil was treated with GML and/or bio-organic fertilizer using Randomized Complete Block Design, with 4 replications. The pH of the untreated soil was 3.78, while the exchangeable Al and extractable Fe were 2.82 cmolc/kg and 211.01 mg/kg, respectively. As a result, the grain yield of rice was only 2.12 t/ha because rice was significantly affected by Al and Fe toxicity as well as acidity. Al toxicity inhibits the root elongation, while Fe toxicity forms a coating area on the root surface. Both phenomena disrupted the plant from taking the available nutrients in the soil solution and eventually reduce the yield. This study showed that the infertility of acid sulfate soils in Malaysia can be ameliorated for sustainable rice production by applying 2 t GML/ha in combination with 0.25 t bio-organic fertilizer/ha. At this rate of applying the amendments, soil pH increased up to 5.25. At this pH, the Al and Fe started to form their inert hydroxides. The yield of rice was found to increase from 2.12 t/ha to 3.99 t/ha. The addition of bio-organic fertilizer supplied NPK and contained microbes that could fix N, which helped increase the rice growth and eventually its yield.

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