Green Manures for Highland Paddy Improvement

Abstract

Two of the most serious problems in northern highland mountainous areas is land degradation and insufficient rice production to feed highland people. Paddy development to grow rice on flooded soil provides a solution to these two problems at the same time. This study was conducted to identify situation of highland paddy system and to evaluate the effects of different types of green manure on improvement soil fertility and rice yield in highland paddy field and to select the suitable legumes green manure for improving their highland paddy yield. Field survey, farmers’ interview with questionnaires, crop cutting and soil sample collection were used to collect the data on typology of paddy field irrigations, soil properties and rice yield of the selected farmers and available legumes in six highland villages of Sob Moei sub-district, Sob Moei district, Mae Hong Son province. The studied villages were Huai Chai Yong, Tiya pur, Ley koe, Huai Nam Sai, Nam Ok Ru and Tee Cha. Data was collected from 16.7% of a total of 432 farming household in the villages. Field experiments were conducted for 2 years in 2007 and 2008 at one of the farmers' highland paddy field in Tee Cha village to study the adaptability of some legumes in highland paddy condition, to evaluate potential of tested legumes as green manure crops and their effects on rice paddy and then select the suitable green manure legumes for highland paddy fields with different levels of available water supply. Furthermore, the nutrient accumulation of the suitable green manure legumes in rotation cropping paddy rice system was investigated on the lowland paddy field in 2008 and 2009 at Center for Agricultural Resource System Research, Faculty of Agriculture, Chiang Mai University.

It was found that the highland paddy fields in the study area could be divided into five typology groups based on degree of slope and availability of water supply. Based on the availability of water for irrigation, there were three types of highland paddy, fully irrigated, partial irrigated and rain fed field which the area size ranged from 0.3-0.7 t ha-1. The average rice yield ranged from 1.8-2.2 t ha-1 which could meet/feed/support household’s need for 1 to 6 months each year. The number of farmers who produced seed yield above average of the villages were within the range of 19-23% of total numbers of household. The soil in the selected fields had suitable pH and sufficient exchangeable K for rice production but with low N and available P contents. The paddy yield limiting factors should be N and P deficiency and water supply. The existing legumes in the studied areas were Sward bean (Canavalia ensiformis), Yam bean (Pachyrrhizus erosus), Lima bean (Phaseolus lunatus), Winged bean (Psophocarpus tetragonolobus), Rice bean (Vigna umbellata), Cowpea (Vigna unguiculata ssp.), Yardlong bean (Vigna unguiculata spp. Sesquipedalis) and Thornless mimosa (Mimosa invisa). All legumes excepted M. invisa are commonly grown with upland rice as a domestic food consumption. Mimosa invisa is used as green manure in some highland villages. For field study, L. purpureus, V. umbellata, C. ensiformis and M. invisa were grown to evaluate soil abundance and rice yield.

In dry season with irrigation supplement until the rainy season, soil condition of the highland paddy field at Tee Cha village was slightly acidic with low contents of total N and available P. All legumes produced significantly more biomass and accumulated more primary nutrients in the above ground part at 115 days after planting than the existing vegetative fallow. There were significant differences in term of biomass and N, P and K accumulation among the four tested legumes. C. ensiformis produced the highest biomass (13.7 t ha-1) followed by V. umbellata (11.5 t ha-1) whereas L.purpureus and M. invisa produced the lower biomass (10.1 and 10.6 t ha-1) than the first two species. C. ensiformis accumulated the highest P contents (41.6 kg N ha-1) whereas L. purpureus and C. ensiformis accumulated the highest P contents (40.9 and 39.2 kg P ha-1) and L. purpureus accumulated the highest K contents (300.3 kg K ha-1).

At 115 days after four legumes were planted, they were incorporated into rice cultivation area, RD 21 rice variety. It was found that, over conventional practice, in V. umbellata, C. ensiformis and L. purpureus plots, 20.7-31% of rice grain yield was significantly increased. For the effect on rice improvement, rice grain yield incorporation with V. umbellata was significantly better than in C. ensiformis plot but not different from L.purpurous and M. invisa plot that could not significantly improve rice grain yield.

Biomass and N and P accumulation in the above ground parts of the four tested green manure legumes grown on the highland paddy soil with low N and available P contents were decreased when the field was not fully irrigated. Different legume types responded to different level of irrigation in biomass product and nutrient accumulation.

By partial irrigation, biomass, N and P accumulation of M. invisa were significantly decreased as same as L. purpureus and C. ensiformis. Among the four tested legumes, C. ensiformis produced the highest biomass and N accumulation at all irrigation levels, however, it accumulated less P than V. umbellata. Followed by C. ensiformis, under full and partial irrigation, V. umbellata produced higher biomass and N accumulation than the others. The level of irrigation influenced not only on the biomass of the four green manure legumes, but also on highland paddy yield under conventional system. Under partial irrigation or no irrigation in conventional rice system, paddy yield was only 1.3-1.4 t ha-1 compared to under full irrigation, the paddy yield was 1.5 t ha-1. Furthermore, levels of irrigation also influenced on rice yield (RD21) and harvest index by different green manure incorporation. Response to different green manure incorporation on Vigna umbellata was considered as the best leguminous green manure to improve highland paddy rice because of its biomass that resulted in increasing of paddy yield and harvest index at all irrigation treatments compared to those under conventional practices although this legume in general, produced less biomass and P content than C. ensiformis. With no irrigation during April to July for 45 days, three legumes, L. purpureus, V. umbellata and C. ensiformis, produced more biomass N than that of fallow vegetation. Besides, incorporation of their biomass were significantly increased paddy yield.

Field experiment conducted on highland paddy field at Tee Cha village incorporated with four green manure legumes, their biomass contained 223-416 kg N, 28.5-40.9 kg P and 215-300 kg K ha-1 that was sufficient for paddy rice cultivation with standard grain yield of 4.37 t ha-1. Rate of N and P accumulated on biomass of each green manure legume from this study were higher than recommended N and P fertilizer rate for rice cultivation in poor soil. However, grain yield of RD21 rice variety obtained from highland field at Tee Cha village was below the standard yield (4.38 t ha-1) of this variety.

Under irrigated lowland paddy field, slightly acidic low N and exchangeable K contents, V. umbellata and M. invisa were evaluated for their potential of nutrient accumulation in rice rotation cropping system. It was found that, in the first rotation with legumes, V. umbellata produced more biomass (21.4 t ha-1) than M. invisa (12.9 t ha-1) which was still higher than biomass from fallow vegetation (7.3 tha-1). The biomass from fallow vegetation accumulated 206 kg N ha-1 Nitrogen accumulation in M. invisa and V. umbellate biomass were higher than fallow vegetation two and three times, respectively. Phosphorous accumulation in V. umbellata biomass was the highest (84 kg P ha-1) followed by that of M. invisa (53 kg P ha-1) whereas the fallow vegetation accumulated only 29 kg P ha-1. Incorporation of the biomass of V. umbellata and M. invisa provided 3.1 and 3.6 t ha-1 grain yield of RD21 rice variety, respectively, that were 71 and 82% of the standard yield of this rice variety and rice harvest index (HI) was 0.4-0.43. In the second rotation with legumes, they produced higher biomass and accumulated more nutrients than the first year round. The biomass N of the vegetation fallow in the second rotation was only half of that in the first rotation. Yield of RD21 rice grown after legumes incorporation produced significantly more grain yield than rice grown after fallow vegetation, however it was not different between rice yield grown after the first and second rotation with legumes although the second rotation with legumes provided more biomass and accumulated N and P than those from one rotation. It was expected that the excess N obtained from legume biomass resulted in lack of transferring the stored carbohydrate in stem and leaf sheath of rice into grain part. Based on the performances of the tested legumes in this study, their adaptation to highland paddy soil under different levels of irrigation, the potential of the legumes as the sources of N and P including the acquaintance of the farmers on useful of each legume, V. umbellata was considered as the most suitable legume to be used as green manure to improve paddy yield on highland of northern Thailand.