Cultivation and Application of Green Manure in Paddy Fields of China

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  • Gu. Rong-shen
  • Wen Qi-xiao

CULTIVATION AND APPLICATION OF GREEN MANURE IN PADDY FIELDS OF CHINA Gu Rong-shen (Jiangsu Academy of Agricultural Sciences) Wen Qi-xiao (Institute of Soil Science, Academia Sinica, Nanjing) China has a very long history of cultivation of green manure crops in paddy fields. Early in the third century, there were records of the rotation of rice and milk vetch in the local chronicles*. In the 1940s1950s, a system of rice-green manure crop rotation was employed as an effective measure to increase nitrogen and maintain soil fertility in the region between the Five Ridges (Mt. WUling) in the south and the Changjiang River in the north. Since 1960, thanks to the improvement of fertilization, inoculation of rhizobium and improved cultivation techniques, the milk vetch, vetch, etc. have been introduced successfully into Guangdong and Guangxi provinces to the south of the’ Five Rjdges, and the Huaihe River valley to the north of the Changjiang River. At the same time, following the solution of such problem as survival through winter and summer for Azolla and the selection of more adaptable varieties of green manure crop in North, Northeast and Northwest China respeotively, the total acreage of green manure crop of paddy fields has reached 8 million hectares. This article deals mainly with a general aspect of the research work of green manure in paddy fields in China. THE MAIN VARIETIES AND THEIR REGIONAL DISTRIBUTION The winter green manure crops are the principal part of green manure crops in China, with an area of up to 83% of the total area of green manure crop of paddy field. The area for Azolla cultivation makes up 8.5% of the total area. The ares of summer green manure crops is not more than 500,000 hectares, though distributed widely. Milk vetch is dominant in the winter green manure crops, covering an area of approximately 74.6% of the total area of winter green manure crops. It is characterized by its higher adaptability to wetness and shade in the seedling stage and is conducive to under-crop sowing before the rice is harvested. Its tender and soft stems and leaves are liable to decompose, therefore, the nutrients it contains are more readily available to the rice plant. According to experimental results from Jiangyin County, Jiangsu P1rovince (l), through a submergence for 24 hours, no dead seedlings of milk vetch were found; and in 120 hours, thereRfter, the rate of dead seedlings began to increase with the longer time of submergence (r = 0.9753**). In accordance with the data of Zhejiang Province(2), the seedlings of milk vetch could be survived only under a condition that the_light intensity at the height of 20 cm from the soil surface is more than 3.5% (2500-3000 * “Annals of Guangzhi” by Guo Grong-yi. 207 Institute of Soil Science, Academia Sinica, Proceedings of Symposium on Paddy Soils © Science Press, Beijing and Springer-Verlag Berlin Heidelberg 1981 Lux, equal to that under a cluster of rice with a yield < 5250 kg/ha); otherwise, the seedlings would either stop in growth or be liable to die. In Jiangsu, a yield of fresh weight of 30 t/ha, has been still obtained from the late rice field with the yield of 4.5-5.0 ton/ha, when the milk vetch is interplanted under the rice plant 40-50 days before harvesting. In the past, azolla was cultured with local varieties belonging to A. imbricata which could not survive over winter without protection in the isothermal area at less than 50 C in January and could not propagate very quickly in spring; therefore, it could not be extensively used for the early rice in the area of double cropping of rice. Recently,~. filiculoides capable of surviving the winter on the natural water surface in the area to the south of the isotherm of 30 C in January has been introduced, with results that the azotase activity of symbiotic blue algae associated with A. filiculoides is 2.7 times higher than that of the local varieties at-150C(3). However, a further extension of cultivation of azolla in paddy field is still restricted by such problems as the method applied, summer surviving and insect injuries, especially Polypedium illinense. Sesbania is characterized by its strong tolerance to wetness. It was reported by Shanghai Academy of Agricultural Sciences that the root system of sesbania developed better under submerged conditions than under upland conditions; and under submerged conditions the weight of nodules per plant, the azotase activity and the dry weight were increased by 137%, 67-80% and 95% respectively as compared with those under upland conditions. Sesbania, if planted between wheat and late rice for about 40 days, will produce fresh plant 7.5 t/ha in weight(4). It is planted either in the interval between early rice and late rice in Guangdong and Fujian or intercropped in the rice seedling beds in Shanghai and may often give a yield of fresh weight of 15 t/ha. Besides milk vetch, there are other important winter green manure crops such as vetches, medic, broad bean. They very in geographical distribution due to their different adaptabilities (Fig. 1). The distribution of azolla is generally similar to that of milk vetch. Sesbania is usually dispersed in the coastal region to the south of latitude 400 N. SOME CULTIVATION TECHNIQUES OF WINTER GREEN MANURE CROPS In dealing with the raising of the yield of winter green manure crops, we use milk vetch as an example, and here is a discussion about it. 1. Suitable Time and Rate of Seeding To postpone the seeding time will lead to the decrease of the yield of milk vetch significantly (Fig. 2). Bur-clover, smooth vetch and vicia cracca are similar to milk vetch in this respect(5). Likewise, excessively early seeding will make the growth of the winter green manure inhibited by its preceding crop, or the germination damaged by high temperature or arid climate; too luxuriance in the growth of green manure in the early stage will aggravate frozen damage in winter. For instance, the safety growth rate of milk vetch in winter in Jiangsu is as follows: 5-12 cm in plant height, with approximately 2-4 branches per plant, coverjng an area of about 80%. In order to reach this growth rate, an accumulated temperature of 550-9000 C above 50 C is needed: therefore, the