Thesis Abstract of AGS Students

Wawta Wassananukul (1991)

The study on replenishment of organic matter for maintaining soil productivity in rice-soybean cropping system was carried out in two stages. An informal survey and formal survey were initially conducted during April to June 1990 in four villages: Tung Or, Mae Kung Bok, Ta and Patan at Sanpatong District of Chiang Mai Province in order to understand farmers' management practices concerning organic matter. Soybean residues were found to be an important source of organic matter currently used by farmers in this system, but the amount of soybean residues was too low to adequately return nutrients and organic matter to the soil. It is necessary to use soybean residues in combination with chemical fertilizer to maintain good yield and soil fertility. The other sources of N and organic matter such as rice straw and animal manure were not adequately supplied in this system since most of the rice straw (79%) was burnt before growing soybean and animal manure was available only in the farm where owners raise the cattle. Considering time, water and labor, green manure crops appear to be a potential source of N and organic matter.

The field experiment was subsequently carried out at the Multiple Cropping Center, Chiang Mai University. The two main objectives of the study were to evaluate alternative sources of nitrogen for maintaining productivity of rice in rice-soybean cropping system and to determine the contribution of soybean residues and green manure on soil organic matter and N-uptake of two varieties of rice. The experiment was factorial in randomized complete block design with two factors, varieties and N sources. There were two varieties of rice, RD 7 and KDML 105. The five N sources were no N fertilizer, urea (50 kgN/ha), soybean residues at 1.25 t dry matter/ha, soybean residues at 1.25 t dry matter/ha in combination with urea (35 kgN/ha), and Sesbania rostrata at 2.9 t dry matter/ha.

The results revealed that the grain yield of two rice varieties was not significantly different but the grain yield was significantly affected by N sources. Incorporation of Sesbania rostrata resulted in rice yield in equivalence to application of urea at 50 kgN/ha with the average rice grain yield of 4687 and 4563 kg/ha, respectively. However, yield from the plots incorporated with Sesbania rostrata were significantly higher than those from soybean residues plus urea, soybean residues alone and no N fertilizer treatments. The average grain yield of these three treatments were 4292, 4122 and 4168 kg/ha, respectively. Difference in grain yield was related to the difference in the number of panicles per square meter. The Sesbania rostrata and the urea treatments contributed significantly to higher number of panicles per square meter than those of soybean plus urea, soybean residues alone and no N fertilizer treatments. There was no interaction between rice varieties and N sources on grain yield.

Economic evaluation of treatments showed that urea, Sesbania rostrata and soybean residues plus urea treatments gave the higher economic returns than no N fertilizer treatment with the marginal rate of return of 221%, 196% and 93%, respectively. While the soybean residue alone resulted in lower marginal rate of return than the control.

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