A long-term field experiment on organic material application has been conducted since 1980 at Phraphuttabat Technical Service Center, Lop Buri Province, Thailand, to clarify the effectiveness of organic materials on chemical and physical properties of soil and yield of corn. Mimosa was the most effective crop for increasing organic matter content of soil. Total and Bray II extractable phosphorus content markedly increased in city compost plots. Rice straw showed significant effect on increasing total and exchangeable potassium in soils. Total Ca, Mg, Cu, Zn, extractable Cu, Zn and exchangeable Ca contents distinctively increased in city compost plots. Extractable Fe content significantly increased in the mimosa plot. Average yield of corn with and without fertilizer from 2000 to 2005 showed that the use of crotalaria, mimosa, rice bean and city compost was beneficial to increase the production of corn yield for one, two, four and three years out of a six year experimental duration, respectively. The 5-year moving average of corn yield for 26 years showed the following trends. Corn yield in the mimosa plot without fertilizer increased but the increase was relatively low. The corn yields in the rice bean plot were relatively high during 1982-1986 and 1999-2003 in both fertilizer plots. The yield of corn in crotalaria plots with and without fertilizer was at a relatively low level from 1980 to 1988, and increased from 1989 to 2005. This long-term experiment suggests that application of green manure improved soil properties. Competition of green manure with corn resulted in a relatively low yield of corn and on the contrary, without the competition of green manure corn yields were relatively high. Rice straw mulch with fertilizer was effective in maintaining corn yield under critical rainfall years, while rice straw mulch without fertilizer did not increase the corn yield like that of the fertilizer plot under critical rainfall conditions. Although city compost application was effective in increasing corn yield, precautions against heavy metal contamination should be taken into consideration.