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Loss of soil fertility is an important problem in field corn production on upland under rain-fed area, which causing a decrease of corn yield. Farmer practice in cropping system, which growing corn followed by sorghum with no fertilizer application for a long time is one of the causes of this loss. Experiments on this cropping system, conducted at Nakhon Sawan Field Crop Research Center from 1981-2002, showed that average corn yield from 1999-2002 (187 Kg/rai) decreased about 56% when compared to that from 1981-1985 (425 Kg/rai).

Application of chicken manure at the rate of 1 ton per rai per year for 13 years not only improved soil fertility but also increased corn yield. Average corn yield from 1990-2002 was 727 Kg/rai in the plot with chicken manure application was higher than that with no fertilizer application (391 Kg/rai). An increase of yield was about 86%.

Growing legume in the cropping system also improved soil fertility and increased yield of main crops (corn). An appropriate cropping system, which was corn-mungbean with a recommend rate of fertilizer application, resulted to a higher marginal rate of return when compared to the same system with chicken manure application. However, the later system improved more soil fertility than the previous system.

In conclusion, a recommendation for farmers was growing corn-mungbean instead of corn-sorghum. For corn-mungbean system, the farmers should apply either with fertilizer at recommended rate or with chicken manure at the rate of 1 ton per rai per year.