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Water saving technology interests crop producers in both the humid and semi-arid regions of the tropics. The amount of irrigation water use could be substantially reduced by the application of compost. The objective of this study was to assess irrigation application and water saving through the optimum rate of compost application. Results show that in both tropical humid and semi-arid regions, compost application on some of the "thirstiest" cash crops is a reliable way of saving water. The net irrigation depths were substantially raised from the soil treated with chemical fertilizer to the compost-amended soil, and as a consequence, lowered the number of irrigation applications in amended soil. The difference of the number of irrigation ranged from 1 to 2 and 1 to 5 applications in tropical humid and semi-arid regions, respectively. With rainfall, supplemental Irrigation was only necessary for rice and cabbage in tropical humid regions, while in semi-arid regions, irrigation constitutes the main source of crop water supply. Without rainfall, the quantity of water saved was estimated at 15.4 and 14.5% of the total water need in tropical humid and semi-arid regions respectively; with rainfall it was 54.7 and 34.9%. In Chad (a semi-arid region), this accounted for about 1.6% on average of the total Lake Chad annual inflow. Copyright (c) 2007 John Wiley & Sons, Ltd.