Abstract: A newly-designed expert fertilizing system based on PDA technology was developed and a knowledge database used for the fertilizing system PDA was designed. By applying a method of modular design, the PDA program was compiled using C programming language for the crop fertilizing expert system and this program was loaded into the PDA memory. This PDA fertilizing expert system consists of six crop subsystems and one extending subsystem, namely, wheat, maize, rice, cotton, cole, tobacco. Because the expert system PDA can distinguish different crops and areas located for planting, and offer the scheme of fertilizing recommendation after obtaining the nutrient contents of N,P,K and trace elements in soil sample, which has solved the problem that farmer cannot scientifically and rationally fertilize for a long time. The efficiency of fertilizer has been improved greatly using the expert system PDA. The crop fertilizing system PDA was verified, validated, and tested in the field, which may be widely used in the countryside of China by its small size, portable, cost-effective, light weight and easy operation.