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One of the major challenges to food security in the developing world is how to provide an adequate but not excessive application of nutrients to crops to ensure food production while maintaining or improving environmental health. This has long been a problem in agriculture and is well known to be a knowledge-intensive process. The problem solution draws on multi-disciplinary information in several disciplines of agriculture and thus provides a challenge for information technology. A handheld decision aid program, Nutrient Management Support System for a Personal Digital Assistants (NuMaSS-PDA), has been developed to support nutrient management for PDA devices for extension officers, farmer leaders, and farmers in the tropics. The program inputs begin with diagnostic field and plant data, the various observations and measured data are combined in a Bayesian combination framework for a summary evaluation of the probability of deficiencies in nitrogen, phosphorus, and potassium. This diagnostic module is one of four that comprise the decision-aid: 1) Diagnosis, 2) Prediction, 3) Economic analysis and 4) A Recommendation Summary. The Economic analysis module will in included in future releases. Efficient use, user-friendliness, cost-effectiveness, local adaptability and environmental concern were specifications for software development. The development framework uses SuperWaba, a Java-like programming tool with GNU General Public License (GPL). This development tool facilitated the implementation for multiple platforms, including Palm OS and various Windows mobile platforms. A flexible routine that permits language versions of English, French, Portuguese, Tetun, and Tagalog languages has been developed for prototype testing. The extreme programming style was followed to facilitate inclusion of multiple disciplines and viewpoints in the software. Pilot testing of the algorithms in Thailand indicates that NuMaSS-PDA improves predictions of nutrient requirements.