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Farmers in developing countries sometimes receive incorrect and misleading information regarding the fertilizer inputs into food production, especially during crises or times of unusually high demand. In the early 2000s farmers of Thailand were sold fake fertilizer or advised to apply excessive amounts of fertilizers by fertilizer dealers. The problem was so pervasive that a protocol was developed to address the problem. This protocol of three steps was based on providing improved information to empower the farmers. The first step was a simplified, field identification of soils, the second was a soil test kit that farmers could use themselves, and the third, described here, was a decision-aid that brought together soil, crop, climate and fertilizer principles with local field data needed to select the correct fertilizer and to calculate the correct amounts to apply. The SimCorn software was designed to estimate fertilizer N, P, and K amounts and how to supply these nutrients using existing fertilizer materials, which seldom come in the desired ratios of N, P, and K. The N algorithm was based on the Decision Support System for Agrotechnology Transfer (DSSAT) 3.5 simulation model which has been adapted to accept soil test kit nitrate as the current N status of the field. Phosphorus requirements were predicted by the Phosphorus Decision Support System (PDSS} program. Potassium fertilizer requirements were based on a Mitscherlich-Bray equation. These algorithms were implemented in both desktop and handheld computers in both Thai and English languages. A blending algorithm was also implemented which calculates the amounts of existing N, P and K fertilizers for a blended fertilizer. This software was first implemented on a handheld platform using a Palm operating system and then subsequently implemented on a desktop computer under a Windows XP operating system. The software is used by extension officers and other interested persons.