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Excess phosphorus (P) is one of the major problems causing algae blooms and disturbing natural equilibrium in the Lake Okeechobee, Florida. Large P loading sources to the Lake Okeechobee are known as dairy farms and beef ranches in the Lake Okeechobee drainage basin. Current P determination through standard laboratory procedures is costly, time consuming, and labor intensive. Developing a portable field sensor will shorten the required time, and reduce the cost of measurement. Therefore, a portable field Raman sensor was developed in order to determine P concentration in soil samples for the Lake Okeechobee drainage basin. The sensor has a laser source at 785 nm with a typical FWHM (full width at half maximum) of 0.2 nm and a laser probe assembly with an 1-m optical fiber. Spectral range for measurement is between 340 and 3640 cm-1. The portable Raman P sensor was calibrated in laboratory conditions. A phosphorus prediction software is written in Visual C++ and incorporated with a prediction model developed using SAS statistical analysis software. The portable Raman P sensor displays and stores Raman spectra, and predicts P concentration of the soil sample for future use. This portable P sensor would be very useful for various applications and the only available tool for on-site P measurement for scientist, agriculturist, environmentalist, and anyone who needs to measure P in soils.