Near-Infrared Reflectance Spectroscopy for Soil Organic Phosphorus.

Organic phosphorus (OP) is abundant in soil and is an essential source of phosphorus for plants. To date, however, there is no direct method to quantify the total concentration of OP in soils. Near-infrared reflectance spectroscopy (NIRS) is a direct, rapid, inexpensive, and accurate analysis technique for a wide variety of materials and it is increasingly used in soil science. The aim of this study was to examine the potential of NIRS to predict total soil OP. Soil samples (n = 120) were taken from an experimental site near Indian Head, SK, Canada, from short-term (8 years) and long-term (>28 years) conservation tillage plots of a field pea ̶ spring wheat rotation receiving  five P fertilizer rates annually. Samples were collected at three soil depths (0-7.5, 7.5-15, and 15-30 cm). Soil total OP was determined by the ignition method. Calibration NIRS equations were developed using 80% of the soil samples and the partial least squares regression while the remaining 20% of samples were used for validation. The predictive ability of NIRS was evaluated using the coefficient of determination of validation (R2) and the ratio of standard error of prediction to standard deviation (RPD). Results show a moderately successful NIRS prediction for total OP (0.80 ≤ R2 < 0.90 and 2.25 ≤ RPD < 3.00). The NIRS predictive ability of OP appears to be related to the relationship with soil organic carbon. Further studies are needed to evaluate the potential of NIRS for predicting the chemical forms of soil OP.