ARS | Publication request: Differences in modified Morgan phosphorus levels determined by colorimetric and inductively coupled plasma methods

Submitted to: Open Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 25, 2012
Publication Date: September 1, 2012
Citation: He, Z., Zhang, H., Olanya, O.M., Frantz, J.M., Larkin, R.P. 2012. Differences in modified Morgan phosphorus levels determined by colorimetric and inductively coupled plasma methods. Open Journal of Soil Science. 2(3):256-262.

Interpretive Summary: Phosphorus (P) fertilization is frequently needed for profitable crop production. Modified Morgan P (MMP) is a soil test P used to estimate plant available P in soils. MMP can be determined by two methods. The critical values of MMP for P fertilization and maintenance recommendations are based on the measurement of a traditional colorimetric (M&R) method. However, an advanced spectroscopic (ICP) method is increasingly used in MMP measurements. In this study, we collected 120 surface soil samples of Caribou Sandy loam under potato cultivation or its rotation crops from Maine. The MMP levels in those soils measured by the two methods were compared. Our data show that the MMP levels in all 120 dry samples were greater when they were measured by ICP method than M&R method. However, the two sets of data were positively and significantly correlated. Based on the observations in this work, we concluded that both M&R and ICP methods can be used for P fertilizer recommendation, but a conversion factor should be applied for ICP data as the current recommendation systems are based on colorimetric M&R data. In future work, more field samples should be tested for setting the conversion factors as the factors could vary with different soil types and cropping management practices due to the presence of different pools of P.

Technical Abstract: Phosphorus (P) fertilization is frequently needed for profitable crop production. Modified Morgan P (MMP) is a soil test P used to estimate plant available P in soils. The critical values of MMP for P fertilization and maintenance recommendations are based on the P concentrations measured by a common colorimetric molybdenum (Mo) blue Murphy-Riley (M&R) method although other P quantification methods have also been used for MMP measurements. In this study, we collected 120 surface soil samples of Caribou Sandy loam (fine-loamy, isotic, frigid Typic Haplorthods) under potato cultivation or its rotation crops from Maine, USA. The MMP levels in those soils measured by different methods were compared. For the 25 field moist soil samples, the MMP levels measured by M&R method were greater than those measured by a modified Mo blue method, in which the modification improved the specificity on soluble inorganic P measurement. The MMP levels in all 120 dry samples were greater when they were measured by an inductively coupled plasma (ICP)-based method, compared to the corresponding data produced from colorimetry. However, the two sets of data were positively and significantly correlated (r=0.900, P<0.001). Based on the observations in this work, we recommend the modified Mo blue and ICP methods be used for investigating inorganic P and organic P partition in soil MMP pools. Both colorimetric and ICP-based methods can be used for P fertilizer recommendation, but a conversion factor should be applied for ICP data as the current recommendation systems are based on colorimetric M&R data.