Open Journal of Soil Science, 2012, 2, 256-262 http://dx.doi.org/10.4236/ojss.2012.23030 Published Online September 2012 (http://www.SciRP.org/journal/ojss) Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and Inductively Coupled Plasma Methods Zhongqi He 1* , Hailin Zhang 2 , O. Modesto Olanya 3 , Jonathan M. Frantz 4 , Robert P. Larkin 5 1 USDA-ARS, Southern Regional Research Center, New Orleans, USA; 2 Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, USA; 3 USDA-ARS, Eastern Regional Research Center, Wyndmoor, USA; 4 USDA-ARS, Application Tech- nology Research Unit, University of Toledo, Toledo, USA; 5 USDA-ARS, New England Plant, Soil, and Water Laboratory, Orono, USA. Email: * Zhongqi.He@ars.usda.gov Received May 2 nd , 2012; revised June 5 th , 2012; accepted June 18 th , 2012 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 recom- mendations are based on the P concentrations measured by a common colorimetric molybdenum blue 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 under potato cultivation or its rotation crops from Maine, USA, and 72 soil samples of Cecil sandy loam with cotton/corn crops under conventional tillage and no-till management with chemical and poultry litter fertilization in Georgia, USA. The MMP levels in all 192 dry samples were greater when they were measured by an inductively coupled plasma (ICP)-based method, compared to the corresponding data produced from colorimetry. Our results show the two sets of data were positively and significantly correlated (r = 0.93, P < 0.001). In average, the ICP-based MMP level of the 192 samples was 23.3 mg·P·kg –1 with standard deviation of 12.9, compared to the average of colorimetric MMP level of 14.9 mg·P·kg –1 with standard deviation of 8.8. Based on the observations in this work, 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. Keywords: Phosphorus Measurement; Soil Test Phosphorus; Modified Morgan Phosphorus; ICP; Molybdenum Blue Method 1. Introduction Phosphorus (P) is an essential plant nutrient. P fertiliza- tion is frequently needed for profitable crop production. Soil test phosphorus (STP) is an estimate of P in soil that is part of soil P available for plant uptake during a grow- ing season. Modified Morgan P (MMP) is one of many STP methods used in the US. This method has been used to evaluate STP in Maine [1,2] and other northeastern states [3,4]. Traditionally, P concentration in a soil ex- tract is measured by a colorimetric method, which mea- sures the soluble inorganic P [5] or molybdate (Mo)-rea- ctive P [6]. With the advancement of technology, induc- tively coupled plasma (ICP)-based methods, which mea- sure total P in a soil extract, are also used in STP mea- surement [7]. As colorimetric P determination methods are initially used to calibrate STP for P fertilizer recom- mendations, the difference in STP data determined by colorimetric methods and ICP-based methods need to be examined and documented in order to appropriately in- terpret ICP-based STP data [7]. Indeed, the difference in Mehlich 3 P (M3P), another STP, measured by Mo-re- active and ICP methods has been rigorously examined by numerous research groups [7-10]. Based on about 6500 soil samples, Pittman et al. [7] reported that the M3P va- lues measured by ICP-atomic emission spectrometer (AES) were higher than those determined colorimetrically. These researchers proposed two regression equations. for P> and < 60 mg·kg −1 respectively. The first was to convert ICP-based data to colorimetric M3P values to make ferti- lization recommendations if STP < 60 mg·kg –1 , and other conversion was for STP > 60 mg·kg –1 for other uses. * Corresponding author. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture. USDA is an equal opportunity provider and employer. Copyright © 2012 SciRes. OJSS