Comparison of different phosphorus extraction and determination methods using manured soils

Abstract

Soil test P (STP) methods were developed for fertilizer P recommendation in crop production. As soil P content of arable land increases due to manure application, the suitability of these STP methods for assessing P runoff potential from a field needs to be evaluated. The objective of this research was to evaluate various P extraction and determination methods using manured soils for their potential use as environmental P testing methods. Eighty one soil samples were taken in 1998 from various manured fields and research plots across Alberta. Phosphorus in the samples was extracted using deionized water, Miller-Axley (0.03 M NH4F + 0.015 M H2SO4), modified Kelowna (0.015 M NH4F + 0.25 M NH4OAc), and Mehlich 3 (M3) (1.5 M NH4F + 0.1 M EDTA) procedures. Phosphorus concentration in all extraction solutions was determined by the molybdenum blue colorimetric method in a Technicon II autoanalyzer. In addition, P content of the deionized water and M3 extractions was determined by an inductively coupled plasma emission spectroscopy (ICP) method. Soluble organic carbon content was measured in the deionized water extraction. The extractable P concentration increased as the acidity of the extraction solution increased in the order: M3 > modified Kelowna > Miller-Alxey > deionized water. The P extracted by Miller-Axley and M3 was strongly related to the deionized water extractable P, suggesting that both methods may be useful in assessing potential P runoff from arable land. The P concentration by the ICP determination was much higher than with the colorimetric method in M3 and the deionized water. The P concentration difference between the two determination methods was largely correlated to the soluble organic carbon in the deionized water extraction (r = 0.828***), suggesting that the undetected P by the molybdenum blue colorimetric method may exist in organic forms. To asses P runoff risk from arable land, both soluble inorganic and organic P need to be considered, and the ICP method can determine the total P, which includes both. In developing a STP for environmental soil P management, both P extraction and P determination methods should be standardized, and our results suggest that M3 and Miller-Axley extraction, and ICP determination are suitable methods for assessing P runoff. Key words: Soil test P, colorimetric P, ICP-P, dissolved organic carbon, soluble organic P, manure application rate