Comparison of extraction methods for the determination of available phosphorus in some selected soils from the Eastern region of Ghana.

Soil testing for phosphorus (P) is used agronomically to determine the amount of phosphorus needed for crop production. To characterize phosphorus in soil system and develop principles and knowledge of its nature and behaviour in soils, developing methods for soil testing and availability of soil Phosphorus to plants are essential. Soil phosphorus tests involve extraction of phosphorus from soils with chemical extractants followed by a quantification of P in the extracting solution. The concentration of available phosphorus in soil needed by plants is usually low compared to the total phosphorus in the soil. The low levels of plant available phosphorus is due to the high reactivity of soluble phosphorus with calcium, iron and aluminium that leads to its precipitation in soil and due to this it is often present in unavailable forms. As a result of the high phosphorus fixation (low soil solution phosphorus concentration) in soils, crop yields are often low. Seven soil phosphorus extraction methods were compared in this study to identify the most efficient method in extracting available phosphorus from the soil samples and to find out if the soil physicochemical properties correlate with the concentration of extracted phosphorus. The extractants include Bray 1, Bray 2, Mehlich 1, Mehlich 3, Olsen, Disodium EDTA and Distilled water. Twelve soil samples were obtained from the Eastern Region of Ghana for this study. The quantities of P removed by the extraction procedures varied for the different soil samples analyzed. The concentration of available P in the extracts was determined by the molybdenum blue method. The pH of the soils ranged from 5.08 to 7.55 that is from slightly acidic to neutral. Percentage organic matter was generally low for all the soils ranging from 0.836% to 3.078%. Texturally the soil samples varied from loamy sand to sandy clay loam. The mean concentration of available phosphorus extracted from the soils ranged vi from 0.100 to 9.926 µg/g soil. The highest concentration of available P was removed from soils collected from Akwadum Cocoa farm and the least from Huhunya plantain farm. The statistical analysis on the mean concentration for all the different soil samples revealed the following order of decreasing extracting performance: Bray- 2 > Mehlich -1 > Mehlich -3 > Bray -1 > Disodium EDTA > Olsen > Distilled water extraction. Correlation analysis between the extractants indicated that there was a strong correlation between Mehlich-3 and Bray-2 P (r = 0.8059), Mehlich -3 and Mehlich - 1 (r = 0.7964), Olsen and Bray -2 P (r = 0.7190), Disodium EDTA and Bray-2 P (r = 0.713) as well as Mehlich-3 and Bray- 1 method (r = 0.7074). Similarly Olsen method showed a close correlation with that of Disodium EDTA method. There was however a poor correlation between Distilled water P and Olsen P (r = 0.2806), Distilled water P and Bray-1 P (r = 0.2828) as well as between Bray- 2 P and Distilled water P (r = 0.2417). There was also quiet significant correlation between distilled water and Mehlich-1 (r = 0.051) and also between distilled water and Mehlich-3 (r = 0.1592). Distilled water and Disodium EDTA on the other hand had a quite significant correlation. Correlation analysis between the extractants and physicochemical properties of the soil was non- significant except for the percentage organic matter which correlated with the extractants

Source: KNUST