Comparison of extraction methods for the determination of available phosphorus in some selected soils from the Eastern region of Ghana.
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Date
September, 2015
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Abstract
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
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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
Description
A thesis submitted to the Department of Chemistry, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of MPhil. Analytical Chemistry