African Scientist Vol. 2. No. 3, September 30, 2001
Printed in Nigeria
1595-6881/2001 $12.00 + 0.00
© 2001 Klobex Academic Publishers
AFS 2000137/2305
Effect of heat treatment on phosphate sorption by the soils
of the Southern guinea savanna of Nigeria
M. O. Aduloju and A. O. Olaniran
Department of Crop Production, University of Ilorin, P. M. B. 1515, Ilorin, Nigeria.
(Received November, 11, 2000)
ABSTRACT: Two soil samples, representing Kulfo and Omu-Aran series from the Southern Guinea Savanna
of Nigeria were first subjected to heat treatment before phosphate sorption studies. The heat treatment
temperatures were 00C, 1000C and 2000C. These two soils were analyzed chemically for pH, organic carbon,
total acidity, effective cation exchange capacity (ECEC), amorphous iron and aluminium. The heat-treated
samples of each soil type adsorbed more phosphate than the control, although there were no significant
differences in the amounts of P sorbed. Omu-Aran, which contains higher values of amorphous iron and
aluminium than Kulfo series, sorbed more P at each level of heat treatment than Kulfo series.
Key words: Heat treatments: Phosphate sorption: Amorphous iron and aluminium; Kulfo and Omu-Aran
series.
Introduction
The term “phosphate sorption” is used to describe any process in which the phosphate ions in solution
react with atoms on the surface of soil particles. Phosphate sorption is commonly measured by shaking
samples of the soil with phosphate solutions, measuring the change in phosphate concentration and calculating
the phosphate adsorbed, plotting the calculated adsorbed phosphate against the observed solution
concentration, then summarizing the result. This plot is known as the Quantity/Intensity plots (Barrow, 1978).
The reaction between soil and phosphate continues for a long time, but at a decreasing rate. Although
the reaction may become slow, it does appear to stop or reach equilibrium. The continuing reactions between
soil and phosphate appear to be important in the decline in the availability of phosphate fertilizers with time
(Barrow, 1979).
Kitur and Frye (1983) highlighted the effect of heat treatment on the chemical characteristics of soils.
Heating of soils to 1100C decreased pH, but the pH increased when the soils were heated to 2000C. Organic
matter and extractable Mg decreased, while extractable NH4 and Na, and electrical conductivity (EC) increased
greatly with heating. The effect on soil test phosphorus and potassium, and extractable calcium were small,
but the soils were already high in these elements. Increases in extractable ammonia, manganese, EC and pH
probably came from both the destruction of soil organic matter and the release of inorganic compounds in the
soil.
High temperatures are known to enhance the rates of phosphate sorption and desorption (Low and
Black, 1959; Muljadi et al., 1966; Gardiner and Jones, 1973; Barrow and Shaw, 1975; Barrow, 1979). Apart
from the physiological effect on plant growth, the net effect of temperature on phosphate availability also
depends on the relative degree to which the process of sorption and desorption change with temperature after
phosphate fertilizer is applied.
Singh and Jones (1977) used phosphorus sorption-desorption isotherms to evaluate phosphorus
requirements of lettuce grown at five different temperature regimes. The desorption and the growth
experiments wee carried out at five different temperatures on soil samples previously treated with phosphate at
different temperature from that used for desorption by aqueous extraction or by lettuce. They concluded that
the effect of temperature on desorption of phosphorus and subsequent plant growth should be more important
than its effect on sorption, and hence, that more phosphorus were required when temperatures were low. Their
conclusion appear to contradict the observation of Beaton and Read, (1963), Barrow, (1963) and Barrow
(1974) that high temperatures lead to an increased in phosphate sorption and hence, reduces phosphorus
availability to plants.
There is little published work on the effect of heat treatment (temperature) on the phosphate sorption
of the highly weathered tropical soils. This paper therefore aims at providing some information on the effect of
heat treatment on phosphate sorption by the soils of the Southern Guinea Savanna Zone of Nigeria.
Materials and Methods
General characterization of the soils
Soil samples collected from Omu-Aran (Omu-Aran series) in Kwara State and Mokwa (Kul;fo series)
in Niger State of Nigeria were used in this study. The pH of the soils were determined by the glass electrode
pH meter method in a 1:1 soil: water ratio. The organic carbon was determined by the chromate wet oxidation
method (Nelson and Sommers, 1982). Organic matter was obtained by multiplying the organic carbon value
by 1.724. The exchangeable bases were extracted with normal, neutral ammonium acetate. Sodium and
potassium in the extract were analyzed by flame photometry method while calcium and magnesium were
determined by the varsenate titration method. Total acidity was determined by extracting 10g of soil with 100
mls of 1M KCl solution for one hour. Ten mls of the extract was titrated against 0.01N NaOH, using
phenolphthalein indicator. The effective cation exchange capacity (ECEC) was calculated as the summation of
the exchangeable bases and the total acidity. Available P was determined by the Bray No. 1 method (Bray and
Kurtz, 1945). The amorphous iron and aluminium were determined by Tamm’s ammonium oxalate method
(Jackson, 1969).
Phosphorus stock solution
Potassium dihydrogen orthophosphate (KH2PO4) weighing 0.44g, was dissolved in 100mls of distilled
water to give a 100-ppm stock solution. This stock was used to prepare the various concentrations of P by
serial dilution for the phosphate sorption studies.
Heat treatment
Each soil sample was subjected to heat treatments of 100 0C and 2000C by keeping the weighed
samples in labeled ceramic crucibles in a muffle furnace for two hours. In the control, the samples were not
subjected to any heat treatment before they were used for phosphate sorption studies. Each treatment was
replicated three times.
Phosphate sorption studies
Eleven 1g – soil samples were weighed sample tubes from each soil treated to 0 0C (control), 1000C
0
and 200 C respectively. Ten mls of 0.04M KCL solution was added to each tube as well as 10mls of different
concentration of phosphate solution. The tubes were shaken for one hour the first day and 30 minutes each day
for the next five days. Equilibrium was assumed to have taken place by the sixth day. The pH values of the
contents of the tubes were recoded daily. On the seventh day, the contents of the tubes were filtered through
Whatman No. 42 filter paper. The phosphate concentration of the clear filtrate was determined by the Murphy
and Riley (1962) method at 600 mm wavelength. The difference iun phosphate concentration before and after
shaking with the soil sample was used to calculate the quantity of phosphate adsorbed by the soil sample. The
adsorption data were fitted into the Langmuir equation for each sample according to Bache and Williams
(1971) as follow: c/q = 1/KQm.
Where c = equilibrium concentration of phosphate ion (mg P mL-1
Q = the quantity of material sorbed by a unit weight of adsorbent (mg P g-1)
Qm = adsorption maximum (mg p g-1)
K = sorption affinity (a constant).
Results and Discussion
The general characteristics of the soils
The general characteristics of the soils are as shown in Table 1. The effective cation exchange
capacity (ECEC) is low in each soil, with a value of 2.42 and 3.25 cmol kg -1 for Kulfo and Omu-Aran seies
respectively. This is indicative of the degree of weathering that has taken place in the soils and the dominance
of Kaolinite in the clay fraction of Nigerian soils. Intensive weathering gives rise to low ECEC values which
results from a predominantly kaolinitic clay (Gallex et al., 1975). The soil reaction of both soils is strongly
acidic and the organic matter content is low. The available P content of the soils is also low. The low values
of organic matter and available P are indicative of active annual bush burning in the two areas and this leaves
little organic matter than can mineralize to boost the available P content of the soil. Amorphous iron (Fe) and
aluminium (A1) values ion Omu-Aran are greater than those recorded for Kulfo soil.
Table 1: The general characteristics of Kulfo and Omu-Aran soils
Characteristics
Kulfo
Omu-Aran
PH
Organic matter (g/kg)
5.3
64
5.4
52
Total acidity (cmol/kg soil)
Effective C.E.C> (cmol/kg soil)
0.18
2.42
0.40
3.25
Available P (mg/kg soil)
2.2
4.50
Amorphous Fe (%)
Amorphous A1 (%)
0.62
0.09
0.69
0.14
Phosphate sorption
Table 2 shows the mean phosphate sorption factors in the soils studied. Generally, Omu-Aan soil
sorbed more phosphate than Kulfo soil. This may be due to the fact that the former is higher in the content of
amorphous iron and aluminium (Table 1). The P values adsorbed by Omu-Aran soil was 2.00 mg/g for 00C;
2.40 mg/g for 1000C and 2.50mg/g for 2000C treatment, while the equilibrium phosphate was 4.2mg/ml;
4.95mg/ml; and 5.30mg/ml for 0 0C, 1000C and 2000C respectively. The equilibrium phosphate for Kulfo soil
was 3.6mg/ml; 4.6mg/ml and 4.95 mg/ml at 0 0C, 1000C and 2000C respectively.
The higher the temperature, the more the phosphate sorbed by each soil (Table 2). This shows that
temperature has a direct effect on the amount of P sorbed. Increased temperature has been reported to decrease
the concentration of phosphate in solution. This indicates that adsorption is exothermic (Barrow, 1979). The
increase in phosphate adsorption with increasing temperature agree with the reports of Chien et al., (1982);
Gardiner and Jones (1973); Barrow and Shaw (1975a) and Barrow (1979) that at higher temperatures,
phosphate is sorbed more than at lower soil temperatures.
Table 2:
Effect of heat treatment on the quantity and intensity factors of phosphate sorption in Kulfo
and Omu-Aran soils
Temperature
(0C)
0
100
200
Soil Series
Sorbed P (mg g-1)
Kulfo
1.71
Equilibrium P (mg
ml-1 )
3.6
Omu-Aran
2.0
4.2
Kulfo
2.20
4.6
Omu-Aran
2.40
4.95
Kulfo
2.40
4.95
Omu-Aran
2.50
5.30
The implication of these results is that in these highly weathered soils of Kulfo and Omu-Aran, high
temperatures are likely to lead to an increase in phosphate sorption, and hence, the phosphate availability in
these soils may be reduced. The annual bush burning exercise in the Southern Guinea Savanna is known to
increase soil temperatures of the plough layer to about 1000C within an hour after the burning (Olofintoye,
1984 – personal communication). The chemical reactions generated by such temperature increase may be
irreversible, especially where compounds of iron and aluminium in the soil are concerned. Continual burning
of bush, which is commonly practiced in the Southern Guinea Savanna, may further increase the phosphate
sorption capacities of the soil rich in sesquioxidic clays, thereby reducing their P utilization efficiency.
ACKNOWLEDGEMENT: We gratefully acknowledge the assistance given by Mr. A. Mohammed for the
analysis of the general characteristics of the soil samples.
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