Abstract:

Studies were conducted to determine the phosphorus status and sorption
characteristics of three Vertisols of Uganda collected from Kamutur in Kumi District,
Olweny Swamp Rice Irrigation Project in Lira District and Sironko in Mbale District.
Soil site and profile descriptions were done to confirm that the soils in the selected
fractions and establishing their ability to sorb phosphorus. The effects of organic
matter application on the phosphorus sorption parameters, and on the availability of
native phosphorus were determined. Changes in the inorganic phosphorus fractions
after incubation with superphosphate were also determined.
There were quantitative differences in the status of native inorganic phosphorus in
the soils. Bray I phosphorus in the composite samples of the surface soils (0-20 cm)
was high (39.5 ppm P), medium (11.6 ppm P) and low (6.6) in soils from Sironko,
Kamutur and Olweny, respectively. Total phosphorus was highest in Kamutur soil
(1322 ppm P), followed by Sironko soil (374 ppm P) and lowest in Olweny soil (335
ppm P). Of the active inorganic phosphorus fractions (aluminum-bonded
phosphorus, calcium-bonded phosphorus and iron-bonded phosphorus), aluminumbonded
phosphorus (Al-P) was the predominant fraction in all the soils; forming
62.2%, 89.7% and 65.8% of the total active inorganic phosphorus fractions in
areas are Vertisols. The phosphorus status and sorption characteristics of the soils
were determined by sequentially partitioning their different inorganic phosphorus
Kamutur, Olweny and Sironko soils respectively. Application of 30 mg P kg'1 soil
active inorganic phosphorus fractions; the increase in Al-P being significant only at
90 days of incubation.
The soils differed in their ability to sorb added inorganic phosphorus and the standard
phosphorus requirements (SPR) were of the order: Kamutur soil (46.61 ppm P) >
Olweny soil (36.08 ppm P) > Sironko soil (8.62 ppm P). Phosphorus sorption in the
soils from Kamutur and Olweny deviated from the conventional Langmuir equation
and conformed to the binary (two-surface) Langmuir relationship. This is an
indication of the existence of two types of sorption sites in the soils having different
affinity for phosphorus; and this results from creation of new sites due to
displacement of structural silicates of clays at high concentrations of phosphorus in
equilibrium solution. For the Sironko soil, phosphorus sorption conformed to
conventional Langmuir equation suggesting sorbed phosphate formed a monolayer
on the sorbent surfaces. Application of organic matter, in form of cellulose and
starch, at 20 g kg'1 soil increased phosphorus sorption maxima in the soils from
Kamutur and Olweny but decreased that for the soil from Sironko. The increases in
Kamutur soil were 5.5, 6.5 and 12.4 ppm P at 30, 60 and 90 days of incubation
respectively. For Olweny soil the increases were 39.9, 42.4 and 47.8 ppm P at 30,
60 and 90 days of incubation respectively. In Sironko soil the decrease (of 4 ppm P)
in phosphorus sorption was significant only at 60 days of incubation.
as single superphosphate in Kamutur and Olweny soils significantly increased all the
No immediate phosphate fertilization is recommended for Sironko soil because of the
high level of available phosphorus, but for Kamutur and Olweny soil there is need
for phosphate fertilization for higher crop yields. The standard phosphorus
requirements indicate that Kamutur soil requires 93.22 kg P ha'1 and Olweny soil
72.16 kg P ha'1 for optimum crop growth. However the actual economic rates should
be confirmed from field experiments.
Incubation studies showed that most of the inorganic phosphorus added in Kamutur
and Olweny soils was recovered as calcium-bonded phosphorus followed by
aluminum-bonded phosphorus at 30 and 60 days of incubation. This implies that
most of the applied phosphorus would be available to crops in the short term.