Citation
Gikonyo, Esther Wakiuru
(2005)
Dynamics of Current and Residual Phosphorus in Tropical Acid Soil.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
The use of phosphate rocks (PRs) has been proposed as an agroeconomically
sound alternative to the more expensive superphoshates
particularly, for P 'recapitalization' based on their purportedly high residual
effects in acid tropical soils. To understand the current and residual dynamics
of P from different P sources, one laboratory incubation trial and two field
experiments were conducted. Experimental treatments comprised factorial
combinations (3x4~2o) f three P sources [Triple superphosphate (TSP), Gafsa
PR (GPR) and Christmas Island PR (CIPR)] at four P rates with and without
manure in three replications. Setaria grass (Setaria Anceps Stapf Cv.
Kazungula) was used as the test crop and was sequentially harvested
bimonthly for 14 months in the field. Results indicated that the amounts of P
extracted from treated soils using three soil tests: Mehlich3 (M3P), BrayI
(B1 P) and Pistrip (PiP) were in the order: M3P>Bl P (85% M3P) >PiP (53%
M3P) and were significantly related ( ~ ~ = 0 . 4to2 0 .83, n=294). Phosphorus
extracted from the different P sources was in the order: TSP>GPR>CIPR and
increased with increasing P rates. When P rates were raised from 0 to 300 kg
decreased substantially at the highest P rate. The degree of phosphorus
saturation (DPS) (defined as a ratio of P already adsorbed to P adsorption
capacity of a soil) also varied with P sources (5.1 to 15.8%) and extractants
(2.1 to 44%) following a similar order to extractable P. The variously
estimated DPS values were all significantly correlated (r =0.91 to 0.98) and
therefore were equally suitable in estimating DPS. Sequential strip P indicated
that P released was described by power (~~=0 . 7to9 0.95) (TSP) and
exponential (PR) functions ( ~ ~ = 0 . 7to7 0.99), while from the field trial, a
power function described RV of P in both PRs and TSP (~*=0.64to 0.96).
Total extracted PiP was related to NaHCOsinorganic and organic P (BicPi
and Po), and HCIP. However, though the Pistrip could estimate P release, it
could not estimate residual value (RV). Residual value is the ratio of amount
of freshly applied TSP required to produce yield X to the amount of previously
applied fertilizer required to produce the same yield X. The dry matter yield
(DMY) exhibited a quadratic relationship with P rates. The maximum DMY (6
11 t ha') was attained at 150200 kg P ha' and over time, DMY increased to
a maximum(11 t ha") and then declined to a constant yield (24 t ha.') after
one year. ManureCIPR integration increased DMY while, manureGPR and
manureTSP integration depressed yields except in the initial harvest. Setaria
DMY was found to be related to NaOHorganic and inorganic P (HydPo and
HydPi, respectively), and BioPo fractions, which are not accounted for in
M3P or B1P thus explaining the low R~ between DMY and M3P (~~=0.0o8r)
B1P (R~=o1.0 ). The current RVs of the three fertilizers determined in the field
were 100, 100 and 140% for CIPR, GPR and TSP, respectively at 100 kg P
ha'. At the same rate, fertilizermanure integration increased initial RV to 180,
160 and 110% in TSP, CIPR and GPR treatments. Wth an exception of
CIPRmanure, RV was depressed by manure in the other fertilizers in
subsequent harvests after the initial one. The RV also declined with
increasing P rates as evidenced by current RV decline to 20, 60 and 50% in
CIPR, GPR and TSP, respectively when P rate was raised from 100 to 300 kg
P ha". The RV declined to 3040% for the first six months and then gradually
to 10  20% after one year (power function). These results indicated that
GPR and CIPRmanure combination were as good as TSP and the optimal P
rate was 100  150 kg P ha". The results did not support P 'recapitalization' in
this soil but annual applications. The RV could be estimated from the P
fractions: BicPo, HydPi and HydPo, and DPS computed from Mehlich
extracted Al, Fe and P. The RV had no relationship with M3P or B1 P.
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