| dc.description.abstract | In this study, the relative phosphorus
availability status of a range of twenty four
mainly acid, Kenyan soils (three Acrisols, six
Andosols, six Ferralsols, three Luvisols and six
Nitosols) was assessed by determining both their
phosphorus supplying power and their response to
added phosphorus from greenhouse experiments
using Serena sorghum (Sorghum vulgare Pers.) as
the test crop. These dat3 were then used to
determine the best chemical methods for the routine
measurement of 'available' phosphorus, both fDr the
whole range of soils, and for different groupings
of. the 50ils.
The phosphorus supplying power was determined
by repeatedly cropping the soils, towards. phosphorus
exhaustionover a period of 308 days. The responses
This experiment is referred to as the exhaustion
experiment
to added phosphorus were assessed over a period
of 50 days using a double-pot technlque in which
thp phosphorus was added in a 'complete nutrient •
solution' to the lower pot, that was separated
from the soil in the upper pot, but still accessible
to the plant's roots. In the controls, the minus
phosphorus nu t r i en t solution was applie d tot he
lower pot. Selected plant parameters were then
determined from these two experiments to reflect
the available phosphorus' levels of the soils.
Extractable phosphorus in the soils was
determined using nine extraction methods i.e.
Mehlich, Bray II, Olsen, Saunder, Legg, resin,
0.2N H2S04, M/500 CaC12 and mineralisable phosphorus.
The extractable phosphorus values were then
correlated with five plant parameters i.e. dry
This experiment is referled to as the response
experiment.
weight of tops, X1 and total phosphorus up Lake of
tops, X2 from the exhaustion experiment, and
percentage relative yield - dry weight (whole plant),
Y2, percentage relative yield - dry weight (tops),
Y4, and percentage relative yield - phosphorus
uptake (tops), Y6, from the response experiment.
From these correlations the extraction mp.thod~that
9 avet h e b est in d ice S 0 f 'a v ail ab 1e' p h 0 s ph a r u s lJJere
determined.
Different soil tests gave the highest
correlation coefficients for different plant
parameters in the simple regression analyses for all
soils for example: resin extractable phosphorus
correlated best with percent relative yield - dry
*** weight (tops), Y4 (r = 0.862 ) wheread Bray II
extractable phosphorus correlated best with percentage
relative yield - dry weight (whole plant), Y2 (r=Oo8l3 ).
Olsen method correlated best with the plant
parameters from the long time exhaustion experiment,
whereas the resin meted W2S generally
the short - term, response experiment. All the
soils gave a response and wen therefore all
deficient in phosphorus.
Higher correlation: coefficients were found
than those obtained by other workers who found ,
values not exceeding 0.52 accounting for only 25%
of the variation in available phosphorus, whereas
in this study correlation coefficients of 0.684
to 0 *** 0862 were obtalned accounting for 47 - 74%
of the variation in available phosphorus.
Saunder, Mehlich and indices of organic
phosphorus generally gave low correlation
coefficients. The low correlation coefficients
with the indices of organic phosphorus suggests
organic phosphorus is not an important source of
available phosphorus under continously moist
conditions in greenhouse experiments. Another
experiment designed to give wet and dry cycles showed
that under those conditions ph2sphorus uptake is
significantly increased presumably because of
organic matter mineralisation and release of
inorganic phosphorus for plant uptake.
Multiple and partial linear regression
analysis in which Quantity, Intensity and organic
phosphorus parameters were correlated
plant parameters showed that the Intensitv and
organic'phosphorus terms diJ not greatly increase
the correlation coefficientc for the better chemical
extractants viz. Olsen, H2S04, resin. The partiql
correlation coefficients showed that the Intensity
term is more important in the shorter, response
experiment than in the longer exhaustion experiment.
When the five plant parameters viz. X1, X2, Y2, Y4
and Y6 were combined and correlated against each soil
test in turn for all the tmenty four soils, reSin,
H2S04 and Olsen methods were best (correlation coefficients
of 0.9.31 *** ; 0.869 *** and 0.862 *** respectively).
The partial correlation coefficienta showed that
V2 and X2 accounted for most of the variation in
extractable phosphorus values.
The sofls were grouped into five groups on
the basis of their physical and chemical properties
viz. pH, ApH (ioe. difference between pH in water
and pH in calcium chloride), percentage carbon
levels, mineralogy and Oil the basis of their
FAD - UNESCO legend classification. Higher
correlation coefficients were generally obtained
when the soils were separated out into groups than
when all the soils were considered together and
different extractants worked best for different
groups of soils and for different plant parameters.
The most discriminating plant parameters from
the short-term response experiment (50 days) i.e.
V2 & V6 gave the best correlation coefficients when
sbils were grouped on the basis of clay mineralogy.
More attention was hO2 over paid to the two plant
parameters which gove the high~st degree of
discrimin3tion bet~een soils and :he lowest
coefficients cf variations i.e. X1 and X2. These
two paramet~rs reflect availability of phosphorus
and how it influenCES dry matter production over
long periods of time (241 or 170 days respectively)
.ullder conditions of intensive phosphorus uptake.
For these two plant parameters, grouping of
soils on the basis of percentage carbon gave the
highest set of correlation coefficient values. Thus
the Olsen method gaVE 0.955 and 00932 for the
low (L2%) and high (>208$) percentagE carbon groups
respectively, whereas S04 gave a value of 00870
for the medium <2.0 - 2.8%) percentage carbon group
Using theSE two extractants 91%, 76%, 87% of the
variation in available phosphorous as reflected by
the X2 parameter were accounted for the low,
medium end high percentage carbon groups,
If all the soils are considered together,
the best extractant was the Olsen method which
accounted for only 66% of the variation in the X2
parameter.
Since in these experimental conditions soils
were moist i.e. no wet-dry cycles, the best Qhemical
extraction methods determined, probably largely
reflect the availability of inorganic Goil phosphoruso
It is suggested that the higher cnrrelation
coefficients obtained when the soils were separated
on the basis of percentage carbon are due to the
three percentage carbon groups reflecting differences
in soil forming process ,hence in the nature of their
soil phosphorus. Thus the different groups require
different chemical extractants to determine their
available phosphorus levels. | en |
