Agriculture Reference
In-Depth Information
where
a
is the amount of P adsorbed (μg/g),
C
is the
concentration (mg/L) and
b
(mL/g), is the buffer power
defined by the slope of the sorption curve at the point
where
P
/
C
= 1 (mL/g).
The parameters
a
and
b
were estimated by regression of
the logarithmic form of the data obtained from adsorp-
tion isotherms. Theoretical doses of P and phosphatic
fertilizers required to develop P levels in soil solutions
under field conditions were calculated from this equation.
The experiment was conducted in a permanent layout
that included two crops of berseem separated by one
mung bean crop (three replications) with 10 treatments
arranged in a randomized complete block design
(RCBD) having a plot size of 5 × 3 m. All agronomic and
recommended cultural practices were followed for
normal growth of the crops. Berseem was sown during
second week of October at a seed rate of 20 kg/ha on
all three sites. A starter dose of nitrogen (20 kg/ha)
along with phosphorus calculated by using a Freundlich
adsorption equation was applied at sowing time. N and
P were used in the form of urea and triple super phos-
phate, respectively. Samples at each cutting were taken
and total yield of berseem was calculated. The total P,
crude fibre and crude protein of each cutting of berseem
were determined. Soil sampling was done immediately
after harvesting of berseem. Adsorption isotherms were
constructed, and P doses were calculated based on the
Freundlich model for each treatment before sowing
mung bean to maintain the same soil solution P level.
After berseem harvesting, mung bean was grown
using seed applied at 20 kg/ha. Phosphorus doses were
applied at sowing. The crop was harvested at maturity
and grain yield was recorded. Post-harvest soil samples
were collected for the construction of an adsorption
isotherm. Again fertilizer doses were calculated and
applied before sowing berseem to maintain the same
soil solution P levels.
After the harvesting of mung bean berseem was again
sown on all three sites. Phosphorus doses calculated
from the Freundlich equation were again to achieve the
same soil solution P levels at sowing time. Samples at
each cutting were taken and the total yield of berseem
was recorded. The total P, crude fibre and crude protein
of each cutting of berseem were determined. The ana-
lytical procedures were based on the methods described
by Bigham (1996) and Richards (1954) or otherwise
mentioned. All the plant samples were subject to
proximate analysis using standard methods. Dry matter
(%), crude protein % (CP) and crude fibre % (CF) con-
tents were determined by standard methods in the
laboratory.
17.3 results and discussion
17.3.1 phosphorus adsorption
characteristics of the selected soils
To the study the adsorption capacity of the three soils,
adsorption isotherms were constructed and the parame-
ters of the Freundlich equation for each soil are presented
in Table 17.2. The buffering capacities (
b
) of the three
soils were 58.25, 42.40 and 39.00 L/kg for sandy clay
loam, clay loam and sandy loam soils, respectively. The
integrity of the fit of the model was ascertained by looking
at the
R
2
values, which were 0.97, 0.98 and 0.96 for
sandy clay loam, clay loam and sandy loam soils, respec-
tively, indicating high conformity of the adsorption data
with the Freundlich model. The relationship observed
between equilibrium phosphorus concentration (EPC)
and
x
/
m
(amount of nutrient adsorbed on soil colloids)
was linear (Table 17.3). The values of the exponents
were found to be less than unity, which relate to the
characteristics of the adsorbent, i.e. soil or CaCO
3
. Kuo
Table 17.2
P sorption parameters of the Freundlich model.
Soil
Amount
adsorbed (
a
)
(mg/kg)
Buffering
capacity (
b
)
(L/kg)
Coefficient of
determination
(
R
2
)
Sandy clay loam
92.59
58.25
0.97
**
Clay loam
104.11
42.40
0.98
**
Sandy loam
70.90
39.00
0.96
**
**
Significant at
P
= 0.05.
Table 17.3
Model and linear parameters of the Freundlich
equation.
Soil
Freundlich equation
Model form
Linear form
Sandy clay loam
P
a
= 92.59
C
58.25
y
= 0.5825
x
+ 1.9666
Clay loam
P = 104.11
C
42.40
y
= 0.3900
x
+ 1.8507
Sandy loam
P = 70.90
C
39.00
y
= 0.4240
x
+ 2.0175
a
P in solution.
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