Agriculture Reference
In-Depth Information
sorption capacities of soils provides an accurate estimate of
the fertilizer requirements of soils. Determining fertilizer
P requirements through sorption isotherms is considered
more accurate than conservative soil testing procedures.
Legume crops are essential components of all agricul-
tural systems worldwide and are important as foods for
human nutrition, as feed and fodder for animal nutri-
tion, and as soil ameliorants for soil nutrition. But they
have high P requirements (Gill
et al
., 1985). Berseem
(
Trifolium alexandrinum
L.) is one of the most important
winter forage legumes in Pakistan, India, Turkey, Egypt
and countries of the Mediterranean region.
As an annual, berseem fits well into crop rotations
and improves the soil for succeeding crops. It has wide
acceptability among farmers due to its merits over other
forage crops in the winter season. The crop gives five
cuts during November to April and its crude protein
content ranges up to 20%, with 70% digestibility, which
results in significant increases in the milk production of
animals. Berseem in fact is known as a 'milk multiplier'.
Being a leguminous crop it fixes atmospheric nitrogen
and thus is excellent for soil fertility.
Food legumes are important components of the agri-
cultural sectors of developing countries due to their
capacity to produce significant quantities of protein-rich
seeds, and to improve soil physical conditions by the
addition of organic matter and biological nitrogen fixa-
tion in the soil (Snapp
et al
., 1998). Mung bean (
Vigna
radiata
L.), commonly known as green gram, is an
important conventional pulse crop and is widely grown
in Southeast Asia, Africa, South America and Australia.
It is an important short-duration grain legume crop with
wide adaptability for different growing areas, low input
requirements and ability to improve the soil by fixing
atmospheric nitrogen. It is well matched to a large
number of cropping systems and constitutes an impor-
tant source of protein in the cereal-based diets of many
people in Pakistan and India. It has dietary advantages
over other pulses because of its high nutritive value and
digestibility and its non-flatulent characteristics. Mung
bean is grown extensively for use as a human food (as
dry beans or fresh sprouts), but also can be used as a
green manure crop and as forage for livestock.
Due to rising costs of P fertilizers and its low avail-
ability in soil to plants, it is crucial for farmers to enhance
yields and P use efficiency by adjusting site-specific and
crop-specific rates for different crops. The balanced use
of fertilizers, their nature and method of application all
play important roles in sustainable crop production.
Good crop production depends upon the apposite tim-
ing and amount of fertilization (Jan & Khan, 2000).
Keeping the above considerations in view, the present
study was undertaken with the following objectives:
1
To assess soil levels of P in solution ('solution P') of
three differently textured soils by developing their
adsorption isotherms for optimum crop growth.
2
To help formulate site-specific and crop-specific
fertilizer P recommendations for farmer's fields.
17.2 Methodology
These investigations were carried out to estimate the
model-based phosphorus (P) fertilizer requirement of
legumes in alkaline calcareous soils. The sites were selected
on the basis of land forms - i.e. recent, sub-recent and old
river terraces - and calcareousness. The soils were normal
from a salinity point of view, alkaline in reaction, low in
organic matter and deficient in available P (Table 17.1).
The sorption isotherms were examined using the
Freundlich model. It is an empirical model and corre-
sponds to a model of adsorption in which the affinity
term decreases exponentially as the amount of adsorp-
tion increases. Over a limited range of concentration,
the Freundlich model often describes adsorption well
(Barrow, 1973). The simple form of the Freundlich
model was proposed by Le Mare (1982) as follows:
P C
b
=
Table 17.1
Some basic physical and chemical properties of the
three soils.
Determinants
Soil I
Soil II
Soil III
ECe (dS/m)
1.11
1.24
1.4
pHs
8.2
8.1
7.9
OM (%)
0.56
0.72
0.45
Olsen P (mg/kg)
4.9
5.8
6.5
Extractable K (mg/kg)
62
81
115
CEC (cmol
c
/kg)
4.2
8.1
6.1
CaCO
3
(%)
8.6
9.2
5.5
Sand (%)
55
36
60
Silt (%)
24
38
23
Clay (%)
21
26
17
Textural class
Sandy clay loam
Clay loam
Sandy loam
CEC, cation exchange capacity; ECe, electrical conductivity of soil
saturated extract; OM, organic matter; pHs, pH of soil saturated extract.
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