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Ftilizer selection for sustainable agriculture
Compiled by:
Mohammad Ali Khaskheli
Agriculture Officer Shahdadpur
Crops require 16 essential
elements to grow properly. The elements include carbon
(C), hydrogen (H) and oxygen (02); those are derived
from air and water. All the remaining nutrients used by
plants come from soil in the form of inorganic salts.
The macronutrients (plant nutrients required in larger
amounts i.e. > 500 mg/kg) obtained from the soil include
nitrogen (N), phosphorus (P), potassium (K), calcium
(Ca), magnesium (Mg), and sulphur (S). The remaining
essential elements needed by plants are known as
micronutrients because plants use them in relatively
small amounts (i-e. <100 mg/kg in plants). They include:
boron (B), chlorine (Cl), copper (Cu), iron (Fe)
manganese (Mn), molybdenum (Mo) and zinc (Zn). Carbon,
hydrogen and oxygen comprise from 94.0 to 99.5 per cent
of fresh plant tissue. The remaining nutrients, which
come from the soil, make up the balance of the tissue.
Fertilizer has played a key role in helping farmers to
achieve their high level of production. Fertilizers
provide essential plant nutrients which are
indispensable for producing sufficient and healthy food
for the world’s expanding population. Plant nutrients
are therefore a vital component of any system of
sustainable agriculture. Nitrogen (N), Phosphorus (P),
and in recent years, zinc, boron and sulfur are the
nutrients of most concern in the grain-production
regions. Unfortunately the soils of Pakistan are
deficient in Nitrogen (100 %), Phosphorus (90%), Zinc
(70%) and Boron (55%). Potassium (K) is generally
adequate but its deficiency is emerging rapidly.
Deficiencies and responses to other nutrients such as
iron (Fe), magnesium (Mg), and other micronutrients are
reported for specific crops and areas. When the soil
cannot supply the level of nutrient required for
adequate growth, supplemental fertilizer applications
become necessary.
Prior to the introduction of fertilizes in Pakistan in
the early1950s, the use of fertilizers have increased
significantly. Total consumption of nutrients in
Pakistan has increased from 5 kg/ha in 1966-67 to
133kg/ha in 2001-2002. However, it is still much lower
when compared with other countries of the world and
highly unbalanced to produce enough and quality food to
meet the country demand. The crop yields in countries
using higher fertilizer rates (e.g. Korea, Japan China,
Egypt etc) are two to three times more than Pakistan.
In Pakistan various types of fertilizers are used, some
are locally manufactured and others are imported. In our
country, most of the fertilizer is used on irrigated
wheat, cotton, sugarcane and rice crops. On these crops
the nitrogen application rate is close to 75-80 percent
of the recommendations, compared with about 20-40
percent, depending on the crop, in the case of
phosphate. Hardly 1-2 percent of farmers apply potash;
that is usually applied to fruit, vegetable, and
sugarcane crops only. Micronutrient deficiencies are
common but less than five percent of the farmers apply
micronutrient fertilizers.
Improper use of fertilizer is a common problem in our
country. Farmers want to ensure good yields, and apply
unsuitable and so much fertilizer that much of it is
wasted. The problem is particularly acute with cash and
other horticultural crops.
Declining land productivity with reduced crop yields has
been also a major problem facing our farmers. The major
factors contributing to the reduced land productivity is
soil impoverishment caused by continuous cropping
without addition of adequate mineral fertilizers and
manures. Moreover, negative soil nutrient balances
(nutrient removal exceeding nutrient application) during
our cropping history have resulted in general
deterioration of fertility levels. Sustained, high yield
agricultural production can be assured once these
negative balances are addressed. The selection of
suitable fertilizer according to the crop demand and
soil type is one of the main tools available for the
solution of above said constraints.
Selection of Fertilizers
What type of fertilizer should buy and apply? The answer
to that question is not an easy one. Economics is
important, but quality, rather than cost per unit of
nutrient, should be the deciding factor in fertilizer
selection. The cost of fertilizer usually is small in
relation to the total cost of producing a crop.
Fertilizer quality cannot be defined in rigid terms. If
you know the composition and properties of fertilizers
and their behavior in the soil, you have a guide for
choosing them for specific purposes.
Characteristics that should be appraised before a
fertilizer is selected are: solubility, effect upon soil
pH, form of nitrogen, the salt index, and cost per unit
of available nutrient.
Solubility. Fertilizer compounds differ greatly
in their solubility in water. These differences are
usually unimportant for application in the solid form.
Recently attention has been given to the production of
fertilizers that have low initial solubility and release
nutrients to the soil and plant gradually over an
extended period of time. High solubility, however, is
one of the major considerations for the grower who
purchases solid fertilizer for dissolving in irrigation
water, or for application in foliar sprays. Let's
compare the solubility of some fertilizers.
Solubility of Some Fertilizers |
Fertilizer |
Solubility (g/l) |
*Acidifying effect |
Ammonium nitrate
Ammonium sulfate
Muriate of potash
Urea
Sulfate of potash
Diammonium phosphate
Monoammonium phosphate
Single Super phosphate
Triple Super phosphate
|
1810
710
350
780
120
430
230
20
40 |
62
110
0
71
0
75
58
0
0 |
|
*The acidifying effect or value indicates the pounds
of 100-score lime needed to neutralize the acidity
produced by fertilizer application of 100 pounds per
acre.
Effect upon soil pH. Fertilizer nutrients may be divided
into three classes according to their influences on the
soil reaction (pH): (1) acidic -- pH below 7.0 (2)
neutral -- pH of 7.0 and (3) basic -- pH above 7.0.
Use of ammonia compounds will eventually have an
acidifying effect on the soil. Due to this soil
acidifying action some minor elements such as iron,
manganese, copper, zinc, and molybdenum become
available. This does not mean that the entire soil
becomes acidic, just the area around each fertilizer
particle or concentration of particles. This acidifying
effect is usually of short duration.
Phosphates are generally neutral in their effect on soil
pH, and potassium carriers are basic.
Form of nitrogen. The form of nitrogen applied
influences directly the nutrition and growth of the
plant. In general, plant roots assimilate two common
forms of nitrogen-nitrate (NO3) and ammonium (NH4). Most
plants seem to prefer the nitrate form.Nitrate and
ammonium are readily absorbed and utilized by plant
roots. Under favorable conditions, ammonium nitrogen is
converted to the nitrate form by nitrifying bacteria in
the soil. Therefore, the ammonium forms are not
available as rapidly as the nitrate forms. Under adverse
soil conditions (temperature below 500 degrees F, too
wet or too dry, high salt concentrations, etc.), the
conversion of ammonium to nitrate is retarded or halted,
and the soil accumulates high levels of ammonium. These
high levels may become toxic to plants. Urea is quite
soluble and is rapidly broken down to a usable form of
nitrogen.
Besides uptake by plants, nitrogen is also removed from
the soil by leaching and volatilization. Leaching means
the nitrogen is washed or flushed out of the soil by
irrigation or rain water. Volatilization means the
nitrogen escapes as gas.
Nitrate nitrogen is sometimes leached from the soil, and
ammonium is sometimes volatilized if it is not converted
to the nitrate form. Ammonium is rarely leached because
it is usually adsorbed to the clay particles of the
soil. Nitrate does not become attached.
Stabilized ammonium fertilizer is slowly converted to
the nitrate form. Since that nitrate is available to the
plants over a longer time, it is less subject to
leaching or conversion to nitrogen gas.
Salt index. Seedling injury or "fertilizer burn" occurs
when the soil solution in contact with the seed or root
contains a high concentration of salts. The plant
seedling, because of the high salt concentration, is
unable to absorb moisture from the soil solution.
Salt injury may result from a high rate of salt-forming
fertilizers, improper placement of fertilizers,
irrigation with saline water, or farming on saline
soils.
Determination of the salt index of a fertilizer is a
means of measuring its tendency to cause seedling injury
or plant "burn." The lower the salt index of a
fertilizer, the less likely it is to cause damage.
Fertilizers with the highest salt indexes generally
supply nitrogen as the primary nutrient, high potash
materials have intermediate salt indexes, and phosphate
materials have the lowest.
Salt Index of Some
Fertilizer Materials
|
Material and Analysis |
Salt Index |
Nitrogen:
Anhydrous ammonia, 82% N
Ammonium nitrate, 34% N
Ammonium sulfate, 21% N,
24% S
Urea, 46% N
Urea-ammonium nitrate
solution:
28% N (39% ammonium
nitrate, 31% urea)
32% N (44% ammonium
nitrate, 35% urea)
Calcium nitrate, 15.5% N
Sodium nitrate, 16.5% N
Phosphorus:
Single superphosphate,
20% P205
Triple superphosphate,
45% P205
Monoammonium phosphate:
11% N, 52% P205
10% N, 50% P205
Diammonium phosphate, 18%
N, 46% P205
Ammonium polyphosphate,
10% N, 34% P205
Potassium:
Potassium
chloride, 60% K20
Potassium
nitrate, 13% N, 44% K20
Potassium sulfate, 50%
K20, 18% S
Sulfur:
Ammonium thiosulfate, 12%
N, 26% S
Ammonium polysulfide, 20%
N, 40% S
Gypsum, 23% Ca, 17% S
Magnesium oxide, 60% Mg
Magnesium sulfate, 10%
Mg, 14% S
Miscellaneous:
Calcium carbonate, lime,
35% Ca
Dolomite, 21.5% Ca, 11.5%
Mg
|
47.1
104.0
88.3
74.4
63.0
71.1
65.0
100.0
7.8
10.1
26.7
24.3
29.2
20.0
116.2
69.5
42.6
90.4
59.2
8.1
1.7
44.0
4.7
0.8
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Cost per unit of
nutrient. Several factors cause fertilizer nutrients to
vary in cost, such as the different manufacturing
processes required in their manufacture, shipping and
handling costs are greater per unit of nutrient in low
analysis fertilizers than in high analysis fertilizers,
the physical form of the fertilizer, whether gas, liquid
or solid affects handling cost. The size of purchase
also affects unit cost. In general a low-analysis
fertilizer in a small package is expensive in comparison
with high-analysis fertilizers in ton lots in bulk form.
The farmer who uses several tons of fertilizer each year
has more options in buying fertilizer than the person
with a lawn or garden.
Here are some important research facts which should be
kept in mind before using or purchasing any fertilizer;
• Ammonia fertilizers, whether dissolved in irrigation
water or not, causes a surface "sealing" when applied to
a calcareous soil. The symptoms of the seal are similar
to a sodium-affected soil-poor drainage, which causes
water to pool or stand on the soil surface.
• Phosphates move very little in the soil, and the
secondary orthophosphate (HPO4) is the phosphorus form
used most by plants in our soils.
• Phosphates are readily and rapidly "fixed" by our
alkaline soils. Liquid phosphates are fixed much more
rapidly than the dry or granulated forms.
• The polyphosphates seem to be no better or no worse
than the usual phosphate forms. There is speculation
that the polyphosphates serve as a chelating agent. If
so, they would have decided advantages.
Suggestions
Fertilizers are one of the very expensive soil inputs;
therefore a soil test should be performed before the
purchase or application of any "special purpose"
fertilizers. It is not possible to make a blanket
statement that one fertilizer is best for all crops
every where. It is true that different crops use
different nutrients at different rates. The unknown is
the reserve of nutrients already in your soil and soils
differ widely in their capacity for providing nutrients,
depending on the amount of total reserves, on
mobilization or fixation dynamics, accessibility of the
chemically available nutrients to the roots, etc.
Therefore it remains necessary to assess empirically the
nutrient status of soils and plants in order to provide
guidelines for effective fertilizer use.
Lack of knowledge is widespread and is usually due to
poor coordination between those working in research and
those in the field working as extension or field
officers. Local research work is required into soil and
crop conditions, balanced fertilization, whether macro
and micro-nutrients are required, the use of animal
manure and compost, the use of improved seed, better
cultivation and harvesting techniques, and the economics
of fertilizer use. Extension workers must make use of
demonstrations, preferably on farmers’ fields and keep
up a constant flow of information by farm visits and by
the use of radio and television. The availability of
proper fertilizers, according to soil type and crop
demand should be assured at right time.
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