Plant Nutrients and Fertilizers
This lecture deals with an addition of soil nutrients
to enhance crop productivity. Given the problems of maintaining fertility
with increased tillage and the resultant depletion of organic matter (O.M.),
the addition of plant nutrients to the system becomes a part of crop management.
II. Learning Objectives
To become aware of the impact of fertilizer addition to crop
To know the basic, primary and secondary micronutrients essential
for crop and plant growth and development.
||To introduce the primary materials used in cropping systems
to deliver nutrients to the field.
||To become aware of different fertilizer combinations
and methods of application of these materials
||To become aware of different fertilizer combinations
and methods of application of these materials
Fertilizer use has led to the ascendance of U.S. agriculture in the
world. After World War II, the munition of plants producing Nitrogen were
converted to plants that could manufacture fertilizer Nitrogen. Cheap,
available Nitrogen was responsible for the substantial increase of corn
production. Fertilizers accounted for about half of the increase
in crop yields in the U.S. between 1941 and 1955.
The primary fertilizer elements are called "primary" because they are
used in the greatest amount by the plant. These elements are Nitrogen (N),
Phosphorus (P), and Potassium (K). A typical
corn plant producing 150 bu/a would need 150 lb.. of N, 55
lb.of P, and 65 lb. of K. Nitrogen is especially
important in the photosynthetic reaction and the formation of amino acids
in DNA/RNA and the cell divisions and growth. Phosphorus
is essential in respiration, providing the "P" in the ATP
cycle, and in photosynthesis by providing the P energy reaction"
for the NADPH2 cycle of photosynthesis.
Potassium is essential to the smooth running of plant enzymes into the
These are several common carriers of these elements, and their use is
ultimately determined by their cost and the concentration of the essential
elements. Application of these materials is often a compromise between
economics and plant intake efficiency. Management of fertilizers is a major
part of a modern agricultural system, and is an important consideration
in a sustainable agricultural system. As fertilizer use becomes more and
more expensive, and application methods become critical for both economically
sound and environmentally responsible agriculture.
In modern agricultural systems, increased emphasis is being made to
properly obtain soil samples to estimate fertilizer weeds. Detailed
soil mapping, and state of the art application equipment is being used
to prescription feed crop fields. These systems address crop yield,
economic, and environmental concerns. Uniformity, in many developing
countries, the fertilizer technology is very expensive, primitive and,
in some places, is nonexistent. Here lies one of the real differences
in agriculture between the developed and developing worlds.
1. Fertilizers accounted for about half the increase in crop yields
in the U. S. from 1941-1955.
2. Increased use of fertilizers/improved technology are major hope
for developing nations.
3. Twenty elements necessary for normal growth of crop plants:
General Information About Fertilizers
12 are frequently deficient in agriculture soils
N-P-K known as fertilizer elements
Ca and Mg constituents of lime
S another major element
1. Three figures written - 10-20-10 refer to percentage of N,
P2O5 and K2O.
2. Acidity and basically of fertilizers
3. Cost of fertilizers.
Fertilizers substantially increase soil acidity
The increase exchangeable and toxic Al
All fertilizers containing ammonium compounds have acidic residual
effects on soils
Acidic fertilizers are generally considered desirable in irrigated
alkaline calcareous soils
Fertilizer Elements and Materials
Cost/unit of element can be estimated by dividing percent of element
Nitrogen is the most extensively deficient and most widely applied plant
Its availability in soils influenced by:
Nitrate is subject to leaching
Nitrogen can be denitrified by soil microorganisms
Can be lost to atmosphere
Formed primarily by catalytic process combining pure N
and hydrogen gas to form ammonia
With high energy cost - nitrogen fertilizers have risen rapidly
- 82% N - most concentrated gas under pressure - converted
to liquid form. Least expensive N fertilizer not widely
used in developing countries, handling high pressure tanks a problem.
Urea - 46% N
- relatively low cost, easily vocalized - should be incorporated, over
1/4 of world's N supply and increasing.
- 34% N - half in ammonium form, half in nitrate form - higher
price, 1/4 of world's N supply, decreasing.
- example 18-46-0 usually thought of as phosphate fertilizer.
- 21% N - declining in usage - from 30% in 1955 to less than
10% now - strong acidifying effect - low N content/marketing
Nitric Phosphates -
currently used in Europe.
The more concentrated the
Nitrogen in the fertilizer carrier, the cheaper the cost of transportation.
High yields of crops most often effected by deficiencies in P
(one of most common limitations).
Phosphorus fertilizers value based on its relative solubility.
- (16-22% P2O5) -treating ground
rock phosphate with sulfuric acid. Declining in use because of low
Phosphoric Acid -
(55% P2O5, 24% P)
can be evaporated to a 72% P2O5)
product. Rock phosphate with sulfuric acid - used in liquid fertilizers
- suited for application in irrigation water.
- (44-52% P2O5, 19-23% P)
one of the most extensively used P fertilizers - rock phosphate
with phosphoric acid - is 95-98% water soluble.
- produced by reactions between ammonia and phosphoric acid high content
of plant nutrients/shipping and handling costs relatively low.
decreases at both high and low pH's. (diagram)
Phosphorus is usually absorbed by plants as H2PO4
and HPO42 - ions
In acid mineral soils, aluminum and iron are absorbed on clay and
react quickly with soluble phosphate to produce compounds that decrease
Ultisols and oxisols generally have high P-fixing capabilities
Soils between pH 6.0-8.5 are usually most favorable for P
nitration of plants
Is absorbed by plants in higher quantities than most mineral elements
K is absorbed as the K+ ion
K deficiencies often occur because of the large and
rapid uptake as farmers create conditions for high yields
- (50-60% K).
- (45-50% K). Source of S where
element is deficient
Potassium is depleated easily
with the rremoval of high yielding crops without retaining residue.
(I.E. corn, silace, hay)
4. Calcium and magnesium and liming.
In strongly acid soils having Al as dominant exchangeable
element, potassium not firmly held and it leaches
Liming leads to replacement of Al by Ca and
K is better retained
5. Micro nutrients
Ca and Mg essential plant nutrients
Both originate in soil from weathering of rocks and minerals
Mg deficiencies are fairly extensive
A high content of exchangeable Al can cause toxic levels
of Al in soil solution
Acid soils do not retain K, Ca, and Mg
well and they leach out
Phosphate ions react with Al and Fe and
their effectiveness is reduces
Treatment of acid soils with lime is multiple purposes:
reduce exchangeable Al, Fe, and Mn
furnish Ca and Mg
improve physical properties of soil
improve P availability
improve capability of soil to retain and release K, Zn, and
Materials for soil liming
finely ground limestone
ground dolomitic limestone
Neutralizing capacities of lime materials expressed in calcium carbonate
pure calcite value = 100
calcium hydroxide = 136
Lime materials evaluated on basis of
fineness of grinding
content of Mg
For oxidized and leached soils such as oxisols and ultisols - pH of 5.0-6.0
In many tropical humid regions with highly weathered and leached soils/local
lime sources not available - shipping cost prohibitive
If you can not afford to
do anything else, LIME! Adjustment of pH improves natural systems
ability to supply nutrients.
In general - pH above 5.0 not necessary in tropics
Estimating Fertilizer Needs
Required by plants in small quantities ranging from a few hundred grams
to a few kilogram/A
Small quantities can be fixed in fertilizer or applied as foliar sprays
1. Soil tests
2. Plant analysis
Soil analysis has become most extensively used method for diagnosing
Must secure a representative sample
Soil testing is an estimate
of the nutrients availabe for crop uptake, tissue analysis. It tells
what plants could take up.
Fresh tissue test in field
Tissue analysis in the laboratory (more accurate)
1. Common practice to mix materials so 2-3 or more elements in
2. Bulk blending common in major farming areas.
3. Small farms in developing countries - bagged, granular fertilizer
easiest to handle
Bulk handling - save $8 - 12/ton
4. In U. S. and Europe, N applied as a separate
material (at least portion).
Nitrogen use efficiency
is higher and loss of Nitrogen to the environment is less when Nitrogen
is applied when plant demand is greatest. (I.E. when corn is 12"
This is because of high level of N use
Application at special times
1. Usually a compromise between ideal and practical.
2. In course soil - split applications reduce leaching.
3. In loams and clays - apply before planting.
4. Best to have a good nitrogen fixing legume crop followed by
crop with high N requirements.
5. N usually applied with each crop.
6. P and K frequently made on selected crops
in the cropping sequence.
Method of Application
1. Most efficient utilization of fertilizer N is when
applied during periods of high plant demand - e.g. tasseling.
2. Best to make nutrients position available to plant roots in
early growth stages.
3. Banding and size dressing advantageous where soils low in fertility
and only limited quantities of fertilizer are used.
4. Broadcast applications used - where soils are maintained in high
states of fertility.
5. Anhydrous ammonia and nitrogen solutions usually injected.
6. Broadcast applications on perennial sods or legume crops.
7. Broadcast where no-till is practiced.
Fertility application method
is dependent on level of fertility, climate, and available management.
Proper use of fertilizer can greatly improve the productivity of crop plants.
Fertilizer can partially overcome the loss of fertility that is usually
associated with tillage, and makes up for nutrients lost by the wind and
water erosion that accompanies even the best crop systems. However,
fertilizer is an exhaustible resource. Nitrogen fertilizer is often
tied to petrochemicals, Phosphorus and Potassium are both salts that are
mined, and deposits will not last forever. The countries in the world
that are using fertilizers heavily, have the cheapest food, and the highest
standard of living. Unfortunately these resources are finite.
In the developing world, atmospheric Nitrogen fixation, soil nutrient recycling
and O.M. addition are the primary methods of providing essential plant
nutrients to crops. The irony is, we might all need to look to these
methods in the future to supply the nutrients needed to support life on
Plowing every 4-5 years helpful to distribute fertilizer
VI. Self Assessment
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||Be able to list primary nutrients and descrive why they
||How do primary elements differ from secondary and micro
elements? Which are more important?
||How would you chose a fertilizer for your crop and how
would you determine the proper application?
||Describe what systems the primary elements impact most
within crop plants.
||Be able to contrast fertilizer usage in the developed
and developing worlds. What problems do you forsee if this trend