TECHNICAL PAPER #33
AND ORGANIC FERTILIZERS
Dr. Kenton Brubaker
Dr. Roy L. Donahue
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
Understanding Inorganic and Organic Fertilizers
[C]1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are urged to contact VITA or a similar
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
voluntary basis. Some 500 volunteers were involved in the
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Maria
as editor, Suzanne Brooks handling typesetting and layout,
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Kenton K. Brubaker,
Professor of Biology and Director of International Agriculture
Eastern Mennonite College, Harrisonburg, Virginia. He
his doctorate in horticulture from Ohio State University and
had experience in tropical agriculture in Zaire, Bangladesh,
Haiti. His current research focuses on the use of organic
in vegetable production. The reviewers of this paper are
also experts in agriculture. Roy Donahue has served as an
and forester in Asia, Africa, and South America. J. Walter
Fitts is President of Agro-Services International, Inc., an
research, analysis, consultation, and planning firm in
Orange City, Florida. Lee Fryer is President of Earth Foods
Associates in Wheaton, Maryland.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries. VITA
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
situations. VITA maintains an international Inquiry Service,
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING INORGANIC AND ORGANIC FERTILIZERS
by VITA Volunteer Kenton K. Brubaker
Every farmer and gardener realizes that plants receive some
their substance from the soil. Just how much plants depend
soil fertility is not always obvious, however, because so
other factors also influence plant growth--water, sunlight,
pests, and plant variety (genetics). In regions of the world
where crop yields are extremely high, farmers add large
of fertilizer, usually in the form of a commercial product,
they purchase at considerable expense from a farm supply
For example, in the corn belt of the central United States,
yields of over 12 metric tons per hectare (200 bushels per
may be achieved by using hybrid corn, more than 125
(kg) of fertilizer per hectare (100 pounds per acre), and
large amounts of irrigation water. Such a farmer may spend
$500 per hectare for fertilizer to produce a crop worth
In much of the world such capital-intensive agriculture is
because of its high cost and often would be unwise due to
the uncertainty of rainfall, insufficient length of growing
or possible lack of demand for the crop at harvest.
addition of some fertilizer may be economically justified.
The decision as to whether or not to use fertilizer will
depend on the answers to the following questions:
Will fertilizer substantially improve the
Will the increased value of the crop cover
the cost of
Are the risks associated with producing the
crop (lack of
rain, short growing seasons, pest damage,
market) low enough to justify the investment
If the answers to all of the above seem to be
"yes," then an
additional set of questions should be asked:
What type of fertilizer is needed, and how
When and how should it be applied?
Will the addition of fertilizer change plant
such a way that other problems may
develop, like increased
susceptibility to drought or pests, collapse of
due to stem weakness (called lodging in
or an undesirable change in quality such
texture, or nutritional value?
Answers to these questions may not be easy to obtain since
is often essential. Usually the farmer or gardener needs
to experiment with fertilizer use in the field in order to
the advantages or disadvantages. However, fertilizer
are often very difficult to interpret due to the many crop
variables, so that information about experiments by local
research stations may be highly desirable.
II. BASIC SOIL FERTILITY THEORY
LAW OF THE MINIMUM
Crop growth and yield depend on a complex set of growth
The law of the minimum states that growth or yield is no
than the factor that is most limiting to growth. Some
such as lack of water or obvious pest damage, are usually
for the farmer to recognize. However, some limiting factors
not as easily detected, like the lack of an essential soil
mineral element (e.g., nitrogen, phosphorus, or potassium),
the lack of good root growth due to poor soil drainage, or
insect or nematode eating the roots. Weed growth or soil
are other factors that may not be obvious to the grower and
are most likely to limit yield.
The law of the minimum may also be applied to the
growth due to the lack of just one soil mineral among the
that are essential. If we consider just three of the soil
minerals--nitrogen, phosphorus, and potassium--and assume
all other growth factors are adequate, the one mineral that
not available in sufficient amount will be the one that
the yield. Figure 1 illustrates the effect of three
soil nitrogen levels on yield.
FACTORS LIMITING CROP GROWTH
The first step in considering matters of soil fertility is
determine what factor or factors are most likely to limit
growth and yield. For example, if lack of soil fertility is
indicated, then one must find out which nutrient is lacking.
Throughout the world, this element is most often nitrogen.
Several factors can limit plant growth:
Lack of water
Lack of sunshine
growing season too short
days too short
too cloudy, or crops shaded by trees
Lack of oxygen for roots
soil water-logged, poor drainage
soil too compact, tight
Soil too cold; may fail to warm up because
Competition with weeds or other plants (too
Pests and diseases that attack leaves,
insects (e.g., beetles, grasshoppers,
diseases (e.g., wilt, mosaic, blights,
birds, rodents, and other animals
Lack of soil nutrients due to
soil erosion with loss of most fertile
soil chemistry, especially improper soil
leaching (removal of nutrients by the
downward in the soil) or cropping removal)
Crop variety, genetics
(*) pH indicates the acidity or alkalinity of the soil, and
on a scale of about 4.0 to 6.5 (acid), 6.5 to 7.5 (neutral)
above 7.5 (alkaline), with the midpoint of 7 indicating the
neutral soil condition. Most plants prefer a pH of about
which is slightly acid.
THE NATURAL CYCLE OF PLANT NUTRIENTS: THE NITROGEN CYCLE
Plant nutrients are neither created nor destroyed; they
change their chemical form and move from place to place. The
movement of nitrogen is interesting, complex, and usually
most crucial to plant growth, so we will deal with it in
detail in this paper.
The earth's atmosphere is the greatest reservoir of
percent of air is made up of this valuable element. Here it
present as a pure element, [N.sub.2], a form that most
use. The most important occurrence in plant nutrition is the
process in which the elemental nitrogen of the air is
into forms of nitrogen that most plants can absorb through
root systems. This process is called nitrogen fixation.
There are three ways nitrogen from the atmosphere can be
for use by plants (see Figure 2):
capture of nitrogen by nitrogen-fixing
algae (a natural process);
fixation of nitrogen by lightning in
industrial fixation of nitrogen in
Nitrogen Fixation by Bacteria and Blue-green Algae
Certain bacteria and blue-green algae are naturally equipped
absorb inorganic, elemental nitrogen from the air and
change it through the addition of hydrogen (called chemical
reduction) to the kind of nitrogen found in the organic
of plants and animals called protein. The nitrogen of
present as amine nitrogen, symbolized chemically as the
By maintaining a well-drained but moist soil, the
nitrogen-fixing microorganisms can be cultivated, providing
cost.-free source of organic nitrogen. However, these
must have an energy source on which to feed, such as straw
other plant residue, and this usually limits the amount of
Other nitrogen-fixing bacteria live in specialized plant
tissues called nodules where they fix nitrogen and make it
to the host plant. Plants that contain nodules are usually
legumes, which include members of the bean and pea family. A
nodule that is active in fixing nitrogen will have a pink
if it is broken open and examined. The bacteria that live in
nodules are called symbiotic because they benefit their host
well as get benefits from the host plant.
The water fern, Azolea, widely used in paddy rice culture,
has nitrogen-fixing microorganisms living in its tissues.
organisms make nitrogen available to both their natural
water fern, and to the rice plant. Thus, a farmer or
who grows legumes or other plants such as Azolea, which have
nitrogen-fixing microorganisms associated with them, is able
convert free elemental nitrogen of the air into organic
of the crop plant.
Nitrogen Fixation by Lightning
Another natural process that converts elemental, atmospheric
nitrogen into a form useful to plants is the electrical
lightning, which occurs in thunderstorms. This process
oxidizes nitrogen (combines nitrogen and oxygen) forming an
nitrogen compound called nitrate ([NO.sub.3]-). This very
fertilizer is readily absorbed through the roots of
plants. Electrical storms may contribute a substantial
nitrogen to the soil in some areas, although the heavy
associated with such storms may tend to wash the nitrate out
the plant root zone fairly quickly. For this reason, a well
developed root system, such as that of trees and grasses, is
essential to capture this form of naturally-fixed nitrogen.
Industrial Nitrogen Fixation
A third process of fixing atmospheric nitrogen is
modern chemical technology in industrial facilities. This
uses natural gas and other hydrocarbon fuels to produce
ammonia ([NH.sub.3]), ammonium ([NH.sub.4]+), and urea
([NH.sub.2] Q/[CNH.sub.2]), both useful
forms of chemically reduced nitrogen. Ammonia can be
inorganic nitrogen, while urea is an organic form of
because it contains carbon.
Table 1 summarizes the forms of nitrogen obtained from the
SOME SOURCES OF NATURAL NITROGEN FERTILIZER
A rich and valuable natural source of nitrogen fertilizer is
oxidized, ancient deposits of bird and bat manure, known as
guano, which occur in various locations around the world,
in coastal regions and caves. The nitrogen in guano,
which is collected and sold as fertilizer, is usually
Forms of Nitrogen Obtained from the Atmosphere
available to plants except
certain bacteria and
Protein or amine
nitrogen produced by
nitrogen-fixing bacteria and
blue-green algae and
into the proteins of
microorganisms or the
plant when the
with the host plant.
nitrogen produced by
Organic nitrogen produced by
nitrogen and hydrogen
natural gas, coal, or
with potassium (K) or sodium (Na), forming potassium nitrate
([KNO.sub.3]) or sodium nitrate ([NaNO.sub.3]).
Another important natural source of nitrogen fertilizer is
or composted animal manure and human wastes. These are a
mixture of several forms of nitrogen including urea
protein (organic, mostly bodies of microorganisms), nitrates
([NO.sub.3]), ammonia ([NH.sub.3]), and, ammonium
([NH.sub.4]+) compounds. The value
of animal and human manures as fertilizer depends on how the
manure is handled, since it is a rich culture of bacteria,
living and dead, and various forms of nitrogen. If the
exposed to oxygen, the reduced forms of nitrogen (protein,
and urea) may be changed to nitrate by bacteria, or the
population of bacteria may increase dramatically and
most of the nitrogen as protein in their own cells. If the
manure is handled so as to exclude oxygen (kept wet or
packed to exclude air), bacteria growth may be limited and
nitrogen will be mainly kept in the reduced forms (ammonia,
ammonium, urea, and protein).
Whether or not the manure is kept under shelter to protect
from rain is also crucial since urea and nitrate nitrogen
easily washed out of the manure. Ammonia nitrogen is also
readily lost to the air as it is quite volatile, but in the
it changes to ammonium ([NH.sub.4]+) and is absorbed by
Since the nitrogen content of animal manures is so easily
several management suggestions should be followed:
Keep the manure under a roof to prevent
that dissolve easily in water.
Incorporate it into the garden or field as
prevent loss of ammonia (or ammonium).
Use a cement floor for storage to prevent
loss of the
portion in which most of the urea and nitrate is
Sufficient bedding to absorb the urine also
Compost human manures thoroughly to ensure
parasites are killed. (A description of
methods of composting human wastes is beyond
the scope of
Another source of nitrogen fertilizer is compost, a
mixture of plant materials and manure. The nitrogen content
compost is usually very low unless it contains substantial
amounts of legumes and manure and is handled with the same
as manure. The state of decomposition would also influence
percentage of available nitrogen it contains.
A final natural source of nitrogen fertilizer is the use of
crops, especially legumes, as green manure. Crops that are
high in nitrogen are turned under and allowed to decay, thus
releasing the nitrogen they obtained from the air through
activity of the symbiotic bacteria in their nodules.
Decomposition microorganisms play an important role in the
natural cycle of nitrogen. Nitrogen may be lost from the
phases of the cycle when certain soil microorganisms
convert nitrates into elemental nitrogen, which then escapes
into the atmosphere. This loss seems to occur most readily
the soil is water-logged and microorganisms are forced to
nitrates ([NO.sub.3], [NO.sub.2], and NO) for their source
of oxygen. Naturally,
this loss of valuable fertilizer nutrients should be avoided
if at all possible by seeing that the soil is well drained
thus well supplied with oxygen from the atmosphere. A well
drained soil that permits good oxygen entrance can be
by good cultural practices, especially by the addition of
To sum up, then, management of the nitrogen cycle may be the
important activity a farmer carries out in relation to soil
fertility. The lack of usable nitrogen is the most frequent
of poor crop growth and yield in most soils around the
The nitrogen of the atmosphere is made available to plants
through nitrogen-fixation. The growth of both free-living
symbiotic bacteria can be managed to increase the amount of
nitrogen in the plant growth cycle. Both symbiotic and
microorganisms grow well in moist, well-aerated soil.
The chemical state of nitrogen must be appreciated to manage
cycle successfully. Organic nitrogen in mainly protein, and
important waste product, urea. Such nitrogen is said to be
chemically reduced or combined with hydrogen. Upon
of protein and urea by bacteria, the nitrogen is released as
volatile gas, ammonia. This reduced form of nitrogen can be
absorbed by plant roots, and it can also be converted by
to an oxidized, non-volatile form, nitrate, which is also
soluble and absorbed by plant roots.
Commercial fertilizers may be in the form of ammonia,
salts, urea, or nitrate, all of which can be quickly
plants. Urea quickly changes to ammonium and can then be
absorbed by plants. Green manures and the protein components
animal manures must be changed to ammonium and nitrate
they can be absorbed by plants. Before conversion to soluble
forms of inorganic nitrogen, the insoluble organic nitrogen
green and animal manures forms a reservoir Of nitrogen that
be released slowly (through bacterial decay) during crop
This slow release prevents its rapid loss during heavy
Highly soluble fertilizers like urea and nitrate are quickly
when leaching occurs. Ammonia can also be lost as a gas, and
nitrate can be changed to elemental nitrogen by
soil microorganisms and lost to the atmosphere.
INORGANIC AND ORGANIC FERTILIZERS
Inorganic fertilizers are generally salts of metals such as
sodium, potassium, calcium, and magnesium. Ammonia can also
as a carrier of other inorganic nutrients when it occurs in
form of a salt of ammonia (ammonium salt). Several important
inorganic fertilizer salts are listed in Table 2.
2. Some Important Inorganic Fertilizer Salts
Source: N. Brady, The Nature and Properties of Soil (New
New York: MacMillan and Sons Publishing Co., 1984).
Note that each of these fertilizer salts contains a certain
of the nutrient element based on the relative weights of
all the atoms in the molecule.
Chemically speaking, organic molecules, and thus organic
are those that contain carbon in organic form. The organic
molecules we have considered so far are protein and urea.
organisms contain many other important organic molecules
carbohydrates and nucleic acids. Any fertilizer whose
are present mainly in organic molecules like urea, protein,
or nucleic acids is called organic fertilizer. In general,
such fertilizers (compost, manure, and cottonseed meal) have
low nutrient content and release these nutrients very
This is because bacteria and fungi must first decompose the
organic molecule for the nitrogen to be freed as ammonia or
phosphorus to be released as phosphate. Urea is an important
exception to this general rule; it has a very high nitrogen
content (46 percent) and is readily available for plant root
absorption after a day or two when it has been converted by
bacteria to ammonium salts.
Some examples of organic fertilizers with approximations of
nutrient content are given in Table 3.
The highly variable nutrient content of organic fertilizers
their use more complicated than that of inorganic
especially if the grower intends to achieve very high
This is because the content and form of nutrients is
only approximately known. Also, the generally low nutrient
of the organic fertilizer makes it necessary to add very
quantities of the fertilizer to the soil. The third
factor in the use of organic sources of nutrients is the
release of most of the organic nitrogen and phosphorus. The
organic matter must first be decomposed by soil
which in turn must also die and decompose, before a
amount of these nutrients is available to plant roots. For
suppose that the organic fertilizer to be used is compost,
green manure, or animal manure--or a combination of any of
If the approximate analysis of the organic material is
(nitrogen-phosphorus-potassium), how much would be needed
hectare to furnish the nutrients to produce 6 metric tons of
(100' bushels per acre)?
One estimate suggests that the following amounts of
nutrients are needed to produce such a yield.
Total needed to produce six
metric tons of corn/hectare
3. Total Nutrient Content Of Some
Urea ([NH.sub.2] [CNH.sub.2])
Guano (bat or bird fecal
Compost (highly variable)
Green manure (legumes)
Horse, cow, or hog manure
Dried fish scraps
Cooperative Extension Service, Organic Vegetable
Gardening, Circular 375-A (Gainesville, Florida: University
of Florida, Institute of Food and Agricultural Sciences, May
If we added 50 metric tons of organic fertilizer per
following amounts of nutrients would be supplied:
250 kg nitrogen,
50 kg phosphorus; and 150 kg potassium
However, only about 30-50 percent of the nitrogen and
would be available the first growing season due to the slow
of decomposition of the organic matter.
About 50 percent or
more of the potassium would be available.
In conclusion, it becomes
obvious that supplying all nutrients in organic form is a
rather uncertain and labor-intensive practice.
As a result, organic
fertilizers may need to be supplemented with chemical
Application of 50 metric tons of organic matter to a hectare
kilograms/are(*)) is a huge job.
Furthermore, availability of that
much material may also be a problem, and working the organic
matter into the soil may require a large expenditure of
Addition of large amounts of organic matter to the soil may
lead to a phenomenon known as "nitrate
depression," where the
soluble nitrogen gets incorporated in the bodies of soil
until the carbon of the organic matter is decomposed.
this reason, the straw (cellulose) of organic matter should
decomposed rather thoroughly before it is used as
Adding nutrients to the soil in the form of organic matter
easy, but it can be done.
The process is an imitation of the
natural fertility cycle of a forest, grassland, or
and wise management plus a lot of hard work are essential
to making the process work successfully.
Alternative methods of adding large amounts of organic
should be evaluated.
Composting is essential to decrease the
carbon content of the plant material that is added to the
heap, thus permitting more rapid release of the nitrogen and
phosphorus when the material is added to the soil.
important technique is to use the partially decomposed
matter as a mulch, thus allowing the composting process to
on the surface of the ground.
The mulch that remains on
the soil surface at the end of the growing season may then
incorporated into the soil as compost.
A third alternative is to
incorporate fresh or partially composted organic matter into
soil just before a fallow period, allowing soil
begin decomposition during a winter or dry season period
crops are not growing.
Little soil microorganism activity occurs
during such a fallow period, but some beneficial
decomposition does take place.
(*) One are = 100 square meters = .01 hectare.
COMMERCIAL FERTILIZER FORMULATION
Suppose we wanted to make a complete inorganic fertilizer,
is, one containing nitrogen, phosphorus, and potassium, all
from inorganic fertilizer salts.
If we mixed potassium
nitrate and ammonium phosphate, we would have such a
To give a simple example, suppose we mixed 100 kilograms of
potassium nitrate ([KNO.sub.3]) and 150 kilograms of
[([NH.sub.4]).sub.2] [HPO.sub.4] to make 250 kilograms of
complete fertilizer. Let
us calculate how much of each element would be present in
batch of fertilizer.
100 kilograms KNO
150 kilograms (NH) HPO
We can now calculate the percentage of each element
this mixed fertilizer as:
45.5 kg/250 kg = 18 percent
34.5 kg/250 kg = 14 percent
39.0 kg/250 kg = 16 percent
We would label this an 18-14-16 fertilizer.
In commercial trade,
this would be considered a high-analysis fertilizer because
contains a fairly high content of nutrients and no filler.
Many commercial fertilizers, at least those that are
inexpensive, have a lower analysis, like 5-10-10.
In such a
fertilizer, the inert material (filler such as sand or
would be 75 percent of the weight.
If one needed to transport
the fertilizer a long distance, this non-nutrient weight
should be considered.
High-analysis fertilizers give more nutrients
per kilogram but they often require special care in
handling and storage.
For example, they must be kept dry because
the salts readily pick up water and so are packaged in
bags and stored in dry areas.
Anhydrous ammonia, a very
high-analysis nitrogen fertilizer, is handled as a liquid
pressure in corrosion-resistant tanks.
Many dry fertilizers are
granulated and coated with clay and wax to make them easier
store and handle.
The coating may also slow the release of the
nutrients when added to the soil; this slower release may be
the inert material may contain some trace
elements that may be absent in high-analysis fertilizers.
DETERMINING THE NEED FOR FERTILIZERS
Observation of Visual Symptoms
Under severe deficiency conditions, a trained plant
can diagnose the need for a particular fertilizer element by
examining the growth of the affected plants and the plants'
example, nitrogen-deficient plants are small and
have a yellowish appearance, especially the lower leaves.
Potassium-deficient plants may show dead tissue around the
of lower leaves and other symptoms such as missing kernels
ears of corn.
Iron-deficient plants usually show a marked yellow
color (chlorosis) at the growing tips of the plant.
use of visual symptoms is not a reliable method of assessing
need for fertilizers.
Many factors limiting plant growth (e.g.,
nematode damage or magnesium deficiency) will cause similar
Also, when several factors are involved, the
visual symptoms can become very confusing.
Even experts have
difficulty identifying a deficiency by visual observations.
Moreover, by the time visual symptoms occur, so much damage
already taken place that correction of the problem is too
be of much value for the current crop.
Soil and Tissue Testing
Analyzing the soil before planting and testing appropriate
before visual symptoms occur are better methods of
the need for fertilizers.
Soil or tissue samples are
usually sent to a central laboratory, which then gives
Portable kits are also available to test soil
and tissues but require a good understanding of their use
general, portable soil-testing kits are used best
in conjunction with a standard soil and tissue testing
Experimental Testing and Crop Yield
The best method of assessing the need for fertilizer is
field trials in which various combinations of plant
applied to the soils and crops in question.
Again, this procedure
needs to be carried out with great attention to experimental
design but finally becomes the basis for other techniques
soil analysis. Such
field trials are usually carried out by research
centers. In most
developing countries, a farmer or gardener
can often determine the need for fertilizer by fertilizing
only a part of a field or garden and observing the results.
SYSTEMS OF CROP FERTILIZATION
NATURAL SYSTEMS USING SOIL-ENRICHING FALLOW
All successful crop production systems that do not rely on
the addition of fertilizers must imitate the natural cycle
existed in the region before the land was cultivated and
to raising crops.
This principle is most clearly seen in the
"slash-and-burn" or "swidden" agricultural
method of the tropics.
With this practice, a forested area that appears to be
for cropping is first selected for clearing.
demonstrates its fertility by the vigor of plant growth,
trees and undergrowth.
The farmer can possibly evaluate the
yield potential by feeling, smelling, and tasting the soil,
by observing forest growth.
A fertile soil feels soft and crumbly,
smells somewhat like new-mown hay, and tastes slightly sour.
In the tropics, larger amounts of plant nutrients are stored
the existing vegetation than in the soil.
With the "slash-and-burn"
practice, this reservoir of plant nutrients is returned to
the soil surface as ash through careful burning of the mass
may also help kill pests in the soil including
weed seeds. A
mixture of crops is then planted, including
legumes as well as many other plants whose size and
imitates the forest structure they have replaced.
After two or three years of crop production, the yield
to the point where weeding no longer seems practical and the
field is allowed, or encouraged, to return to mature forest
rapidly as possible.
Many slash-and-burn farmers cherish the
sprouting trees that will regenerate the nutrient stores of
mature forest. The
roots of these trees and vines will penetrate
deeply into the soil and retrieve nitrogen and other soluble
nutrients that will have leached from the topsoil during the
brief period of cropping.
This forest fallow (regrowth) may require
12-20 years to regenerate soil fertility.
such as the planting of tree legumes could possibly hasten
regeneration, but the cycle cannot be shortened too much or
soil will be permanently damaged.
pressures in many areas force farmers to re-use fields
they have fully regenerated, and crop yields have declined
Other cropping systems such as wet rice paddies also imitate
natural swamp ecosystem, but these may be associated with an
annual flooding cycle, and so are not dependent on a
The flooding brings a substantial quantity
of nutrients from the eroding hillsides farther up the
Flooding also makes soil nutrients such as phosphorous more
CROP ROTATION WITH GREEN MANURES
A system widely practiced before about 1950 in the temperate
agricultural regions is crop rotation.
Here cash crops such as
corn and wheat are rotated with soil building crops such as
clover, alfalfa, or beans, usually soybeans.
Some of the soil-improving
crop may be removed as hay or, for beans, seeds to sell,
but as much as possible is returned to the soil as a way of
building up the nitrogen content of the field.
Before the wide
use of commercial fertilizers, this was one of the most
practices of temperate agriculture.
In combination with the use
of manure (the next alternative discussed), it is still
by a small group of farmers known as "organic"
farmers may also use limited amounts of commercial
(the last alternative described below).
COMBINING CROP PRODUCTION AND ANIMAL HUSBANDRY
Many farmers find that the incorporation of animals into
agricultural system is crucial to crop production.
from these animals is carefully placed on the fields.
with a smaller area to cultivate, may incorporate animal
into a composting system, thereby increasing the quantity
quality of the organic fertilizer they use to fertilize their
farmers have developed especially intricate
systems of using both animal and human manure (known as
soil) in the production of crops.
The integration of hogs and
fish into these systems is also crucial to food production
To make compost, a partially decayed mixture of mostly plant
material, the following points should be kept in mind:
Use plant residues as rich in nitrogen as
with animal manure. Materials rich in
nitrogen include legumes and animal
Chop as finely as practical and mix the
time, if you wish to achieve more rapid decomposition.
Keep moist but not saturated so that air
Add superphosphate or rock phosphate to
the loss of
Add a small amount of already partially
rich-garden soil to promote favorable decomposition.
inoculate the compost with useful
Keep the compost heap large enough to
not so large that air is excluded (a minimum
two square meters). A compost heap that
will not heat adequately enough to destroy
and pathogenic organisms.
APPLICATION OF COMMERCIAL FERTILIZER
When it is impossible or impractical to use natural methods
maintaining soil fertility, the addition of commercially
fertilizers is necessary.
They can also be used to supplement
any of the above alternatives.
Applying the proper kind and amount of fertilizer is
since these materials are highly concentrated and often
The kind and amount of fertilizer must usually be determined
experimentally and should be adapted to the soil and
Usually the fertilizer is placed in the soil below and
beside the seed so that the growing roots can quickly begin
feed on the nutrients.
Under no circumstances should chemical
fertilizers be mixed with seed; to do so will kill the
of fertilizers, especially nitrogen,
may be spaced out over the growing season in regions of very
IV. CHOOSING THE
BEST SYSTEM OF CROP FERTILIZATION
ADVANTAGES AND DISADVANTAGES OF THE FOUR ALTERNATIVE SYSTEMS
Natural-Soil Enriching Systems
On the plus side, these systems
Are inexpensive because a free service of
annual flooding, natural reseeding.
Provide many benefits in addition to
that the farmer may not even be aware of,
recycling of trace minerals and pest control
Offer ecological stability and genetic
part of a complex natural system with
species cooperating with one another.
On the other hand, such systems
May require years to regenerate fertility,
substantial percentage of land in fallow.
deficiency occurs, such as very low levels of
in the soil and soil-forminq materials,
soil-enriching systems do not replenish these
Are difficult to manage if poor or
undesirable tree or
Are not easily adapted to mechanized crop
natural soil-enriching systems are labor intensive.
Will not support large populations.
Crop Rotation with Green Manures
The advantages of crop rotation with green manures include:
Free source of nitrogen through
where legumes are grown in the
Green manure crops control soil erosion
and may control
Green manure crops not only improve soil
dramatically improve soil structure and increase
May be combined with animal production.
Some of the disadvantages include the following:
A considerable amount of land must be used
taking it out of production.
Incorporating the green manure crop into
the soil may
considerable animal or mechanical power to turn
The cost of good seed may be prohibitive.
Inoculation with suitable bacteria may be
Green manure crops often deplete soil
a dry soil
for the succeeding crop.
Integration of Crop Production and Animal Husbandry
Integrated systems have a number of advantages.
Animals provide valuable manure; they can
also graze on
unsuitable for cultivation and eat roughage unsuited
consumption, turning these materials
and animal products.
Animals can help diversify the range of
and give work when crops do not require attention.
example, fences can be repaired and manure
times when work in the crop fields is not
Draft animals help work the land and carry
market. Cattle may also be
driven to market for sale.
Animal products (meat, milk, cheese, eggs)
quality of the human diet.
Animal manure will improve the composting
nitrogen for microorganism growth and ensuring
completion of the decomposition process.
Like green manures, animal manures also
On the other hand,
Animals may be expensive and require
special skills and
not readily available, such as veterinary
and high protein feed supplements.
Animals require that a certain amount of
land be devoted
or other animal feeds; this land must
to protect crops.
Animals require constant care, which may
during busy crop production periods.
Animal manure may be a source of
insects, and some disease organisms.
Application of Commercial Fertilizers
Some of the advantages of the use of commercial fertilizers
A fertility program can be designed
especially for a
crop under specific soil conditions.
By selecting the proper fertilizer, rapid
or slow release
nutrient can be regulated.
High yielding plant varieties can be used,
called "miracle hybrids."
These new hybrid
are designed to produce higher yields in
additional fertilizer and water. Their
potential has been increased through plant
Land that has been depleted of nutrients
can be rapidly
in many cases.
Irrigated lands can be farmed intensively.
Large urban populations can be sustained.
As with the other systems, commercial fertilizers have
These include the following:
The cash investment may be prohibitive.
Often other supporting technologies are
fertilizers, such as irrigation and pesticides,
increasing the cash investment. This
"package" of technology may be required as
increased through new programs of fertilization.
The fertilizer may be applied incorrectly
wrong type, incorrect placement, or wrong
Commercial fertilizers add only nutrients;
they do not
soil structure. Unless good soil
maintained, the soil will deteriorate, and increasing
commercial fertilizers will be required
a given level of production.
Facilities for handling and proper storage
of the fertilizer
ASSESSMENT OF LOCAL CONDITIONS AND RESOURCES
In choosing a new crop fertilization system, or more likely,
modifying a current system, one must realistically assess
resources. First, it
is important to analyze carefully the
system currently being used.
It may be useful to concentrate on
the movement of nitrogen through the cycle, and note where
improvements of nitrogen availability to plants can be
Perhaps commercial nitrogen fertilizer could be applied on
crops to find out if additional nitrogen will increase crop
yield. It may also
be useful to determine the value of a phosphorus
or potassium fertilizer on each of the important crops in
Second, the nature of the soil or soils in the region should
to consider here would be the depth, texture
(soil particle size), structure (crumbs, blocks, plates),
matter content, drainage, slope, and nutrient content of the
soil, including the acidity or alkalinity (pH).
The third factor to consider is the suitability of the crop
crops to the local soils, rainfall, temperature, length of
season, ease of production, and marketability.
arrangement of crops on the farm and the best planting and
harvesting sequence also need to be assessed.
The final factor to be considered is the availability of
of plant nutrients.
Are local deposits of nutrient-rich materials
available? If the pH
needs to be modified, is ground
limestone available locally? If organic matter is needed,
good sources available?
How could animal husbandry be better
utilized to furnish humus and nutrients to the soil?
If resources are not available locally, then nutrients may
to be imported into the region.
The organization of such supply
systems may be carried out by private businesses, the
or community cooperatives.
Again, careful assessment and management
is necessary to make certain such resources are both
and economically justified.
Rainfall and Irrigation
Many of the new high-yielding crop varieties require large
of water and irrigation is often essential to increase
yield. This may
require great expense if water must be pumped
from a well or river.
Many agricultural development schemes have
run into considerable difficulties as water supplies became
or fuel costs increased sharply.
An additional consideration
is the expense of leveling the land to allow efficient
Also, for some soils, farmers need to prevent
the buildup of sodium and other salts caused by the
of water after several years of surface irrigation.
Soil Texture and Drainage
Soil texture, which is the percentage of sand, silt, and
particles in the soil, must be considered in the management
soil fertility. A
sandy soil (coarse texture) will not hold
fertilizer nutrients against leaching.
should be added in small amounts and fairly frequently.
such a loose soil is well drained and thus permits good
of both plant roots and soil organisms.
Organic matter (humus)
added to a sandy soil may increase the humus content and
Many tropical sandy soils will not
hold humus for very long because of the extremely high rate
organic matter decomposition.
For such soils, the amount of clay
minerals is crucial since these tiny clay particles will
most fertilizer nutrients by adsorption (physical and
Silt particles, intermediate between sand and clay in size,
also intermediate in fertilizer-holding capacity.
Soils with a
high clay content may be tight and poorly drained, thus
the oxygen availability to roots.
The addition of organic
matter to such soil will often greatly improve the crumb
of the soil, permitting better water drainage and an
supply of oxygen.
Unless a soil is well-drained, addition
of fertilizer will have little value in yield improvement.
Soil reaction refers to the hydrogen ion content of the
which can be measure using the pH scale.
A pH of below 6.5 is
considered an acid soil and is unsuitable for many
addition of lime or limestone (calcium carbonate) will help
replace the hydrogen ions on the soil particles with
raising the pH to a desirable level.
Again, the higher the clay
content or organic matter in the soil, the more calcium is
to replace the hydrogen on the clay or humus particles.
Some old soils that have been leached for centuries are
acid and may require considerable treatment to make them
for certain crops.
Such soils may be suited to what are called
acid-loving crops (such as bermuda grass, cotton, cowpea,
pineapple, sweet potato, coffee, and orchids).
Previous Experience and Available Plant Varieties
The importance of research experience cannot be
considering the soil fertility system.
Such experience is
difficult to obtain because demonstrations and experiments
which just one variable at a time is being examined are hard
design, but there is no better way to determine plant
needs. When new
varieties of plants are being considered for use
in the cropping system, their response to soil fertility
examined under each type of field condition.
Such research should
be done at an agricultural research center, if possible.
V. FUTURE DEVELOPMENT OF FERTILIZATION SYSTEMS
New methods of supplying nutrients to plants are emerging.
promising is the genetic modification of plants other
than legumes to accept nitrogen-fixing bacteria into nodules
their roots. With
the advent of this technology, a major milestone
in plant nutrition will have been reached.
however, this type of genetic engineering is proving to be
complex than first anticipated.
Continued research in genetic engineering may produce
genetic potential in crop plant growth and yield.
type of plant breeding using tissue culture and haploidy
should make possible new genetic advances whose nature is
culture takes single cells from a plant and grows
them into new plants.
If these single cells come from tissue
with one set of chromosomes (haploid), such as the cells
give rise to pollen grains, then the hidden or recessive
traits will appear.
This helps plant breeders deal with one gene
at a time.
Research on the interactions of plants in mixed culture
more than one crop in a field at a time) is still only in
beginning stages, mainly because the industrialized,
type of cropping patterns have tended to overshadow the more
labor-intensive mixed culture technology.
Mixed culture requires
more harvesting and hand weeding since machines cannot
distinguish among the plants.
As certain regions of the world
concentrate more on multiple cropping (growing more than one
together), the symbiotic effects of such systems will become
Symbiosis occurs when both crops benefit by being
grown together. One
crop may help the other (e.g., corn can
support climbing beans), while in return the second crop may
furnish nutrients to the first (beans fix nitrogen, which
corn may use).
The economics of food production in the future is a major
for many persons attempting to forecast agricultural
cost of industrially-based resources, so essential for much
agriculture, is escalating rapidly.
Many North American
farmers find their labor-efficient products to be priced
the amount hungry nations can afford to pay.
For this reason,
the poorer countries are often advised to develop a national
policy of self-sufficiency, based on local soil fertility
The population pressure in most nations of the world is a
threat to many agricultural systems, especially those
fallow and crop rotation (different crops in different
the same field). In
countries with land reform programs where
landless peasants are becoming landowners, the problem of
production for export often follows.
on the nation for increased export earnings often are felt
new landowners in the form of federal decrees.
For example, a
national government may require farmers to grow export crops
coffee or bananas, rather than food crops for local use;
farmers will resent these decrees.
Economic factors often frustrate
such programs because the new farmers are unable to produce
the export crop successfully.
As a result, the land returns to
the creditors and landlessness is again established.
There is a constant struggle for farmers to care for their
and their families while at the same time trying to adjust
international economic realities beyond their control.
and improvement of soil fertility is basic to farmers,
However, there is no guarantee of success
because factors beyond individual control may render all
futile. In the
last-analysis, the protection of soil fertility
and the economic viability of the agricultural sector must
part of the food policy of every national government.
BIBLIOGRAPHY/SUGGESTED READING LIST
Brady, Nyle. The
Nature and Properties of Soil. New York, New
and Sons Publishing Company, 1984.
Donahue, Roy L., Miller, Raymond W., and Shicklum, John C.
to Soils and plant Growth. 5th edition.
Cliffs, New Jersey: Prentice-Hall, Inc., 1983.
The Fertilizer Institute. The Fertilizer Handbook.
Fertilizer Institute, 1982.
Follett, Roy H., Murphy, Larry S., and Donahue, Roy L.
Amendments. Englewood Cliffs, New Jersey:
McCune, Donald L. Fertilizers for Tropical and Subtropical
Alabama: International Fertilizer
Olson, R.A. Fertilizer Technology and Use. Washington, D.C.:
Science Society of America, 1971.
Fertilizers and Their Use. New
York, New York: