Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville -
John T. Woeste, Dean :A
DEC 07 199a
University of Florida I
Originally printed as Experiment Station Bulletin 506, 1952, revised
1955. First printing of Extension Bulletin 177, 1962; second printing, 1964;
third printing, 1965; fourth printing and revision, June 1966; fifth print-
ing, April 1967; sixth revised edition, May 1975 and the 1977 revised print-
ing as Bulletin 183; revised printing 1989.
The Fertilizer Tag/Label ........................ 2
Sources and Characteristics of Plant Nutrients ........ 5
Other Common Fertilizer Concepts . . . ... 11
Manures and Composts ...................... 14
Pollution Potential of Fertilizers . . . ... 16
Appendix ........................ ...... 18
Fertilizers and Fertilization
Jerry B. Sartain1
Plants need at least 16 different chemical elements for their growth
processes. They require large quantities of carbon, hydrogen, and
oxygen, which they obtain from air and water. Nitrogen, potassium,
phosphorus, calcium, magnesium, and sulfur also are used in consid-
erable quantities by the plant. These elements may not be present
in sufficient supply nor in proper form, however. For example, air is
four-fifths nitrogen gas, but only the clovers and similar leguminous
plants with nitrogen-fixing bacteria in their roots can use it. All
other plants must have nitrogen in combined form, such as is found
in nitrate salts, ammoniacal salts, and certain other fertilizer mate-
In addition to the elements mentioned above, there is a group of
elements-iron, copper, manganese, zinc, boron, chlorine and
molybdenum-that are all essential for normal plant growth but
needed only in very small quantities. The majority of soils supply
enough of these elements, but plants grown on Florida's sandy soils
may show deficiencies. In some instances the nature of the soil itself
also limits nutrient availability to the plant. Whenever any element
is in short supply for plant growth, it must be furnished by some
type of fertilization for economical crop production.
The chemistry of fertilizers and their reaction with the soil is very
complex. The following discussion purposely omits the use of chemical
terms and symbols other than those appearing on the fertilizer tag
or as used in the Florida Commercial Fertilizer Law. However, this
simplified presentation covers most of the essential facts.
The chemical symbols used on the fertilizer tag to designate various
constituents may be confusing to readers who have no introduction
to chemistry. For example, nitrogen is listed on the tag as the element,
N; phosphorus is given as the oxide, P205, and called phosphoric
acid2; and potassium is listed as the oxide, K20, and called potash.
Florida Fertilizer Law designates the fertilizer constituents simply
as Primary Plant Nutrients or Secondary Plant Nutrients. This ter-
minology will be used in the discussion which follows.
1J.B. Sartain is a Professor and Extension Specialist Soil Fertility, Turf and
Ornamentals; Department of Soil Science, IFAS, University of Florida,
Gainesville, FL 32611.
2Phosphate is a more accurately descriptive term.
Soil reaction (pH) plays a vital part in fertilizer efficiency. It refers
to the degree of acidity or alkalinity of a soil. To have a simple
numerical measure, the pH scale has been adopted. At pH 7.0 a soil
is neither acid nor alkaline, but neutral. As values decrease from
pH 7.0, soil acidity increases. As values rise above 7.0, soil alkalinity
Virgin Florida soils range from pH 3.6 (very acid) to pH 8.0 or
slightly higher (alkaline). The strong acid conditions are encountered
in certain acid peats, mucks and palmetto flatwoods. Alkaline condi-
tions are usually associated with marl or other calcareous soils.
Most crops grow best in the range pH 5.5 to 6.5. However, in some
instances, a different pH may be required for disease control.
THE FERTILIZER TAG/LABEL
There is a wealth of information on the Florida fertilizer tag. This
information, if understood by the consumer, can often prevent need-
less expenditure for materials containing unnecessary elements, or
crop losses resulting from the use of materials lacking in some essen-
tial plant nutrient.
Fertilizers differ in composition depending upon how much of each
of the different plant nutrients they contain, along with the source
of these nutrients. A 100-pound bag (though only 20% of Florida's
fertilizer is now distributed in bagged form) of 16-4-8 analysis fer-
tilizer contains 16 pounds of nitrogen, 4 pounds of available phos-
phoric acid, and 8 pounds of soluble potash.4 The first figure always
indicates nitrogen, the second available phosphoric acid, and the
third potash. These are the primary plant nutrients. The figures are
percentages, and should not be confused with the "units" (%'s of a
ton) which still are used in the fertilizer industry.
Some fertilizers are quite similar in analysis; from these, approx-
imately the same amounts of nitrogen, available phosphoric acid,
3 The importance of the pH determination is in its indicator significance of
lime needs and also with respect to current or potential availability of
nutrient elements in the soil (The fact that pH, per se, is a quantitative
measure of hydrogen ion concentration is of little practical significance in
the buffered soil system).
4 In certain instances other numbers may be added to the grade, but the
element to which they refer must be included then, to avoid ambiguity.
and soluble potash may be applied to the crop by changing the amount
of fertilizer per acre.5
The guaranteed analysis for the available primary plant nutrients
in a 16-4-8 fertilizer would appear on the fertilizer tag in percentages
(%) as follows:
Total Nitrogen, not less than ................... 16.00%
. . . . . . . . %
. . . . . . . .. %
Available Phosphoric Acid, not less than ........... 4.00%
. . . . . . . %
Soluble Potash, not less than ..... ............ 8.00%
In addition to the primary plant nutrients there are also secon-
dary plant nutrients. These are reported in similar manner at the
bottom of the tag if they are guaranteed to be present. The commonly
recognized secondary nutrients include calcium, magnesium, copper,
manganese, zinc, boron, iron, sulfur, and molybdenum.6 Of these,
manganese, copper, zinc, boron, iron, and molybdenum are more
correctly referred to as "micro"nutrients,7 and are so designated
throughout most of the United States. There are certain required
statements of forms and sources of both primary and secondary plant
nutrients which must appear on the tag. An example of the above
tag filled in for a typical mix of nutrients might read as follows:
5 When comparing one fertilizer grade with another, it usually is best to
keep the nitrogen constant, because plants are very sensitive to deficiencies
or excesses of this nutrient. For example, if twenty 100-pound bags of
16-4-8 have been used in the past, 20 times 16-4-8 or 320 pounds of nitrogen,
80 pounds of available phosphoric acid, and 160 pounds of potash-were
being used. To determine how this compares with the amount of nutrition
provided by a 10-10-10 formulation divide the 320 pounds of nitrogen by
10 (the pounds of nitrogen in a bag of 10-10-10). This gives 32 bags of
10-10-10 material as necessary to provide the 320 pounds of nitrogen. At
10 pounds each of phosphoric acid and potash per bag of 10-10-10, the 32
bags would also supply 320 pounds of available phosphoric acid and 320
pounds of potash, which is 240 pounds of P205 and 160 pounds of potash
more than was applied in the past. The consumer must decide whether
such a substitution is permissible and economical.
6All secondary nutrients are reported in the elemental form.
7The use of the terms 'minor' and 'trace' is now discouraged.
Nitrogen 50% Organic (35% Synthetic, 15% Natural)
Total Nitrogen, not less than .................... 16.00%
Nitrate Nitrogen ................... 2.25%
Ammoniacal Nitrogen .................. 5.75%
Water-Soluble Organic Nitrogen . . .. 5.60%
Water-Insoluble Nitrogen ................ 2.40%
Available Phosphoric Acid, not less than . . ... 4.00%
Insoluble Phosphoric Acid .................. 0.20%
Soluble Potash, not less than .................. 8.00%
Chlorine not more than .................... 3.00%
Derived from: Castor Pomace, Tankage, Ammonium Nitrate,
Ammonium Sulfate, Urea, Ammoniated Superphosphate,
Potassium Sulfate, Potassium Chloride and Potassium-
Statement of Secondary Plant Foods:
Total Magnesium as Mg ..................... 2.00%
Water-Soluble Magnesium as Mg ................ 2.00%
Sulfur (combined) ................ ....... 2.00%
Sulfur (free) ................ .......... 0.00%
Copper as Cu ........................... 0.50%
Derived from: Magnesium sulfate and Copper sulfate
When the term "organic" is used on the label, the specific organic
nutrient must be identified and quantified as either synthetic or
natural, with the respective percentage of each also specified.
If the total of the primary plant nutrients is relatively high, it will
indicate a high-analysis fertilizer. There is no set dividing line, but
fertilizers with a total of more than 24% primary nutrients would
definitely be classified as high-analysis fertilizers. If the total does
not equal 16%, the fertilizer must be labeled in bold letters as "Low-
Chlorine ordinarily is not valued as a plant nutrient in the fer-
tilizer. It may be injurious to the quality of certain crops, such as
tobacco, tomatoes, and potatoes, or in plant-bed fertilizers if high
percentages are present. However, small amounts may be beneficial
under some conditions.
Sources used in formulating the primary nutrients N, P, and K
are listed in the "derived-from" section immediately following the
Chlorine statement. This statement is of value to the consumer in
determining the source of a given plant nutrient.
A knowledge of the sources of the secondary plant nutrients is
also important in order to know their availability to the plant. Sources
used in formulating the secondary nutrients are listed in the "derived-
from" section immediately following their guaranteed percentage.
Conditioners and Fillers.-The amounts of various fertilizer
materials which are required to supply the primary and secondary
plant nutrients in a ton of material having a given formula usually
do not total 2,000 pounds when mixed together. The remaining por-
tion is made up of conditioner or filler, or both. A conditioner is a
material that helps prevent a fertilizer from becoming so moist or
so hard on standing that it will not flow properly. Conditioners may
also supply some of the plant nutrients in the formula.
Filler is the term applied to a material added to fill out the ton of
weight. It often consists of dolomite, raw phosphate or sand. It usually
has certain conditioning value, but contains no significant amounts
of available plant nutrients.
SOURCES AND CHARACTERISTICS OF PLANT
Fertilizer materials commonly contain more than one plant nu-
trient or form of plant nutrient (Table 1). For example, ammoniated
superphosphate may contain nitrate nitrogen, ammoniacal nitrogen,
and phosphoric acid. Properties of the various common forms of plant
nutrients, keyed to fertilizer-label terminology, are as follows:
Nitrate Nitrogen.-This form of nitrogen in mixed fertilizers
comes mainly from ammonium nitrate, ammoniated superphos-
phates, sodium nitrate and sodium-potassium nitrate (Table 2). Am-
moniated superphosphate is prepared by spraying ammoniating so-
lutions on phosphates. Ammoniating solutions used at present gen-
erally contain ammonium nitrate or urea, or both, and ammonia gas
dissolved in water.
The nitrogen in ammonium nitrate is one-half nitrate nitrogen.
All of the nitrogen in sodium nitrate and in sodium-potassium nitrate
is in the nitrate form. Calcium nitrate and potassium nitrate also
carry all of their nitrogen in the nitrate form. Crops can use the
nitrate nitrogen from different sources equally well.
Nitrate nitrogen dissolves readily in water and moves freely in the
soil with the movement of water. It is not held by the soil particles.
For this reason, heavy rains may cause leaching loss of nitrates. On
the other hand, this characteristic of nitrate nitrogen makes it a
favored top- or side-dressing when rapid movement into the root zone
is needed, or as an inclusion in the band to serve as a mobile starter
fertilizer at planting8.
8 Solonaceous crops (potatoes, tomatoes, tocacco, etc.) usually respond to a
higher-than-average ratio of nitrate to ammonium than most crops, appa-
rently as a requirement for mobilizing calcium in the plant.
Table 1.-APPROXIMATE ANALYSIS OF SOME SOURCES OF PLANT NUTRIENTS.
(See Table 2 also for other nitrogenous materials.)
Percentage Composition (See text for meaning of symbols.)
Material N P205 K20 Ca
Mg S Other
Ammonium sulfate ................
Ammonium sulfate-nitrate ..........
Ammonium phosphate ............
Ammonium nitrate plus lime ........
Basic slag (Thomas) ...............
Basic slag (Open hearth) ............
Calcium sulfate ...................
Calcium nitrate ...................
Copper sulfate ....................
Emjeo (Magnesium sulfate) .........
Guano (Peruvian) ..................
Iron sulfate copperass) ..............
Magnesia (seawater) ..............
Magnesium sulfate ................
Manganese sulfate .................
Phosphate rock ...................
Potassium chloride ................
Potassium magnesium sulfate .......
Potassium nitrate ................
Potassium sulfate .................
Sewage sludge (activated) ...........
Sodium molybdate .................
Sodium nitrate ....................
Sodium-potassium nitrate ..........
Superphosphate, Ordinary ..........
Zinc sulfate ......................
* If conditioned with dolomite.
2.4 8.5 0.6 1
1.4 0.5 1
TABLE 2. NITROGENOUS FERTILIZER MATERIALS
Sodium nitrate ........................
Sodium-potassium nitrate ...............
Potassium nitrate .....................
Calcium nitrate .. ........... .......
Ammonium nitrate ....................
Ammonium nitrate plus lime ** ..........
Ammonium nitrate solutions ............
Ammonium phosphate .................
Anhydrous ammonia ...................
Aqua ammonia ........................
Ammoniated superphosphate ............
Ammonium sulfate ....................
U rea ................................
Isobutylidene diurea (IBDU) ............
Sulfur-coated urea .....................
Cottonseed meal* ................... ...
Peruvian guano* ......................
Sewage sludge (activated)* ..............
Processed tankages* ...................
Raw bone meal* .......................
Garbage tankage* .....................
16.2 Very rapid
15.0 Very rapid
15.5 Very rapid
33.5 Very rapid
20.5 Very rapid
16-21 Very rapid
26 Very rapid
3.3-4.1 Very slow
2.5-3.3 Very slow
* Natural organic.
** A-N-L, Cal Nitro, Calcium ammonium nitrate, etc.
Ammoniacal Nitrogen.-This form of nitrogen comes mainly
from ammonium phosphates, anhydrous ammonia, ammonium sul-
fate, and ammonium nitrate. All of the nitrogen in ammonium sulfate
is in the ammoniacal form. Ammonium phosphate also contains only
ammoniacal nitrogen, and is often used in high-analysis fertilizers.
The ammoniacal nitrogen from different sources is equally usable
Ammoniacal nitrogen dissolves readily in water. It differs from
nitrate nitrogen in that it is held to a greater or lesser extent by the
soil particles, so free movement through the soil is retarded. Move-
ment of ammoniacal nitrogen through a strongly acid soil usually is
more rapid than through a slightly acid soil. Therefore, proper liming
to reduce soil acidity will help reduce the leaching loss of ammoniacal
nitrogen by heavy rains. Most plants can use ammoniacal nitrogen
directly, but bacteria usually convert it to readily available nitrate
form in a period of one to four weeks. If the soil is strongly acid, the
bacteria are not efficient, and certain types of plants may suffer from
lack of the nitrate form of nitrogen. Soil that is saturated with water,
very dry, or too cold also results in slow conversion of ammoniacal
nitrogen to nitrate nitrogen. For this reason nitrate nitrogen is fa-
vored for rapid penetration of cold soils when side dressing in the
late fall, winter, and early spring.
Water-Soluble Organic Nitrogen.9-This form of nitrogen is
supplied mainly from urea. The urea is made by chemical processes,
but is identical with urea nitrogen found in the urine of animals.
Water-soluble urea nitrogen changes to ammoniacal nitrogen
within a few days after application to the soil, when applied in the
amounts usually found in mixed fertilizers. For this reason, the water-
soluble organic nitrogen (normally urea) reported on the fertilizer
tag should be considered as the equivalent of ammoniacal nitrogen.
Since urea is an organic material which is non-ionic when in solu-
tion, it will leach through the soil rapidly if a heavy rainfall occurs
immediately after application. After three to four days most of the
urea will have been converted to the ammoniacal form and less leach-
ing loss should occur as the result of a heavy (leaching) rainfall.
Water-Insoluble Nitrogen.-This form comes almost entirely
from natural organic sources, such as seed meals, tankages, and
sewage sludge products. A small amount of water-insoluble nitrogen
comes from synthetic materials primarily urea-form.
Relative amounts of insoluble nitrogen from different sources are
not reported on the fertilizer tag. This makes it impossible to deter-
mine which forms are being used in significant quantities if more
than one source is reported. Water-insoluble nitrogen cannot be used
directly by the plant, but must be converted to ammoniacal nitrogen
by soil organisms. The conversion proceeds rather gradually. For this
reason, insoluble nitrogen is more slowly available to a crop and less
subject to leaching loss by heavy rains.
The conversion of insoluble nitrogen proceeds more rapidly in some
materials than in others. In some it is so slow as to make them of
little value as fertilizers. A list of nitrogenous fertilizer materials is
given in Table 2, along with estimated rates of availability of their
Most natural organic nitrogen carriers have part of their nitrogen
in soluble form. This portion is listed as the nitrate, ammoniacal,
and water-soluble forms on the fertilizer tag. The usable insoluble
nitrogen in a fertilizer is very high in cost compared with other
sources, since only one third to one half of the insoluble nitrogen
9 The name "organic nitrogen" refers to nitrogen of the type found in
organisms such as plants and animals. It includes certain of the nitrogen-
ous compounds now made synthetically.
now used in fertilizers is available within a reasonable length of
time. It should be requested only if a real need exists. Side-dressing
with cheaper nitrogen while the crop is growing, along with the
control of nitrogen release afforded by decomposing crop residues,
usually eliminates the need for insoluble nitrogen at planting.
Insoluble nitrogen is valuable for lawns, turf, and certain ornamen-
tals, as uptake of nitrogen by the plants is less rapid. This reduces
the intense flush of growth immediately after fertilization. Much of
the beneficial response to natural organic may be due to the second-
ary elements which they usually contain.
Available Phosphoric Acid.-This form of phosphorus comes
mainly from superphosphate, ammonium phosphates, and ordinary
or concentrated superphosphate. These materials are made by treat-
ing raw rock phosphate with acids to make the phosphorus more
available to the plant. High-analysis fertilizers may contain consid-
erable amounts of ammonium phosphate. The available phosphoric
acid from these different materials is about equally usable by the
All forms of phosphoric acid are strongly held by the soil against
loss by leaching, unless the soils are 1) strongly acid and 2) white
or gray sands. Moderate liming largely corrects this condition for
flatwoods soils. Soils that are red or yellow in the surface or subsoil,
on the other hand, tend to hold phosphoric acid so strongly that, in
the second or third year after application, the availability of the
residual phosphorus may be much lower than for black or gray soils.
Thus, the amount of phosphorus to be added in order to maintain
fertility of the red and yellow soils is somewhat larger than for black
or gray soils.
Insoluble Phosphoric Acid.-This form of phosphorus must not
be confused with available phosphoric acid. It is largely composed
of rock phosphate or of waste-pond phosphate added as filler, or
constitutes the part of the rock phosphate that was not converted to
the available phosphoric acid form by acid.11 This form of phosphoric
acid is only slowly available to the plant but, over a long period of
10 However, recent information raises a question as to the degree of reversion
of phosphate by relatively high ammoniation of superphosphates, with
subsequent reduction in availability of the phosphorus. The effect would
be least with band placement in acid soils, and greatest in broadcast use
on calcareous soils.
11 It also indicates whether or not raw phosphate was the form of filler used,
and whether the amount is significant. Multiplying the value for insoluble
phosphoric acid by 100 will give an estimate of the amount of waste-pond
phosphate filler that might have been used per ton, whereas multiplying
by 60 gives the equivalent in terms of high-grade rock phosphate.
time, does have certain value and is, therefore, reported on the tag.
It is more rapidly available on moderately to slightly acid soils than
on slightly acid to neutral or alkaline (high lime) soils. The use of
ground rock or waste-pond phosphate has been confined largely to
Many soils in the Southeast are deficient in sulfur. The main source
of this element in the past has been ordinary superphosphate, which
is about half calcium sulfate. If ground rock phosphate or concen-
trated phosphates are to be used continuously on a given area, even-
tually sulfur may have to be supplied from another source to replace
that removed by the crops.
Soluble Potash.-Potash comes mainly from potassium chlo-
ride (muriate of potash), potassium-magnesium sulfate, sodium-
potassium nitrate, potassium nitrate, and potassium sulfate. Cotton
bur ash contains potash in the form of carbonate. The soluble potash
from different sources is equally usable by the crop.
Potash is held in the soil much as is ammoniacal nitrogen. In
sandy soils it may be leached out of the root zone of shallow-rooted
crops to some extent, but apparently much of it can still be recovered
by deep-rooted crops.
Chlorine.-This element comes almost entirely from potassium
chloride (muriate of potash).12 Other forms of potash, such as potas-
sium sulfate, potassium-magnesium sulfate, potassium nitrate, and
sodium-potassium nitrate, are used if it is desirable to reduce the
chlorine content of a fertilizer.
Secondary Plant Nutrients. -Considerable care must be used
in reading a fertilizer tag to determine the amounts of secondary
plant nutrients guaranteed. These secondary nutrients may be from
various sources that differ widely in availability to the plant. The
water-soluble forms usually are the most important sources in a
mixed fertilizer,13 but certain less-soluble forms are now receiving
wide acceptance as well.
Magnesium always is given as both water-soluble and total, with
the symbol Mg. Water-soluble magnesium usually is derived from
potassium magnesium sulfate or from magnesium sulfate. Other
secondary nutrients of importance, with their symbols and commonly
used water-soluble sources, are as follows:
12 When potash is supplied from crude salts such as kainit, manure salts,
or sylvinite, the amount of chlorine added with the source of potash usually
is considerably higher than when supplied only from potassium chloride
(muriate of potash).
13 Some common sources of primary and secondary plant nutrients are listed
in Table 1.
Plant Food Symbol Derived from
Copper Cu Copper sulfate
Manganese Mn Manganese sulfate
Zinc Zn Zinc sulfate
Boron B Borax, borate
Iron Fe Iron sulfate, chelated
Molybdenum Mo Sodium molybdate
Sulfur S Superphosphate,
Calcium Ca Calcium sulfate,
Insoluble forms of secondary nutrients quite widely used include
copper oxide, magnesium oxide, manganese oxides, and calcitic and
dolomitic limestone. Calcium, magnesium, copper, zinc, manganese,
iron, and molybdenum are held in the soil in various manners. Cop-
per, zinc, manganese, iron, and molybdenum are held so strongly
under certain conditions that they may be relatively unavailable to
crops. Availability of these elements tends to be reduced as pH in-
creases into the alkaline range, except for molybdenum. This element
may be rendered increasingly unavailable as the soil approaches
Chelates have been developed to partially offset this strong inac-
tivating ability of the soil. In iron chelate, the only chelate which
has been of practical success for soil usage, the iron is protected in
soluble form within a complex molecule until taken into the plant,
where it is broken down and the iron utilized.
Water-soluble sulfur and boron may move much like nitrate nitro-
gen and be leached away. Almost all of the primary and secondary
plant nutrients may be tied up to some extent in soil micro-organisms
and/or organic matter (humus), only to be released to a crop when
the organisms die or the organic matter decomposes.
OTHER COMMON FERTILIZER CONCEPTS
Bulk Blending.-This term refers in general to dry mixtures of
straight, high-analysis materials intended primarily for immediate,
short-haul, bulk delivery and broadcast application before significant
deterioration of physical or chemical makeup takes place. The present
14 These usually are not selected deliberately to supply the element indi-
trend is toward the use of materials which have been selected for
granule sizes and gravities that will not unduly segregate during
bulk transit, and which will respond uniformly to centrifugal broad-
Fertilizer Injury.-High chlorine levels will reduce crop quality
or injure sensitive crops such as tobacco, tomatoes, and potatoes, or
cause injury to plant beds.
Too much usable nitrogen in the soil at one time may cause leaf
burn and injury to the roots. The accumulation of soluble salts from
fertilizers or by irrigation with salty water can prevent intake of
water by the plant, and reduce yield.
In general, the more sandy the soil and the drier the soil, the more
severe will be the injury from a given amount of excess chlorine,
nitrogen, or total salts. Irrigating the crop to keep the soil moisture
high and to wash out some of the soluble salts is the best method
of correcting high salt conditions in the field or in the plant bed.
Fluid Fertilizer.-One of the most rapidly growing segments of
the fertilizer industry today is that of fluid fertilizers. This segment
of the industry encompasses three areas: gasses, solutions, and sus-
Anhydrous ammonia, containing 82% N, is supplied as a liquid
under pressure. Upon soil injection and exposure to soil conditions,
anhydrous ammonia transforms initially to a gas prior to its eventual
conversion to ammonium ions. This transformation necessitates the
injection of this material below the surface of a moist soil to prevent
volatilization. Because of the crops produced and the dry sandy na-
ture of the soils, large amounts of anhydrous ammonia are not used
Liquid or true-solution fertilizers are increasing in use in Florida.
These liquid fertilizers are generally of relatively low analysis, con-
taining 8% or less of potash. Sources of nitrogen in liquid fertilizers
include: uran (a mixture of ammonium nitrate and urea containing
28, 30, or 32% N), ammonium polyphosphate (10-34-0 or 11-37-0),
and ammonium nitrate or ammonium sulfate. Use of ammonium
polyphosphate facilitates the production of higher analysis liquids
because it serves as a source of both N and P. The pholyphosphate
ions also tend to serve as a chelating agent, which permits the inclu-
sion of more potash. Even with the inclusion of the polyphosphate,
however, the potash content will seldom exceed 8% in a true liquid
Phosphorus may be supplied by ammonium polyphosphate, phos-
phoric acid, concentrated superphosphate or ordinary superphos-
phate. In true liquids the ammonium polyphosphate is the most
popular. Depending upon the source of the ammonium polyphos-
phate, it will impart a light-green to dark-brown appearance to the
Potash is almost always added as potassium chloride (muriate of
potash) or as potasssium sulfate (K2S04). The finely ground, so-called
'soluble' forms of these salts are used to facilitate their dissolution.
As mentioned above, the percentage potash in a true solution is
limited to about 8%, because of salting-out problems.
Suspensions are solution fertilizers which contain small amounts
of clay. Higher-analysis fertilizers can usually be formulated in the
suspension form, because the clay suspends the undissolved salts of
the fertilizer in solution. The primary disadvantage of a suspension
is that it must be continuously agitated. A wider range of micronu-
trient sources at higher addition rates may also be mixed with sus-
pensions than with true solution fertilizers.
Fluid fertilizers are increasing in popularity in Florida, and par-
ticularly the true solutions and suspensions. Most of the golf courses
in the state fertilize by injecting solution fertilizers into the irrigation
system: This process is called fertigation.
Fertilizer Acidity.-Fertilizers which contain ammoniacal nitro-
gen are potentially acidifying when added to the soil. For every
molecule of ammoniacal nitrogen mineralized in the soil, four hydro-
gen ions are released. Since soil reaction (pH) is a measure of the
hydrogen ion activity, these released hydrogen ions tend to increase
the concentration of hydrogen in the soil and to reduce the soil pH.
The most acidifying source of nitrogen is ammonium sulfate. For
every pound of N applied as ammonium sulfate, 5.35 pounds of lime
are required to neutralize the acidity produced. Diammonium phos-
phate (DAP), ammonium nitrate and urea require 5.0, 1.8 and 1.8
pounds of lime to neutralize each pound of nitrogen applied, respec-
Crops being irrigated with deep-well water, which has a high bicar-
bonate content and a typical pH greater than 7.0, could actually
benefit from the application of acid-forming nitrogen fertilizers,
which tend to neutralize the effects of the high-pH water.
Not all fertilizers are acid-forming, with some actually even increas-
ing the soil pH. These include potassium nitrate, calcium nitrate
and sodium nitrate. Their use should be limited in situations where
the soil pH is already high and a potential micronutrient problem
Salt Index.-Fertilizer salt index is a relative value placed on all
fertilizer materials based on the nutrient content of the material
and the potential osmotic pressure which a gram-molecular weight
of the material will exert when placed in water. All salt indexes are
expressed relative to a standard, which is sodium nitrate. This was
a common fertilizer material until ammoniated materials became
available following World War II. The higher the salt index of a ma-
terial the more salt applied when using the material as a nutrient.
A few examples of the salt index of common fertilizer sources include:
ammonium nitrate, 2.99; urea, 1.62; sodium nitrate, 6.06; ammonium
sulfate, 3.25; concentrated superphosphate, 0.21; potassium chloride
(muriate of potash), 2.19; and potassium sulfate, 0.85.
In general, the higher the nutrient content of a fertilizer material
the lower its resultant salt index. To reduce the total salt addition
through the application of fertilizers, it is recommended that high-
analysis fertilizers be used instead of low-analysis ones. The same
nutrients can be applied with 1000 pounds of a 10-20-20 mix contain-
ing 711 pounds of salt as with 2000 pounds of a 5-10-10 mix containing
952 pounds of salt.
MANURES AND COMPOSTS
Animal manures and organic composts are used primarily for their
soil-conditioning value, and only secondarily for their plant nutrient
content. An exception is chicken manure, which if unleached is quite
high in plant-nutrient percentages and requires judicious usage to
avoid crop "burning".
The rate at which the nitrogen in manures and composts becomes
available depends to a large extent on the ratio of nitrogen to car-
bohydrates (cellulose, etc.) in the material. Until the carbohydrates
have been reduced by micro-organisms to a relatively low level,15ac-
tivity of the micro-organisms will continue to keep the nitrogen tied
up in their life processes and not allow it to become readily available
to growing plants.
Composting is the process of allowing or aiding micro-organisms
to reduce the carbohydrate content of a material until it becomes so
low as to be limited as an energy source. In the process, the material
is much reduced in volume. Near termination of the process, decom-
position of the dead organisms releases the nitrogen they have been
recycling. The controlled release of nitrogen afforded by decomposing
field crop residues is essentially a composting process.
A true compost is a material in which carbohydrates are reduced
to the extent that, when added to the soil, the material will release
available nitrogen at a significant rate, or at least not reduce the
effectiveness of added fertilizer nitrogen. Organic materials which
15 In general, a carbon to nitrogen ratio of twenty to one (C/N = 20) is consid-
ered to be the dividing point, although this can vary widely depending
on the composition and coarseness of the material.
have not reached this stage of decomposition should not be called
composts. For example, so-called garbage compost which has been
allowed to compost only a few days, until the readily active waste
proteins are destroyed and the material made relatively sanitary for
further disposal, is only a slightly composted product without the
characteristics of a true compost.16 The majority of the cellulose is
still intact and the material will cause acute nitrogen starvation of
plants, similar in effect to applications of straw or sawdust, unless
supplemented heavily with fertilizer nitrogen. It is impractical to
compost low-nitrogen high-carbohydrate materials for agricultural
usage without initially adding fertilizer nitrogen to them. A well
composted material usually is black in color, with visual identity of
all but the coarsest fractions of the original materials being lost
Following are approximate analyses of four animal manures:
Percentage Composition on a Dry Basis* (litter-free)
Manure Nitrogen Phosphoric Acid Potash Rate of Nitrogen
Type (P205) (K20) Availability
Horse 3.2 1.1 1.4 Very slow
Cow 4.2 1.1 3.2 Slow
Sheep 3.0 1.1 3.1 Slow
Chicken 6.2 4.0 2.0 Rapid
*About 4 pounds of fresh manure equals 1 pound of dry product.
Horse manure is called a 'hot' manure because it is high in undi-
gested carbohydrate and will heat excessively in a pile. Cows and
sheep are ruminants. Carbohydrate is much reduced in the rumen
of the animals; therefore, the nitrogen in ruminant manure is more
readily available to a crop. Fresh cow, sheep, and chicken manure,
undiluted with litter, can cause salt damage but will not cause nitro-
gen starvation of the crop.
Commercial dried cow, sheep, and chicken manures are excellent
products for direct usage. Manures which have been taken from un-
16 The composting process of even readily compostable materials such as
manures or green plant residues takes from 6 to 8 weeks.
covered corrals or feedlots where leaching has occurred, and that
have been diluted with litter, may be relatively low in nitrogen and
potassium but nevertheless are good soil conditioners. However, weed
seeds may be introduced by them.
All manures, and especially horse manure, are improved by com-
posting. Inter-layering about 6 inches of manure with 2 inches of soil
into a pile several feet high, maintained in moist condition for 6 to
8 weeks, makes an excellent product when the pile is then mixed for
The practice of incorporating non-toxic organic rather than con-
centrated fertilizers when transplanting fruit trees and ornamentals
is excellent as well. Sewage sludges, manures, steamed bone meal,
and mature composts can be mixed directly with the soil before
setting the plants. The nutrients in these materials are primarily
in a form that resists leaching during the frequent waterings essen-
tial to successful transplanting, yet are released at a controlled rate
more suitable to the initially limited requirements of the plant as
compared with the ready availability of nutrients in concentrated
commercial fertilizers, with their high toxicity potentials.
POLLUTION POTENTIAL OF FERTILIZERS
The pollution potential of the various fertilizers is of considerable
interest, especially in connection with eutrophication of natural lakes
and impounded waters. The potential for detrimental pollution by
plant nutrients or fertilizer salts, other than phosphorus and nitro-
gen, generally is of negligible importance to the quantities originat-
ing from the fertilizers per se. Nitrate nitrogen moves readily through
the soil, and its contribution can be significant where an extensive
root system is lacking or dormant, especially under conditions where
a shallow water table is controlled by ditch or tile drains. Penetration
of nitrate through an extensive soil profile to a deep aquifer under
judicious fertilization is not likely to be a problem, in Florida or
Only a small percentage of nitrogen passes an active, well-de-
veloped root system. Thus, broadcasting fertilizer over an area only
partially covered by the plant roots, such as in immature orchards,
promotes greater leaching loss than where the fertilizer is confined
to the area of root expansion. Nitrogen application during periods
of crop dormancy would also enhance leaching losses.
There is an initial difference in mobility of ammoniacal, nitrate,
and urea nitrogen when first applied to the soil, but the mobility of
ammoniacal and urea nitrogen prior to its nitrification is delayed
only temporarily. This constitutes a significant factor only where
heavy leaching rains follow application of the fertilizer within a few
The movement of phosphorus through mineral soil profiles usually
is negligible in neutral to slightly acid soils but, in acid sands, move-
ment becomes progressively more active as acidity increases below
about pH 5.5. The movement of fertilizer phosphorus from cropped
organic or sandy soils following heavy fertilization within a few feet
of a drainage ditch or immediately over shallow tile is an unevaluated
possibility. The major way that phosphorus moves into drainage
channels is by surface-soil erosion.
Soluble constituents, such as nitrates, may be moved in significant
quantities by surface runoff following initially intensive rains. This
is especially true under ridge-row culture. With gentle rains, nitrates
usually are moved down into the soil before surface water runoff
occurs. A major contributor of both nitrogen and phosphorus to drain-
age water is surface movement of manure from pastures, or following
manure top-dressing of the soil where incorporation is impractical.
What could be termed pollution of the soil by buildup of retained
fertilizer constituents to a toxic level applies primarily to copper and
possibly molybdenum under improper usage. The buildup of phos-
phorus in the soil is not detrimental, per se, to plants. In fact, phos-
phorus compounds apparently have a desirable buffering action in
sandy soils. However, the increased pollution potential resulting from
movement of enhanced concentrations during surface erosion must
GLOSSARY OF FERTILIZER TERMS
Ammonium Nitrate.-This material is used as a top-dressing or
side-dressing where only nitrogen is needed. It contains about 32-
34% nitrogen and is used in much the same manner as ammonium
sulfate, though it is more concentrated and thus should be used at
a lower rate. One-half of the nitrogen is the same as that in calcium
nitrate, whereas the other half is the same as that in ammonium
Ammonium Nitrate plus Lime.-This material, sold under vari-
ous trade names (A-N-L, Cal-Nitro, Calcium Ammonium Nitrate,
Nitro Lime), is ammonium nitrate to which either calcitic or dolomitic
lime has been added to condition the material and reduce its nitrogen
content to 20.5% nitrogen. The mixture is non-acid-forming and
supplies calcium or, in the case of dolomite conditioner, both calcium
and magnesium in addition to the ammoniacal and nitrate nitrogen.
Ammonium Phosphates.-These materials are produced by
reacting ammonia with phosphoric acid or with a mixture of phos-
phoric and sulfuric acids. Some of the more widely used ammonium
phosphate fertilizers include monoammonium phosphate (MAP),
diammonium phosphate (DAP), and ammonium phosphate-sulfate
(16-20-0). Fertilizer grade MAP is of grade 11-48-0. Two grades of
DAP (16-48-0 and 18-46-0) are used in fertilizers. The 18-46-0 is more
common, however, because it is more economical to produce.
The ammonium phosphates are completely water soluble. Ulti-
mately, they have an acid effect on the soil because of the ammonium
they contain, even though the initial reaction of DAP is alkaline.
These materials have the advantages of a high plant nutrient content
and a low salt index, which minimizes both shipping, handling, and
storage costs, and the possibility of salt injury.
Ammonium Sulfate (Sulfate of Ammonia).-This material con-
tains 20.5% nitrogen, all in ammoniacal form, but no other primary
plant nutrient. Ammoniacal nitrogen does not penetrate to the root
zone as readily or give as quick a response as does nitrate nitrogen
when used as a side- or top-dressing. On the other hand, ammoniacal
nitrogen does not leach out as readily when heavy rains occur. For
this reason, it is used where quick response is not vital to the crop.
This is one of the most acidifying sources of N, in that 5.35 pounds
of lime are required to neutralize every pound of N applied as am-
monium sulfate. Because of its acidifying effect it is often used in
situations where the soil pH is at or approaching too high a level,
such as in pots watered with high pH-water.
Anhydrous Ammonia.-This material consists of ammonia gas
under pressure and cannot be used except with special apparatus.
Usually it is injected into the soil behind a chisel-like implement. It
contains 82% ammoniacal nitrogen. Water must be present for reac-
tion with the ammonia upon injection, to trap the ammonia in the
soil in ammoniacal form. A sterilizing effect occurs around the injec-
tion site because of the toxic nature of low concentrations of ammonia
gas. This sterilization usually lasts for only a few days, however.
Basic Slag (Thomas Slag).-Basic slag averages about 15% avail-
able phosphoric acid. In addition, 100 pounds of basic slag are equal
to about 70 pounds of limestone, for the correction of soil acidity.
Because the primary ingredient in this material is calcium silicate,
and since sugarcane has been shown to respond to applied silica
under certain conditions, it has been used as an amendment to
increase sugar production.
Open hearth Basic Slag averages about 10% available phos-
Calcium Nitrate (Prill Cal).-Crystalline calcium nitrate takes
up moisture from the air too readily to be conveniently used for side
dressing. Prill Cal (15.5% N) is a calcium nitrate product that con-
tains 1.2% ammoniacal nitrogen. It is quite stable and makes an
excellent nitrate side dressing.
Calcium Sulfate (Landplaster, Gypsum).-This material is used
primarily as a source of sulfur when sulfur is needed but is not
supplied in the usual manner by ordinary superphosphate or as the
sulfates of ammonium or potassium. It is not a liming material for
reducing soil acidity. When applied to acid soils the immediate effect
is an increase in the acidity of the soil solution, though the ultimate
effect after leaching is an effective reduction of soil acidity. This
becomes apparent even for strongly acid subsoils.
Chelates. Chelates of micronutrients, of which iron EDTA is the
most common, consist of the element held in the center of a large
organic molecule in a manner that protects the element from im-
mediately reacting with the soil and becoming unavailable before
the plant has a chance to absorb it. Apparently the plant takes up
the entire molecule under some conditions and then releases the
iron or other nutrient within the plant.
Colloidal Phosphate (Waste Pond Phosphate).-This is finely
divided raw mineral phosphate or phosphatic clay. If the phosphoric
acid content is 20% or more, it is approximately equal to ground rock
phosphate on a phosphoric acid basis. For example, 300 pounds of
20% colloidal phosphate would equal 200 pounds of 30% ground raw
rock phosphate. (See Rock Phosphate.)
Concentrated Superphosphate (CSP) (Triple Superphos-
phate).-This material is 95-98% water-soluble and contains 46%
P205. It is principally monocalcium phosphate [Ca(H2PO4)2], which
is produced by reacting rock phosphate with phosphoric acid. This
material does not contain sulfur.
Frits.-Frits of micronutrients consist of one or more of the ele-
ments dissolved in molten glass-like material. This mass is cooled
by dropping it in water to fracture it, and then grinding it to a powder.
The nature of the glass-like matrix and the fineness of grinding
determine the rate of solubility in the soil and the rate of release of
micronutrients from the frits. Because of high energy costs, frits are
not as commonly used today as they were during times of low energy
cost. A commonly used material of the past, FTE 503 (FTE stood for
fritted trace elements), is no longer available. Three micronutrient
products closely identified with FTE 503 are currently available,
however. They are: F-503 (powder) which contains fritted iron and
boron along with oxides of Cu, Mn, Mo and Zn; F-503 Oxide which
does not contain fritted materials but contains borax and oxides of
other nutrients; and F-503 G which is granular and contains borax
and oxides and sulfates of other nutrients. It is not a fritted material.
Gypsum.-(See Calcium Sulfate.)
Isobutylidene Diurea (IBDU).-This is a synthetic organic ni-
trogen source with slow-release properties. It contains 31% N. The
rate of N-release from the material is determined by its particle size
and its degree of dissolution in water. Soil factors such as pH and
temperature do not affect the release of N greatly, as is the case with
Ureaform and Milorganite. Thus, this material can be used effectively
during cool-season growth.
Milorganite.-This is activated sewage sludge containing about
6.2% nitrogen and 3.5% phosphoric acid. Its nitrogen is only about
60% as available as the nitrate and ammoniacal forms. Most forms
of natural organic nitrogen used at present have a much lower rate
of availability. The direct use of high-cost natural organic usually
is not an economical practice except for landscaping, turf mainte-
nance, plant beds, or certain specialty crops.
Nitrogen Solutions. -A number of sources of nitrogen solutions
exist, but the most popular ones are Uran (a combination of am-
monium nitrate and urea containing either 28, 30 or 32% N), and
ammonium nitrate (21%). Other common base solutions of nitrogen
and phosphorus include 10-34-0 and 11-37-0. These products are
nonpressure solutions which are typically used in Florida. Numerous
other low-pressure and high-pressure solutions are marketed, but
they are more common in agronomic production systems of the Mid-
Ordinary Superphosphate (OSP) [Superphosphate, Normal
Superphosphate, Single Superphosphate].-This product con-
tains 20% P205 and is 85-90% water soluble. It is a mixture of
monocalcium phosphate and gypsum; therefore, it contains 8-10% S
as calcium sulfate. It currently is not widely used because of its low
Potassium Chloride (Muriate of Potash). -Approximately 95%
of all K consumption is in the form of KC1. This material contains
60-63% K20 and is a beneficiated product of mining. Its high chlorine
content may be damaging to some crops.
Rock Phosphate.-This material consists of finely divided phos-
phate rock and contains from 27 to 44% phosphoric acid in insoluble
form. It is moderately available to plants on slightly to moderately
acid soils, and almost unavailable on highlime soils. It is used to
some extent on pastures but is not suitable as a sole source of phos-
phorus for rapidly growing row crops.
Rock Potash.-Pulverized granite, green sand, and similar ma-
terials carry potassium in the form of feldspars or glauconite, which
are highly insoluble potassium-bearing minerals. The rate of avail-
ability of potash is so low as to render these materials economically
valueless as compared to the usual sources of water-soluble potash.
Sodium Nitrate (Nitrate of Soda); Sodium-Potassium Nit-
rate; Nitrate of Soda Potash; Potassium Nitrate (Nitrate of
Potash).-These materials all carry only the nitrate form of nitrogen
and are widely used for side or top-dressing. They contain from 13-
16% nitrate nitrogen. In addition, sodium-potassium nitrate contains
14% potash, and potassium nitrate contains 44% potash.
Solution Fertilization.-Numerous fertilizer materials and mix-
tures on the market are intended to be dissolved in water and applied
as sprays to the leaves while the crop is growing, by dipping the roots
into solution, or by pouring solution into the holes in transplanting.
They are made from ordinary fertilizer materials which have been
purified and concentrated to reduce insoluble residues. These prac-
tices are of economic value only under very special conditions. The
materials may be used also in the dry state like ordinary fertilizers,
but they usually are so concentrated that great care is necessary to
avoid injury to the plants. Solutions of ammonium nitrate and urea
ammonium nitrate have come into quite widespread use for direct
Starter Solutions.-(See solution fertilization).
Sulfur-coated Urea (SCU)-A slow-release nitrogen product
which has proven to be effective in vegetable, ornamental and
turfgrass production. Prilled urea is coated with sulfur to control
the release of N from the product. It contains 32-38% N and the rate
MARSTON SCIENCE LIBRARY
of release is primarily governed by the thickness and biological degra-
dation of the sulfur coating. The rate of N release is slowed by cool
temperatures, so the product has generally been shown to produce
responses inferior to IBDU when used during the cool season. Mot-
tling of turfgrasses has been noted when SCU was used as the sole
N source during the cool season.
Urea.-Urea contains 45-46% water-soluble organic nitrogen that
is usually converted to ammoniacal nitrogen within one to three days
after application to the soil. It is currently called an "organic" nitro-
gen, but should not be confused with natural organic or with other-
wise-insoluble nitrogen. It is a very concentrated form of N, which
is difficult to use at a light rate of application.
Urea is converted to ammonium by the enzyme urease, produced
by microbiological activity. If urea is surfaced-applied to moist soil,
or if surface application to dry soil or turf is followed by only light,
non-penetrating rains, appreciable nitrogen loss as ammonia gas
An impurity, biuret, usually is found in prilled urea as a result of
heating during the pelleting process. Fertilizer urea for general usage
contains not more than 2.5% biuret, because the compound is toxic
to plants if present in significant quantities. For crops known to be
exceptionally sensitive, and for spray usage, special low-biuret urea
such as so-called "crystal" urea is available.
Ureaform.-(Ureaformaldehyde, UF, Nitroform, Blue Chip,
Powder Blue, Methylene Urea). This is a material containing
about 38% nitrogen, made by chemically combining urea and formal-
dehyde for fertilizer use. It releases nitrogen to a crop more slowly
than do soluble forms of nitrogen. Approximately one-third of the
nitrogen is rapidly available, one-third moderately available, and
one-third so slowly available that it must be built up to considerable
quantities in the soil if it is to release nitrogen in significant amounts.
Tests of the various ureaforms now offered for sale show that the
best are superior to natural organic in prolonging the supply of
available nitrogen. The current high cost of ureaform largely restricts
usage to golf greens, ornamentals, and similar cultures. Release of
N is by microbial action, so factors which influence the biological
activity of soil microbes (such as temperature, pH, nutrients, etc.)
affect the release of N. Generally speaking, it has not been shown
to be effective during cool-season growth.
Urea Phosphates.-(Urea-Urea Phosphates). This new class
of nitrogen and phosphorus materials has been developed by the
Tennessee Valley Authority (TVA). They are not widely available at
present, but may be introduced into the speciality market in the
future. These sources are noted for their high analysis (27-9-0, 32-21-
0, 16-41-0, 28-28-0 and 34-17-0) and for their acidifying properties.
Some of these products produce a pH of less than 2.0 when dissolved
MARSTON SCIENCE LIBRARY
MARSTON SCIENCE LIBRARY
This publication was produced at a cost of $1565.90, or
31.3 cents per copy, to provide information on fertilizers
and fertilization. 9-5M-89
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTI- :
TUTE OF FOOD AND AGRICULTURAL SCIENCES, G.L. Zachariah, director,
in cooperation with the United States Department of Agriculture, publishes this
information to further the purpose of the May 8 and June 30, 1914 Acts of ...... ...
Congress; and is authorized to provide research, educational information and
other services only to individuals and institutions that function without regard to
race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding
4-H and youth publications) are available free to Florida residents from county extension offices.
Information on bulk rates or copies for out-of-state purchasers is available from C.M. Hinton,
Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611.
Before publicizing this publication, editors should contact this address to determine availability.