Soil, Plant, and Fertilizer
KANSAS STATE UNIVERSITY
COOPERATIVE
EXTENSION
SERVICE
Phosphorus (P) does not exist in soils in the
simple elemental form, but is found combined with
other elements forming complex minerals (inor-
ganic) and organic compounds. The total phos-
phorus content of the surface six inches may be as
little as 200 pounds per acre on very sandy soils to
over 2,500 pounds per acre on fine textured soils.
Equilibrium of solid phase phosphorus with solu-
However, only a small fraction of this total phos-
tion phase phosphorus available for plant use.
phorus is in a form that is readily available to plants.
Thus, application of phosphorus fertilizer, or agri-
Thus, the amount of phosphorus available to
cultural or municipal wastes are necessary on
crops depends on the quantity of phosphorus in
many soils to meet plant phosphorus needs.
the soil solution and on the continued dissolving of
Soil Considerations phosphorus minerals to maintain the soil solution
level of phosphorus. Phosphorus soil tests mea-
Research into the chemistry of soil inorganic sure the ability of the soil to supply phosphorus to
phosphorus has shown a very complex system of the soil solution for plant use. Soil test methods
reactions and compound formation dependent on used by laboratories do not measure the total quan-
such factors as soil pH, type and amount of soil tity of plant available phosphorus in the soil, but
minerals, amount of phosphorus in the soil, and
rather measure a part of those compounds that
other soil factors. Likewise, the chemistry of or- maintain plant available phosphorus in the soil so-
ganic soil phosphorus is very complex and proba- lution. The amount of phosphorus measured with a
bly less understood than inorganic soil phos-
soil test is therefore an index that is related to the
phorus chemistry; however, breakdown (mineral- fertilizer needs of crops. The amount of phos-
ization) of organic matter and crop residue by soil phorus measured with a soil test is related to crop
microorganisms is recognized as being a ma-
response to phosphorus fertilizer by conducting
jor contributor in many soils to plant available many phosphorus fertilizer rate experiments on
phosphorus. many soils and with various crops.
Phosphorus moves very little in most soils, be-
cause of the low amount dissolved into the soil wa- SOlL TEST RATING
ter (solubility). For this reason, phosphorus move-
ment is much less than for nitrogen, potassium, or
calcium. Erosion and crop removal are the major
ways soils lose phosphorus.
As the soil test values for phosphorus increase, the
amount needed from fertilizers becomes less.
Plant Considerations
Phosphorus (P) is an essential part of meta-
bolic processes that occur within the plant, such
The low solubility of phosphorus bearing min-
as photosynthesis, the synthesis and breakdown
eral and organic compounds means that there is
of carbohydrates, and energy transfer. If the soil
very little phosphorus in the soil solution available
level of available phosphorus is not adequate for
for plant absorption at any one time. For most soils,
these plant processes, then production will be re-
the amount of phosphorus dissolved in the soil so-
duced unless fertilizer phosphorus is added.
lution is no more than a fraction of a pound per
acre. Crops need much more phosphorus than
Plant Needs
what is dissolved in the soil water and a rapid re-
plenishment of the solution phosphorus occurs as Plants contain less phosphorus than nitrogen
plants absorb phosphorus (equilibration). This or potassium yet removal of phosphorus by good
maintenance of phosphorus in the soil solution by yielding crops is sizeable, especially if the total
dissolving of phosphorus minerals is the key to the above ground portion of the crop is harvested as
plant available phosphorus status of the soil. can be seen from the nutrient removal table.
2
PLANT NUTRIENTS REMOVED BY VARIOUS CROPS phosphorus that it will accumulate in the entire
season has been taken up while only 49 percent of
Yield N P2O5 K2O
Crop the dry matter has accumulated. In spite of this rel-
atively low amount of the total uptake that occurs
early, nutrient availability early in the season is im-
6 ton 270* 60 270
Alfalfa
portant and phosphorus fertilizer should be ap-
4 ton 140
140 46
Cool-season grass plied early to get the most benefit.
Corn, Grain 150 bu 135 54 39
144
Stover 9,000 lb 101 36 Plant Uptake
100 bu 81 44 25
Sorghum, Grain Several factors need to be considered when
156
7,500 lb 106 31
Stover thinking about phosphorus uptake by plants:
50 bu 188* 44 66
Soybeans, Grain 1. Plants take up phosphorus almost entirely
3,500 lb 82 10 50
Straw as the orthophosphate anion (HPO or
60 bu 75 28 23 H2PO ). The relative amount of each ionic
Wheat, Grain
53
Straw 4,500 lb 30 8 species in the soil solution depends on soil
pH. Acid soils favor the H 2PO species,
*Legumes get part of their nitrogen through symbiotic
and alkaline soils favor presence of HPO
nitrogen fixation.
Plants are able to absorb both species
effectively.
Phosphorus removal figures can be useful in
combination with other inputs to determine phos- 2. Absorption of the orthophosphate anions,
phorus application rates. On soils deficient in like other nutrients, occurs primarily from
phosphorus, application rates need to exceed re- the soil solution. Phosphorus uptake is re-
moval if you expect to increase the available phos- duced considerably by dry soil favoring
phorus in the soil. Very low phosphorus rates ap- deeper incorporation where dry surface soil
plied as starter fertilizer may stimulate growth, but conditions develop.
cannot be expected to maintain the available phos- 3. Because the soil solution concentration of
phorus in the soil. To maintain the available phos- orthophosphate ions is quite low, absorp-
phorus in a soil, the amount of phosphorus fertil- tion of the orthophosphate ions in most
izer applied must at least equal crop removal of cases occurs against a concentration gra-
phosphorus. On soils testing very high in available dient (known as active absorption) as the
phosphorus, removal can exceed application and phosphorus concentration is greater within
not be detrimental to the soil. the root than in the soil solution phase. This
active absorption requires energy derived
Accumulation by Plants from root respiration of carbohydrates.
Thus, conditions like wet or cold soils that
Concentration of phosphorus in plants varies
reduce root metabolic activity will also
due to such factors as stage of growth, plant spe-
slow phosphorus absorption.
cies, part of the plant sampled, and environmental
4. Absorption of nutrient ions and moisture is
conditions. Plant uptake of phosphorus, like other
greatest near the actively growing tip of
nutrients, proceeds at a faster rate than does dry
each root. Root exploration through the soil
matter production in the early stages of plant
as plant roots develop allows for new areas
growth. For example, in the first 20 days, only 3 per-
of the soil to be nutrient sources. The total
cent of the phosphorus needed by a mature grain
root surface area and the mass of soil ex-
sorghum crop has accumulated. By early bloom
plored by the roots both are important for
(about 60 days after emergence), 60 percent of the
DRY MATTER AND NUTRIENTS ACCUMULATION AS GRAIN SORGHUM PLANTS DEVELOP
(MEDIUM MATURITY HYBRID)
Rapid Early Grain
Seedling Growth Bloom Formation Mature
0-20 days 21-40 days 41-60 days 61-85 days 86-95 days
% % % % %
N 5 33 32 15 15
P2O5 3 23 34 26 14
K2O 7 40 33 15 5
Dry Matter 2 15 32 32 19
From: R. L. Vanderlip. 1972. How a Sorghum Plant Develops. Kansas Extension Circular. C-447.
3
potential nutrient absorption. Plant roots
have been estimated to occupy no more
than one to two percent of the total soil
volume.
5. Other nutrient ions are absorbed by plants
at the same time as the orthophosphate
ions. Although most ions are thought to be
absorbed independently, there are some
demonstrated interactions between nutri-
ents as competition for absorption sites on
roots occurs. The plant strives to maintain
electrical balance by absorption of posi-
tively and negatively charged ions and by
exchange of ions from the roots with the
soil solution. The presence of ammonium
ions (NH ) has been shown to have an en-
hancement effect on phosphorus uptake
especially with starter fertilizers.
6. The concentration of the orthophosphate
Fertilizer Terminology
ions in the solution also is important to po-
The Kansas fertilizer law requires that any
tential absorption. Thus, placement of fer-
product sold as a commercial fertilizer must show
tilizer in bands or as granules develop tem-
on the label, or bill of sale for bulk material, a guar-
porary zones of higher solution phos-
antee of minimum percentages of total nitrogen,
phorus concentration allowing for greater
(N), available phosphoric oxide (P2O5) and water sol-
uptake.
uble potassium oxide (K2O), commonly referred to
7. Phosphorus is known to move very little in
as nitrogen, phosphate, and potash.
soil (nonmobile nutrient) and roots must be
Although phosphorus does not exist as P2O5 in
very close to phosphorus in the soil for up-
fertilizer materials, phosphorus recommendations
take to occur. Diffusion of phosphorus to
are made for rates of P2O5. There are, however, many
roots will occur over no more than a quarter
other situations where the phosphorus content is
of an inch. For this reason, placement of
expressed as elemental phosphorus (P) and under-
phosphorus fertilizer close to the seed is
standing which units are used is very important in
important for efficient early season use of
discussing phosphorus amounts. (To convert from
the applied phosphorus.
P to P2O5, multiply P by 2.29 and to convert from P2O5
to P, multiply P2O5 by 0.44.)
Terms frequently used in discussing fertilizer
phosphorus are water-soluble, citrate-soluble, cit-
Fertilizer Considerations rate insoluble, available, and total phosphorus.
When phosphorus containing fertilizers are
Water-Soluble: Fertilizer samples analyzed by
applied to soil they are first dissolved by the soil the State Control Laboratory are first dis-
water. Next, much of the newly dissolved phos-
solved in water under standardized condi-
phate is rendered less available by a phenomenon
tions. The amount of phosphorus dissolved is
called phosphorus fixation, or adsorption. The ex-
measured and expressed as a percentage P2O5
tent of phosphorus fixation and the compounds
by weight of the sample.
formed depends on many factors. Two very impor-
Citrate-Soluble: The fertilizer not dissolved by
tant factors mentioned earlier are type and amount
the water is then placed in a 1 normal ammo-
of soil minerals, and soil pH. In general, high clay
nium citrate solution and the amount of phos-
content soils fix more phosphorus than sandy
phorus dissolved is measured and expressed
soils. A low soil pH reflects the likely presence of
as a percentage P2O5 by weight of the sample.
soluble iron and aluminum which readily react with
Available: The sum of the water-soluble and
fertilizer phosphates to cause fixation as iron and
citrate-soluble phosphate is considered phos-
aluminum phosphates. A high soil pH reflects
phorus in the fertilizer available to plants and
abundance of calcium for fixation as calcium phos-
is the amount guaranteed on the fertilizer
phates. In Kansas soils, the least fixation occurs in
label.
the pH range of 6 to 7 as illustrated below. There-
fore, soils should be limed for better fertilizer phos- Citrate Insoluble: The phosphorus fertilizer
phorus availability. Little can be done economically not dissolved by the normal ammonium citrate
to alter the pH of calcareous, high pH soils, but fer- is measured and expressed as a percentage
tilizer placement can be used to reduce fixation. P2O5 by weight of the sample.
4
Total: The sum of the available and citrate in- quired on the label of rock phosphate, basic
soluble phosphate is the total phosphorus. slag, bone meal, tankage, and other natural or-
Most commercial fertilizers have very little of ganic phosphate materials sold as phosphate
the phosphorus in the citrate insoluble frac- sources.
tion. Total phosphate analyses are only re-
COMMON PHOSPHORUS FERTILIZER SOURCES
Percent of Available
P2O5 Content
Phosphorus that is
Source N Total Available Water Soluble
% % % %
85
0 21 20
Superphosphate
85
0 45 45
Conc. Superphosphate (Triple)
92
11 49 48
Monoammonium Phosphate (MAP)
18 47 46 90
Diammonium Phosphate (DAP)
100
10 34 34
Ammonium Polyphosphate (POLY)
100
0 54 54
Phosphoric Acid
0 34 3 to 8 0
Rock Phosphate
Source: Ohio Agronomy Guide. Ohio Cooperative Extension Service. Bull. 472.
some use of furnace acid in fertilizer manufacture.
Manufacture of Phosphorus Fertilizer
One such fertilizer uses furnace acid, potassium
Rock phosphate is the raw material used in the
hydroxide, urea and aqua ammonia. These prod-
manufacture of most commercial fertilizers on the
ucts are generally more expensive to manufacture
market today. Ground rock phosphate has been
due to the added cost of the materials used.
used as a phosphorus fertilizer for acid soils, how-
The wet process involves treatment of the rock
ever, due to low plant availability of the phosphorus
phosphate with sulfuric acid producing phos-
content coupled with high transportation costs, lit-
phoric acid (green or black acid) and gypsum which
tle is used today.
is removed. The resulting wet process acid con-
tains some impurities and sulfur but is less expen-
sive to produce. The impurities in wet process acid
Phosphate Manufacture
have not been a problem in the production of dry
fertilizer. Improved technology in recent years has
developed processes for removal of some impuri-
ties from the wet process acid so that excellent
quality liquid fertilizers are manufactured. The
amount of impurities in either dry or liquid fertilizer
is quite small and causes no toxic effects. Either
treatment process (dry or wet) produces orthophos-
phoric acid -the phosphate form absorbed by
plants. No difference in availability to plants exists
due to the method of converting rock phosphate to
phosphoric acid even though there are fewer impu-
rities in furnace acid.
Phosphorus Sources
Selection of a phosphorus fertilizer source
can be confusing as many products exist on the
market, with each having unique characteristics
that are touted by the manufacturer. Degree of wa-
Manufacture of most commercial phosphorus
ter solubility, liquid versus dry fertilizer, and ortho
fertilizer begins with the production of phosphoric
versus polyphosphates are characteristics fre-
acid. Phosphoric acid is produced either by treat-
quently mentioned.
ment of the rock phosphate in an electric furnace
(dry process) or by acid (wet process). The dry pro- Water solubility: Water solubility of phos-
cess produces a very pure and more expensive phorus fertilizer has been studied for years and it is
phosphoric acid (white or furnace acid) used pri- generally agreed that water solubility is a desirable
marily in the food and chemical industry. There is trait in phosphatic fertilizer. Water solubility is
5
most important where availability of the phos- Most polyphosphate fertilizers will have 40 to
phorus for immediate plant uptake is needed, such 60 percent of the phosphorus remaining in the
as starter fertilizer. Even with starter fertilizers, ortho form, thus sufficient ortho ions exist in these
100 percent water solubility of the phosphorus is materials to meet immediate plant needs. Re-
not necessary. Based on research with phosphatic search has shown polyphosphates to be equal to
fertilizers of varying water solubility, phosphatic orthophosphates as starter fertilizers.
fertilizers with water soluble phosphorus contents In the soil, polyphosphate ions are readily con-
of 50 percent or greater are equal for the most re- verted to orthophosphate ions by adding water
sponsive crops even on soils testing low in availa- back to the polyphosphate ions (hydrolysis) which
ble phosphorus. Most phosphorus fertilizers on the is the reverse of the process in its manufacture.
market today have a water soluble phosphorus con- This process is enhanced by an enzyme, pyrophos-
tent of 75 percent or more, making water solubility phatase, abundant in most soils. The conversion
as a selection criteria unimportant. of polyphosphates to orthophosphates is fast.
With broadcast applications to warm, moist soils,
Liquid vs. Dry: Utilization of the phosphorus is
much of the polyphosphate will be converted to
not changed by the fertilizer being applied as liquid
orthophosphate within two to three days after
or dry. Utilization is related to such factors as appli-
application.
cation method, crop growth characteristics, soil
Polyphosphates are primarily marketed as liq-
phosphorus level, and climatic conditions. The
uid ammonium polyphosphate fertilizers. The re-
amount of water in a liquid fertilizer is insignificant
moval of water in the manufacturing process gives
compared to the water already in the soil. If fertil-
a substantially higher phosphorus analysis com-
izer is applied to extremely dry soil, liquid fertilizer
pared to ammonium orthophosphate clear liquid,
will not stay in solution and dry fertilizer will not
reducing transportation costs and volume per acre
dissolve. If the soil has good moisture, then the liq-
to get the same phosphorus rate. The polyphos-
uid fertilizer will stay in solution and dry fertilizer
phate liquids also are more convenient for the fertil-
will dissolve.
izer dealer to handle and allow for formulation of
Liquids do have some advantage in preparing
blends not possible with orthophosphate clear liq-
specific blends of more than one nutrient in that a
uids. However, no consistent advantage has been
homogenous mixture can be formulated, whereas,
shown between ortho and polyphosphates in crop
dry bulk blends involve mixing individual materials.
response. Phosphorus availability from ortho and
Dry bulk blends rely on uniform sized particles of
polyphosphate fertilizers should therefore be con-
each component of the blend to minimize segrega-
sidered equal.
tion and uneven fertilizer spreading in the field. Se-
lection of a liquid or dry phosphorus source should
Phosphorus Application
be based on adaption to the farmers operation and
price rather than expected agronomic advantage. Incorporation of the phosphorus into the plant
Ortho vs. Polyphosphate: Polyphosphate fer- root zone is important to assure good utilization un-
tilizers are manufactured from orthophosphoric der Kansas climatic conditions which frequently
acid by heating the acid, water is split from the result in extended periods of dry surface soil. An
ortho ions bonding them into chains of various exception is broadcasting of phosphorus on per-
lengths (primarily two phosphate chains). The term manent sod and alfalfa fields. These perennial
鈥減oly鈥? is used to indicate that various length phos- crops have an extensive surface root system and
phorus chains exist in the fertilizer. The manufac- are able to effectively utilize surface applied phos-
turing process does not convert all of the ortho ions phorus. No improvement in utilization has been ob-
(plant available form) into polyphosphate ions. served from incorporation.
Certain advantages and disadvantages are as-
sociated with each method of application with no
method being superior in all situations. Applica-
tion method differences are most likely to occur on
STEAM
soils deficient in phosphorus and are more likely at
low rates of phosphorus. Under these conditions,
application methods that reduce soil fertilizer con-
tact (bands) and place the fertilizer relatively close
to the seed perform well. Methods of application
that reduce soil-fertilizer contact include starter
banding, deep banding (dual application with nitro-
gen as ammonia or liquid) and surface strip applica-
tion. Starter banding and dual application generally
perform better than broadcast applications on
phosphorus deficient soils. Where soil test levels
6
are medium or higher, differences among methods plants is usually less than 30 percent in the first
are less likely; however, in years of cool-wet soil year after application and in some cases is 10 per-
conditions after planting, starter fertilizer can be cent or less. This low recovery is because soil reac-
advantageous for early growth. tions discussed earlier reduce phosphorus con-
With the use of conservation tillage practices, centration in the soil solution and plant roots
less opportunity exists for incorporation of fertil- occupy only a small percentage of the soil pore
izer. Under these conditions, the use of starter fer- space. Some of the phosphorus not recovered in
tilizer or deep band placement may be better than the year of application will be taken up by crops in
broadcasting. Another alternative on soils of me- succeeding years. Band application of phosphorus
dium phosphorus soil tests or above is to apply can improve utilization of the fertilizer phosphorus.
higher broadcast rates in years where incorporat- All phosphorus sources have similar availability
ing tillage operations are done with no application when applied at the same rate and with the same
or low rates of starter in other years. method of application.
The first step in putting a phosphorus fertiliza-
tion program together is to soil test to determine
Putting it all Together the need for phosphorus. Once the need for phos-
Soils lacking in available phosphorus need fer- phorus is established, application methods should
tilizer phosphorus application. The illustration be- be considered. Band placement can improve up-
low summarizes the general cycling of fertilizer take on low and very low testing soils. Knowing that
phosphorus through soils and plants. phosphorus is generally equally available from vari-
Phosphorus fertilizer does not remain dis- ous products, selection of a phosphorus fertilizer
solved in the soil water for long since soils adsorb should be based on its suitability for the applica-
phosphorus, making it less available. As a result, tion method selected and its cost per pound of P2O5
recovery (utilization) of the fertilizer phosphorus by applied.
We wish to gratefully acknowledge the review of this publication by David Kissel, Soil Fer-
tility Research, KSU, Ray Lamond, Extension Soil Fertility and Management Specialist,
and Larry Murphy, Plains Director, Potash and Phosphate Institute, Manhattan.
David A. Whitney
Extension State Leader, Agronomy Program
Cooperative Extension Service, Manhattan, Kansas
October 1988
C-665 Revised
Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State
University, County Extension Councils, and United States Department of Agriculture Cooperating, Richard D. Wootton,
Associate Director. All educational programs and materials available without discrimination on the basis of race, color,
national origin, sex, age, or disability.
10-88-3M; 12-93-M
File Code: Crops & Soils 4-1
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