xt7x959c7412 https://exploreuk.uky.edu/dips/xt7x959c7412/data/mets.xml   Kentucky Agricultural Experiment Station.  journals kaes_circulars_004_602 English Lexington : The Service, 1913-1958. Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Circular (Kentucky Agricultural Experiment Station) n. 602 text Circular (Kentucky Agricultural Experiment Station) n. 602  2014 true xt7x959c7412 section xt7x959c7412   ( §) G4; ? 
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INTRODUCTION ...................................................................... 3 l
SOURCES OF SOIL PHOSPHORUS .................................... 4 A
PHOSPHORUS FIXATION AND RELEASE ...................... 5 .
AVAILABLE PHOSPHORUS .................................................. 5 A
PHOSPHORUS COMPOUNDS IN THE SOIL .................. 6 A
MAKE PHOSPHORUS MORE AVAILABLE ...................... 8 4

 Phosphorus in Kentucky Soils 1
l Sources, Fixation, Release
By GEORGE D. CORDER
Phosphorus is 1 of the 16 or more plant food elements that are
essential for crop growth. When plants do not get enough phos-
phorus, root and top growth are stunted; blossoms, fruits, and seeds
“ do not develop properly; yields are low and often maturity is delayed.
Knowing how chemical reactions affect the availability of phos-
A phorus to plants will help farmers develop better soil fertility pro-
grams, thus resulting in 1T1OI`€ economical CI‘O]_) production.
The phosphorus contained in the plow layer of Kentucky soils
will range from a few hundred to several thousand pounds per acre.
‘ Table 1 shows the total phosphorus content from experiment fields
located throughout Kentucky.
Table 1. Total Phosphorus Content of Some Major Soil Types in Kentucky“
Location of Total Pliosphorous Content
Soil Class Experiment Field per Acre Plow Layer
Pounds
L Maury silt loam Lexington 12,400
‘ Crider silt loam Princeton 780
Tilsit silt loam Princeton 900
Monongahela silt loam Berea 880
\Vellston silt loam Fariston 820
Bedford and Dickson silt loam Campbellsvillc 1,100
Tilsit catcna silt loam Greenville 660
Grenada silt loam Mayfield 960
¤Kentucky Agricultural Experiment Station Bulletin $397, Soil Manugenu·nt Experinient.r
(out of print—c0pies available only at libraries)
f The total phosphorus content of most of the other soil types in
Kentucky falls somewhere between the lowest and the highest re-
corded in Table 1. Relatively few, however, approach the Maury
silt loam.
A 100-bushel corn crop (stover included) contains at maturity
about 25 pounds of elemental phosphorus. Four tons of alfalfa hay
requires about 20 pounds of this element. Other crops grown in
Kentucky likewise require relatively small amounts of phosphorus.
Even though the total phosphorus content of Kentucky soils is well
3

 above the amounts found in crops, additional phosphorus is required
on most of them for good crop production. i ·
The reason for this is the wide difference between the amounts of _
“total” and “available” phosphorus in the soil. The “available” phos- l
phorus is that part of the “total" phosphorus that is potentially usable .
by a crop during a growing season. The “total” phosphorus, on the
other hand, includes that part that is available plus the much larger ` ‘
amount that is not immediately available to the crop. Terms that A
are commonly used by soil scientists to describe this phosphorus are
“insoluble,” “fixed” or “unavailable." However, a small part of this
phosphorus is slowly soluble and can convert itself to an available »
form. This process is called “phosphorus release." Likewise, available
phosphorus can revert to the unavailable forms. This process is com-
monly called “phosphorus fixation.” These two processes will be
discussed more fully later. A
SOURCES OF SOIL PHOSPHORUS
The wide variations of the phosphorus content of Kentucky soils
can be attributed to two sources:
1. Native Phosphorus This is the phosphorus that was in the l
parent rock from which the soil was formed. Originally it was mostly I
a constituent of the phosphorus-bearing minerals generally referred
to as apatites. Chemical and biological reactions in the soil slowly X .
released it from the apatites as the parent rock weathered and de- .
composed to form the soil. Thus, a soil may be low or high in phos-
phorus depending on the parent rocks from which the soil originated.
The Maury silt loam (Table 1) is a good example of a soil derived
from parent rocks with a high content of phosphorus-bearing minerals. A
Likewise, the Tilsit catena silt loam (Table 1) is a good example
of the opposite.
Alluvial (deposited by water) soil materials may have been trans-
ported some distance from their area of origin. However, they too
may be low or high in native phosphorus content depending on the ·
composition o‘f the parent material.
2. Applied Phosphorus Phosphorus in commercial fertilizers has
been applied, sometimes in large amounts, to many Kentucky soils.
Thus, the original phosphorus content, particularly that in the plow
layer, may have been increased by these phosphorus applications.
Much of the tobacco land is a good example. On the other hand,
the phosphorus content may have decreased where plant removal has
been greater than phosphorus applications.
4

 K PHOSPHORUS FIXATION AND RELEASE
I ’ The sand and silt fractions of a soil are made up largely of quartz
A and other minerals which are resistant to weathering or decomposi-
tion. They may contain phosphorus and other nutrient elements;
A · however, since they are generally insoluble, their ability to release A
— . phosphorus is very low. Also, because of their physical and chemical
nature, their ability to attract or Hx phosphorus is very low. Hence,
` in very sandy soils, there may be little phosphorus fixation or release.
On the other hand, the clay minerals of a soil, often called colloidal
material, are relatively active chemically. They are capable of ad-
i sorbing phosphorus that is in the soil solution (soil moisture) be-
cause of the electrical charges that surround them. The clay min-
erals also are capable of releasing the adsorbed phosphorus back
‘ to the soil solution (phosphorus release). This reaction of phos-
phorus and the soil colloids is often called “anion exchange.”
In the process of soil formation, iron, aluminum, and calcium
are released from the parent rock. Phosphorus will, under certain soil
conditions, combine with these elements to form insoluble compounds
(phosphorus fixation). If soil conditions change (for example, due to
· liming acid, soils), these compounds can release phosphorus to the
soil solution (phosphorus release).
Growing plants obtain phosphorus from the soil for their food
supply. When these plants die and return to the soil as organic
A matter, this phosphorus will be returned to the soil solution through
the processes of decay (phosphorus release). However, because of
the electrical charges surrounding the humus particles (also called
collodial material), these particles may adsorb phosphorus just as
· the clay minerals do (phosphorus fixation).
The soil conditions under which phosphorus fixation and release
occur will be discussed more fully later.
AVAILABLE PHOSPHORUS
As already stated, “available phosphorus” is that part of the total
phosphorus in a soil that is potentially usable by a crop during a
growing season. It includes the phosphorus in the soil solution plus
part of the phosphorus that is adsorbed by the soil colloids. Some
of the adsorbed phosphorus is so loosely attached that it can be picked
off by plant roots if these roots grow in close proximity to thc phos-
phorus. Also an electrical imbalance may develop between the plant
root and the soil colloid which is strong enough to pull the phos-
phorus away from the eolloid.
5

 Phosphorus in commercial fertilizers is guaranteed to be in an
“available" form, meaning that it is soluble in a standardized ex- ,
tracting solution (water or citric acid) that supposedly simulates .
a normal soil solution. This phosphorus is “available” until it is
placed in the soil complex. lt does enter the soil solution as it is '
guaranteed to do, but in the presence of the soil colloids and other
elements, it may become fixed as described above. This chemical
reaction is sometimes called “phosphorus reversion.” This phosphorus `
“fixation” or “reversion” explains why soils may test low in available _
phosphorus even though phosphate fertilizers have been recently
applied.
Chemical reactions in a soil are such that somewhat of a balance II I
is maintained between the levels of available phosphorus and fixed
phosphorus. As growing crops remove the available phosphorus from
thc soil solution, fixed phosphorus will move into the soil solution to
maintain the soil balance. A standard soil test will show the amount I
of phosphoms available at the time the sample was collected. It
does not show all the fixed phosphorus that can move into the soil
solution over a long period as the available phosphorus is removed.
This explains why a soil that has had recent applications of phos-
phorus may test low but yet have enough to produce good crops.
It also shows the need for knowledge of the soil itself, recent phos- `
phorus applications, past crop growth, along with the results of a ‘
soil test, before phosphate fertilizer programs are planned.
The preceding discussion and Fig. 1 point out that phosphorus _
in the soil may be a part of several complex compounds. The amounts
of phosphorus “fixed” in a soil depend on the amount and kinds of
clay minerals and organic matter present and the reaction (pH) of
the soil. llenee, a heavy clay soil or one high in organic matter will
"fix" more phosphorus than a sandy soil or one that is low in organic ‘
matter. Likewise, more phosphorus will bc “fixed" in an acid or
alkaline soil than one that is neutral in reaction.
PHOSPHORUS COMPOUNDS IN THE SOIL
Iron, .—\Iaminun1, and Manganese Plzosphates In acid soils, large
amounts of iron. aluminum. and manganese are present in the soil
solution. Phosphorus from the native supply or from fertilizer ma-
terials will combine with these minerals to form iron. aluminum,
and manganese phosphates. This phosphorus is “tied up" and
relatively unavailable for plant use. The process may be represented
as follows using aluminum in the example:
6

 hearing Soil
· Mineral;
E E ’“°‘’*°  
— $·¤· Aviiiim -- $?Iil?.$ (
Organisms PHOSPHORUS (PH 1.0 Ind up)
I on Iron-aluminum
¤5·-ZW °·:*·*·*· an [6`ZYZ$"¤.....»
I Sin of the arrow indicates value ol phosphorus movement.
Fig. l.—This schematic drawing shows the sources of available phosphorus, phosphorus
fixation, and how the levels of available phosphorus are depleted and replenished.
A1+++ + H2POi· + 2HgO Z 2H+ + A1(OH)2H2PO4
aluminum soluble water hydrogen insoluble
_ phosphorus phosphorus compound
( aluminum phosphate)
Note that the soluble (available) phosphorus, in the presence of
water and aluminum, combines with the aluminum to form an in-
· soluble compound (phosphorus fixation) and sets some hydrogen
free in the soil solution making the soil more acid. Also note that
the chemical reaction can be the reverse, starting with the insoluble
aluminum phosphates and ending with soluble phosphorus (phos-
. phorus release).
Tricalcium Phosplzatcs In alkaline soils, large amounts of calcium
and magnesium and perhaps sodium and potassium will appear in the
soil solution. However, most of the phosphorus in such soils will be
"Hxed” by combining with the calcium. This process often occurs
( in overlimed or naturally alkaline soils and may be illustrated as
follows:
Ca(H2PO,)u —}- 2CaCO;, 2 Ca3(PO4)2 —}— ZCO2 —l— 2HgO
soluble calcium insoluble
phosphorus carbonate phosphorus
compound compound
(tricalcium phosphate)
Note that the soluble phosphorus compound (monocalcium phos-
phate) combines with the calcium carbonate (dissolved limestone) to
7

 form an insoluble (unavailable) phosphorus compound (tricalcium
phosphate). This process also sets some carbon dioxide and water p
free. It, too, can be the reverse, starting with insoluble tricalcium K
phosphate and ending up with soluble monocalcium phosphate. .
Organic Phosphorus In organic matter, phosphorus is a constituent I
of plant materials and the soil micro—organisms which make up the
organic matter in soils. This organic phosphorus is released to the a
soil solution only as the organic matter decays. But it may quickly _ _
revert to the insoluble compounds described above if the soils are
either acid or alkaline.
Adsorbed Phosphorus As stated earlier, phosphorus may be ab-
sorbed by the soil colloids (clay and humas particles). This reaction
may be illustrated as follows:
( clay > ( clay 5 V
colloid OH·—l- HZPOJ 2 colloid H2PO4 —}—OH‘
soil in soil in soil soil
solid solution solid solution ‘
Note the negative charge on the phosphorus (H2PO{) in the soil
solution. Then note that the phosphorus has become electrically
attracted to the clay colloid at the right, displacing the OH" ion. -
This is the phosphorus which, if loosely adsorbed, can be picked .
off by plant roots if they grow in close proximity to it. °
MAKE PHOSPHORUS MORE AVAILABLE
Phosphorus fixation can work to the farmer’s advantage. Since
phosphorus becomes easily fixed, it does not leach from the soil. Thus,
it can be stored without fear of its leaving the soil except by crop ·
removal or soil erosion.
Fixed phosphorus slowly but gradually becomes available for
plant use. Enough of it may become available during a year to grow
good crops, especially on soils fairly well supplied with it. How-
ever, phosphorus can be made more available by good soil manage-
ment practices.
Liming acid soils will reduce the amounts of iron, aluminum. and
manganese in the soil solution and will likewise reduce the amounts
of the insoluble phosphorus compounds that are formed in the soil.
Also. liming such soils will reduce the acidity level to a point that
favors the growth and activity of soil micro-organisms which. in turn,
will hasten the decay of organic matter. thus releasing organic phos-
phorus for plant use.
8

 Avoid Overliming Phosphorus is tied up when there is an excess
' of calcium and magnesium just the same as it is when there is an
_ excess of iron and aluminum.
Figure 2 illustrates the phosphorus compounds at various acidity
levels, and that more phosphorus is available for field crops between -
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  .-_,   _.r_.     A 0 P,. up.,
i    /A ’7·?‘/ VY  U  
    " ‘ y'/ ' f / "·*i
·x%:2;‘::e ·    ·
. pH 4.0 $.0 6.0 7.0 8.0
  Iron and aluminum phosphorus compounds that are relatively
·   unavailable to field crops.
ig   Tricalcium phosphate and other forms of phosphorus that are
`·‘*' · " ’ relatively unavailable to field crops.
I Forms of phosphorus that are potentially available to most field
  crops, Normally this is a small part of the total phosphorus
in a soil.
Fig. 2.—SoiI pH is a factor determining how much phosphorus is available for plants.
· Soil pH between 6.0 and 7.0 is best for most field crops.
a pH of 6.0 and 7.0. Some horticultural plants, however, will make
greater use of the phosphorus in somewhat more acid soils. This
figure also illustrates that there is an exchange of phosphorus from
the available forms to the relatively unavailable forms and vice versa.
Applying heavy rates of phosphate fertilizers will increase the
amounts of phosphorus “stored” in the soil complex. As total phos-
phorus in the soil increases, so does the amount that is available
to growing crops. Hence, eventually enough phosphorus will be con-
verted from the fixed forms to the available forms so that smaller
applications of phosphate fertilizers will supply crop needs.
Row or band applications of phosphate fertilizers will leave the
material in contact with a smaller portion of the soil body. This prac-
9

 tice will result in less phosphorus fixation than when the fertilizer
is mixed thoroughly with the soil. This method may be used when _
working with low-phosphate soils or limited amounts of phosphate
fertilizers. ‘
10

 
 Cnoperative Extension Work in Agriculture and Home Economics: College of Agriculture
and Home Economics. University of Kentucky, Lexington, and the United States Depart-
ment of Agriculture, cooperating. William A. Seay. Dean and Director. Issued in further-
ance of the Acts of May B and June 30, 1914.
(Filing Code 1) 5M-6-65