xt7zcr5nbh6t https://exploreuk.uky.edu/dips/xt7zcr5nbh6t/data/mets.xml   Kentucky Agricultural Experiment Station.  journals kaes_circulars_004_622 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. 622 text Circular (Kentucky Agricultural Experiment Station) n. 622  2014 true xt7zcr5nbh6t section xt7zcr5nbh6t Q
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 POTASSIUM IN KENTUCKY SOILS
By George D. Corder and Harold F. Miller
Extension Specialists in Soils
' Potassium is o11e of the I6 or more plant food elements that are
essential for crop growth. An adequate supply of available
potassium in the soil helps to insure the health of plants and `
improves the quality of the crop. It insures greater elliciency in
photosynthesis, assists in the functioning of chlorophyll and aids '
in the formation and translocation of sugars, starches and oils. It
increases the plumpness of·cereal grains and stillness of straw.
Potassium tends to offset the effects of excessive nitrogen.
POTASSIUM CONTENT OF KENTUCKY SOILS
Unlike nitrogen and phosphorus, the quantity of total potassium
found in Kentucky soils is relatively high. Table I shows the
potassium content of the plow layer (the surface 7 inches) of a
few Kentucky soils.
The potassium in Kentucky soil can be attributed to two sources
—native and fertilizer potassium.
Native potassium
Tl1is is the potassium that was in the parent lllilL(’l`l1llS—I`()(`l(S
containing feldspars and 1l1l(`1lS—l>1`()lll which the soil was formed.
l Table `I.-—-Total Potassium Content of the Surface 7 Inches of Soils on Experi-
ment Fields in Kentucky*
TTTTTTT TCTTTT iiii T I V T T T Ti  
Potassitnn
Location of (Iontcnt
Soil Class lixpeiitnent I·`it·ltaTitaiTQiTinfi€l7Tf`\tiirln$Ianz1geii1<·nl
Iixpcriinents." (Out of print; copies available only at lIl)I`2IlIC\)
3

 Table 2.—Potassium Uptake by Millet Grown on Six Kentucky Soi1s*
  7777777 1  
(1)Ol£1SS1LlH1 lb/A) 1’<)l11SS1lllll
1)L11`(‘|l1 $011 Test Lcvcl Rcinox 011
l)Cl`lCll1 11011110 .»\1101· -1 i11 4 XIi1l0t
Soil T1110 Xl1lll'1`1il1 (1l`()1)1)1llg (ll`()I)S Crops (lb/A)
1i(1Cl1 (1ll1(`il1`C()11S 511:110,
Si1tst0n0 111111
1.1l11CS10l1€ 300 193 1100
1)ClI11)l`()1(C T:1111CS1()l1C 173 7-1 275
Ali1111`}' 1)1l1>S1)11ill1(` 11i1110st11110 11-1 73 125
l1C(1l`()1`(1 111lll(ZSl()l1€ 01 -19 125
(1l`Cl11l(12l 1.110ss 78 5-1 75
Tilsit SZl11(151()11C :11111 511:110 51 -15 50
7·UH1K11]111111·1— 111 1·:.111YHH}7L{ 
1)Ulil\\|llI1I 1{('|1l(}\('l1 111 N1i11t·1 :11111 1{(‘l1 (ZI0101 :11111 11IC l1lll1l€%1llII1 1‘1Xl1`2lC1C(l 111
l"Ull1` (ilI(’III1(1l1 x1(`llI(1(1*} 1-1`(HII Six l{('Il11l(`1i§' Soils."
\%S.\1’22:l10
T110 1iC1(1S1)2ll`S uml l1l1(`1lS :110 101:1ti1011 high 111 1111t:1ssi11111 (Z()I1[CI1[
uml thus soils (1C1`1\'(‘(1 1111111 1)L11`Cl1[ 111:1t01i:11s (`()l1[1l1111l]g t110s0
IIl111C1`2l1S 1l1`C :1111 111 11:110 :1 high Il&ll1\'C 1)1)l1lSS1ll111 101110111. O11 1110
()[1lC1` 11:11111, s01110 11:110111 111:1101i:11s. s:11111st0110 1i()1` 0x:11111110, :110 low
111 1)()[1lS>1l1l11 1111110111 :11111 soils 110111011 l`1`()l11 1110s0 :110 :111t t11 1010:1s0
l)U[llSS1lll1l to 1110 1l\'2l1lil1)1C 1()1`I11 slowly. .·\ll 111llSLl`2l[1()I1 01 this is
S1l()\\'ll 111 T:11110 2. ()11 :111 l.i(1Cl1 soil 1l1l\'1llg1‘i11(1 110111111s 111 0x1;11:111g0-
:1b10 1101:1ssi11111 1)C1` :11:10, 1i(>ll1` l1ll11C( 01011s 1011101011 1100 ])Ol1l1(1S ,
111 1)()121$S11llll, 1111110 011 il Tilsit soil with 51 1111111111s ()1-CX(`112l11gC‘211)1€
1)()[1l$$1l11ll 1)C1` il(`1`(‘. 1110 li<)111` 11111101 (`1`()1)$ 1`ClI1(>\'C(1 only 50 11o111111s
01 1101:1ssi11111.
.·\11111i:11 (\\'1l1(‘l` [l`1111S1)()]`1(‘(l) soil 111:1t01i:11s 111:11 11:110 1)€€1l
lt`1ll41`1(`(1 gl`Cil[ 11is1:1111·0s 110111 111011 u10:1 01 origin. b1<)\\'€\'CI`. 11101:
lllily 11:110 \'§l1`}'1llg1(‘\`(T1>1)1 1)<)1il%\111Ill, 1101101111i11g1111 1110 1I11Il€1`l11()g1-
1::11 (`()l1l1)()S1111)l1 0I 1111- 1)Jl1`CI1l 111:11011ul 111 1110 21l`C(l 111 111011 origin.
Fertilizer potassium
1’01:1ssi11111 (`U11lLl111(‘l1 111 (`()I1l1I1(‘1`(`11l1 1011111/01s hus 110011 :111111101l.
$()ll1C[1lI1(‘$ 111 ll\l`g(‘ 1l1I1<>l1111%. 10 lllilllf 1{C11[1l(`1§\` soils. This 111:11
11I1\'C 111l1`(`1l5(`(l 1110 1)1)l2l%N11lI1l 101110111. 1)l11`[1(`ll1il1`1}' 111 :11111 just
1)(‘1<)\\' 1110 111011 1il1`C1`. 10 1C\'(‘1s _g1`(‘LllC1` llltlll 111:11 01 1110 11:110111
111il[Cl`1il1\. xllllll l{(`l1llI(li} 1011:1110 1:11111 is :1 00011 0x:11111110 01 this.
()11 1110 011101 11illl(1. 1111010 l)l)11l\\ll1111 1011101:11 111 (`1`O1)5 hus 110011
4

 gI"€2IE€I` [1]}.1]] [lT€ I)O[ZTS5llllTl 2ll)l)ll(`Zlltl()lIS, [IIC l)()[2lSSlLllTl (`OITICITI ITIZIF
have been decreased below the original level.
Fig. I illustrates the potassium cycle in the soil. During weather-
ing, physical, chemical, and biological forces act on the parent
materials antl break them into liner lractions, largely sand. silt. antl
clay particle sizes, Such breaktlown is accompanied by the release
of chemical elements, inclutling potassium, as well as by the lorma-
tion ol clay minerals. -
Most native potassium that is releasecl during the soil»lorming
processes will be in the exchangeable and non-exchangeahle lorms, .
and some lertilizer potassium will revert to these lorms. Note in
Fig. I that both exchangeable anal non-exchangeable potassium
are sources ol reatlily available potassium anal that the process is
reversible. The process is cliscussecl on pages 7-9.
The relative amounts ol sancl, silt anti clay lractions Iiouncl in a
soil tlepentl on the kincl ol parent material (santlstone, limestone,
shale or mica) lrom which the soil was tlerivecl. The relative
Potassium-
Bearing
Parent
Materials
II Soil-Forming Processes
I CVOP Clay, Minerals and Humos
Residues
Exchangeable and
Non·Exchangeable
Potassium (
Commercial
Crops I Fertilizers
  Available /
Potassium
I
Harvested V
Cro  Leaching
WIDTH OF ARROW INDICATES THE VALUE OF POTASSIUM MOVEMENT
Fig. `l.——Potassium Cycle in the Soil.
This schematic drawing shows the sources of soil potassium, potassium
fixation, and how the levels of available potassium are depleted and replenished.
There is little or no leaching ot potassium except in sandy soils.
5

 amounts of these fractions and the kind of clay minerals in a soil
deterrrrirre its ability to fix or release potassium.
Sand and silt fractions
The sand and silt fractions of most soils are rrrade up largely
of quartz, wlriclr is resistant to decomposition by weathering. The
other minerals in tlrese fractions may contain potassium and other
rrtrtrient elernerrts but, since the particle size is relatively large, the
rate of potassium release is low. Also, because of the physical and
chemical rrattrre of sand and silt, their ability to fix potassium is
low.
Clay minerals
()rr the other hand, the clay minerals (the dominant materials
in the clay or colloidal fraction) of a soil are relatively active in
fixing and releasing potassirrrrr.
(Lenerally there are forrr kinds of clay minerals in Kentucky
soils. l,isted in the order of their abundance, they are kaolinite,
soil mica or illite, vermicrrlite, and rrrorrtrrrorillorrite. No soil is
conrposed of only one of these and rrrore often a soil may contain _ ‘
as many as three or forrr. Lach clay mineral has its own character-
istics with respect to potassium fixation aml release. ln addition,
each clay mineral corrtairrs different arrrorrrrts of native potassium, `,
which is bonded between the clay layers.
Because of this crystal structure and the location of the negative
clrarges within the crystals, the illite and verrnieulite clays are
capable of absorbing potassirrrrr from the soil solution and entrap—
ping it between neighboring clay particles. These "fixed" potassium
cations can be errtrapped in this way because of the relationship
of their size of the "lrexagonal" cavities in the silica sheets of two
adjoining mica or vernriculite layers (see l·`ig. 2). Fixed potassium
is not as available to plants as is exchangeable potassium, btrt may
be gradually released as levels of exchangeable and soil solution
potassirrm become low.
.\ knowledge of the types of clay minerals in a soil is important
in interpreting soil test results and in planning potassium fertiliza-
tion programs. Response to potassium fertili/atiorr of crops grown
on soils that contain different kinds of clay minerals may go in
seeyral interesting directions. (Zrops grown on soils containing
predornirrantly kaolin-type minerals generally respond to potash
lertili/.rtiorr wlrerr a soil test indicates a need for additional
G

 potassium. Crops grown on soils with a high content of illite or
vermiculite clay minerals may show little or no response even
though a soil test indicates a need for potassium. Such behavior
is thought to be due to the release of enough potassium from the
crystal structure or from previously fixed potassium to meet the
requirements of the crop.
Few Kentucky soils, except the Purchase area and slack water
bottoms, contain appreciable quantities of montmorillonite clay.
The potassium reactions with this clay are complex, since potassium
may or may not be fixed, depending upon the pattern of soil
acidity to which the clay mineral has been exposed.
Since Kentucky soils contain mixtures of the various types of
clay minerals, plant behavior following a given treatment will
usually fall somewhat between the extremes mentioned above.
Soils derived from calcareous shales can release potassium that
usually is not reflected in a soil test. These soils contain some
illite and vermiculite in their line silt and clay fractions as well
as some weatherable )olassium-bearin felds Jars.
l c
Kentucky soils derived lrom limestone are medium in their
ability to release potassium. This suggests that they are composed
` {gis ·.•.j· ,_‘:;3u_j·‘·.:,;j·;· .·.‘·*.; :_ {-".·2f:·gT_134 .11 
' ‘»~"i' ' '* H'-? "~!'·TK?’·~' »  rlijyijlluhlkiyli TF-;IA1:-nfLT-·7x%iT:<'.F
  K 4 LZ] K K K
A _  .\.. . : · »  · ·   ¤_·(.M·. ·,·•··` .»·-.
   me    ' K C
· I K 1 ,:{·;j}[Cli}1’·.;I'Z}}';;jf   M9
Kcclinne CG  “{··'#Z‘1`·Y;=*·‘?‘i··¥;;Z€H-IJif Cu  
S 'l M' . .
Ol [CG Or Montmorrllomre
lllite ond
Vermiculite
~:‘.·T ' .
*-2:2;% Silico Loyer M Aluminum Loyer
Fig. 2.—-The Structure of Four Types of Clay Minerals Found in Kentucky Soils.
The potassium held by the lrillonite and vermiculite clay
minerals are relatively high (S0 — 120 med. 100 grame). The CEC
of humus is about 110 mec].`l00 grams. The above values are
for pure clay minerals or lmmus. \\`hen they are mixed with each
S

 other and with sand and silt, the CEC of the soil body will be
the sum of the CEC`s of the various fractions that comprise the
soil. Thus, if the humus content and the kind and amount of
clay minerals in a soil are known, a rather close approximation
of its CEC can be made. .
Cation exchange in soils is a reversible chemical reaction in
which one cation adsorbed on the surface of a soil colloid is re- .
placed by another cation. These cations are rather loosely held
on the edges of the clay minerals or humus particles or between
the layers of clay minerals. They occupy exchange sites because
they are balancing the negative charges of the clay minerals or
humus.
C0 K C0 K K K
cl; dgfliaa K +2 KG " cqflgaa :C°' + C°C'2
CG CG K Cu K
Fig. 3.-Cation Exchange.
Note that when potassium chloride is added as fertilizer, the potassium
replaces calcium on the clay colloid, allowing the calcium to combine with
the chloride in the soil solution. The process is reversible.
The importance of cation exchange capacity is that it prevents
, or reduces leaching of 1`a·t111m· components, such as potassium,
ammonium, magnesium and calcium. Cation exchange is a means
by which the soil can store potassium and other cations that may
be released later to plants.
AVAILABLE POTASSIUM
Terms commonly used by soil scientists to describe the different
categories of potassium in the soil are "non-exchangeable" or
"fixed," “exchangeable" or "available," and "readily available"
(see Fig.  
Like nitrogen and phosphorus, potassium must be in an avail-
able form to be used by plants. Even though the total potassium
content of most Kentucky soils is well above the amounts required
or removed by crops, many of these soils need potassium applica-
tions to obtain highest crop yields. This is because only a very
small amount of the total potassium is in the readily available
forms cfuring a cropping season.
Exchange reactions of the soil are such that a balance is main-
9

 l
l2lil]€(l between 1l1e 1l11‘ee (`ll[Cg<)l`iCS ol p<>1:1ssi11m (Fig. 4). As
gmwing crops l`€lll()\’C rezulily 1l\'&lll2ll)lC 1><>1:1ssi11111 l`1‘<>111 1l1e soil
s<>l111i<>11, CX(`ll1lIlgC2ll)lC ]><>1:1ssi11111 will Ill()\'C imo s1>l111io11. .-\lso
some ol 1lie 111m-ex1·l1:111ge:1l>lc or lixed po1;1ssi11111 will move into
:111 CX(`lHlIlgCill)lC li(>l`ll1. bl()\\'C\'Cl`, 1l1e rate :11 which l]OlT·CXCl12llTg€—
11l)l(? 1>r>1z1ssi11111 IIIOYCS to :111 cx(‘l1:1ngeal>le l-<)T`lll is slower 1l1:111 1l1e
1::110 :11 \\'lll(l1 Llie CX(`l1illlg€1ll)l€ l)()[1lSSllllll moves 10 1lic l`C2l(lll}'
;1v:1il:1l>lc l<>1‘111. 'l`l1is ]>1‘o1·css is reversilnle ill 1l1e1‘e is :111 excess ol
l`C1t(lll}' :1v:1il:1l>lc })()[2lSSllllIl over exclizuigezible ])()l.2lSSlllIll (see
Fig. tl).
Non·E>lc :1111l l`C1l1lll\’
.1x.1il.1l>l1· 1>i>1;issi11111 I)li(`\('lll \\'ll('Il ll1(* ‘»ilIlll)l(‘ is 11·s1<:t>1:1ssi11111 1l1:11 1*:111 Il11>\'('
i1111» 1l11· <‘\lllLlI\L{('Qtl)l(' llllilll 1>x:1·1` :1 lung |><·ri1¤1l. 'l`l1is (’Xl)l1tlll» wliy
s1¤1111· stills 111:13 l1:1x1· :1 l1>\\'l<‘sl l>111 301 s111»]>l1‘ 1·111»11gl1 ])(llLl\\llllll l1>1
l1‘l.1ll\<‘l\ liigli trol) iielils. l1 ;1ls11ill11s11;1I1‘sll11‘111*L*1l l<1l`1\l·§Il1l\\`l-
<·1lg1· til lll1‘ :111i111l:11` soil. l`(‘(`CIll Pfllitvllllll
ll)

 l
:1pplic:1tions, :1nd p:1st crop growth, as well :1s the soil test results,
when planning potassium lertilixer programs.
Available l)()[1lSSlll]I1 is only a part ol the tot:1l potassium in il
soil that is potentially available to il crop during a growing season.
:\\'illl2ll)l€ potassium includes the potassium in the soil solution
plus some ol the potassium that is adsorbed on tl1e soil colloids.
Some ol the adsorbed (exchangeable) potassium is obtained by A
pl:1nt roots il` they grow close to it. Plant roots :1lso possess negative
charges and may :1tll`:\t`[ potassium ions lrom the cl:1y mineral i
surfaces or edges.
Potassium in commercial lertilizers is gu:1ranteed to be soluble
in :1 standardized extracting solution. \\’hen applied to the soil,
it dissolves and enters the soil solution lor :1 short period and is
readily av:1ilable. However, in the presence of the soil colloids,
much ol it will revert to the exchangeable lorm :1nd some llllly
revert to the non-exchangeable or lixed lorm.
HOW TO MAKE POTASSIUM MORE AVAILABLE
Potassium fixation is not necess:1rily bad. It can work to the
l`armer`s advantage, since fixed potassium :1s well as that held in
the exchangeable lorm almost never leaches below the plow l:1yer
except perhaps in s:1ndy soils. More potassium can be made
:1vail:1ble by good soil management practices.
‘ Lime ucid soils
,»\s soil pl-I is adjusted to ne:1r neutral (pl·l ($.0-7.2) certain
pl:1nt nutrients. particularly phosphorus, :1re made move il\'illlill`)lC
:1nd the supplies ol c:1lcitun and magnesium :1re obviously increased.
A Since crops need il b:1l:1nce ol all the nutrients, they will use
potassium more elliciently when the soil pH is :1djt1sted so th:1t
the other nutrients are available in abundant supply.
(jalcium or lime :1dded to soils composed ol` dillerent clay
n1iner:1ls may react dillerently with respect to tl1e release ol
pot:1ssium. The addition ol calcium to 2l(`l(l soils with large :1mounts
ol kaolinite ZlH(l//l()l` montmorillonite clay minerals may increase
the availability ol potassium to plants, whereas adding c:1lcium to
vermiculite andlfor illite soils may have little or no eflect on the
availability ol potassium. Liming acid soils llllly increase root
growth and thus the :1n1ount ol available potassium that plants
can get.
11

 Avoid over-liming
\\’hen soils are over-limed (to near pH 8,0), a`high concentra-
tion ol calcium around the plant roots may reduce the amount
ol potassium cations in solution or in close proximity to the plant
roots. This may inhibit the uptake ol potassium. Furthermore,
there is a limit to the total amount of cations (calcium, potassium,
magnesium, sodium, ctc.) that a plant will absorb. Plants need
a proper balance ol these cations to make maximum growth. ll
there is an excessive amount ol calcium around the roots, the plant
may absorb a disprn-exchangeable or lixed lorm. Then
smaller applications ol potassium lertili/ers will be needed to
supply trop needs.
Return crop residues V
.s\ ltlt)-bushel corn crop (stover and cobs included) contains.
at maturity, about lllll pounds ol` elemental potassium. ()nly about
22 pounds ol` this will be in the grain. llence about 78 pounds · i,
ol potassium will be returned to the soil il the stoyer and cobs
are returned. Likewise, lll bushels ol wheat contain about l2.5
pounds ol potassium, whereas the straw alone contains about Ill)
pounds. 'lhese examples illustrate the importance ol returning -
trop residues to the soil.
\\`lien the whole crop is harvested. as in the case ol corn
silagc or hay. larger amounts ol potassium are removed. lake this
lxut into consideration when planning potassium lertili/ation pro-
grams.
.»\;;ricultural and Home Economies Extension Service of the Univtrsity of Kentucky. the
United States Department 0[ Aiiriculture eooperatinti. \\'. .»\, Seas". Director, Issued ii.
furtherance of the Acts of May 8 and June 30. 1914. mM#12)G8