Mouhamad et al / Chemistry International 2(1) (2016) 47-58

 

Behavior of Potassium in Soil: A mini review

 

Raghad Mouhamad1,*, Ameerah Atiyah1 and Munawar Iqbal2

1Department of Soil Fertility, Ministry of Science and Technology, Baghdad, Iraq

2National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar-25120, Pakistan

*Corresponding author’s E. mail: raghad1974@yahoo.com

ABSTRACT

 

Potassium (K) is vital to many plant processes and is available in different forms i.e., freely available K in soil is utilized by plant, whereas fixed K (minerals, zeolites, taranakinte, alunite, aluminosilicates) and structural K (feldspars, micas and minerals) is not available for plant uptake and these both forms are regarded as exchangeable and non exchangeable forms of K. Furthermore, the availability of K to plants also depends upon soil properties i.e., moisture, aeration, temperature, tillage system as well as K dynamics. Therefore, rate of K exchange vary among different soils and ultimately, the uptake of K affect the growth and yield in plants.

Keywords: Potassium, Exchangeable, Non exchangeable, Minerals, Plant uptake, Crop yield

 

Capsule Summary: The behavior of potassium (K) in soil and its interaction with plant is reviewed in this review article.

Cite This Article As: Raghad Mouhamad, Ameera Alsaede and Munawar Iqbal. 2016. Behavior of Potassium in Soil: A mini review. Chemistry International 2(1) 47-58


 

INTRODUCTION

 

The role of K in soils is prodigious. Of the many plant nutrient-soil mineral relationships, those involving K are of major if not prime significance. Since the middle of the 17th century, when J.R. Glauker in The Netherlands first proposed that saltpeter (KNO3) was the principle of vegetation, K has been recognized as being beneficial to plant growth. Glauker obtained large increases in plant growth from addition of saltpeter to the soil that was derived from the leaching of coral soils. The essentiality  of  K  to  plant  growth  has  been  known  since  the  work  of  von Liebig published in 1840 (Sparks, 2000).

Of the major nutrient elements, K is usually the most abundant in soils. Igneous rocks of the Earth’s crust have higher K contents than sedimentary rocks. Of the igneous rocks, granites and syenites contain 46  to  54,  basalts  7,  and  peridotites  2.0  g  K  kg–1.  Among the sedimentary rocks, clayey shales contain 30, whereas limestones have an average of only 6 g kg–1. Mineral soils generally range between 0.04 and 3% K. Total K contents in soils range between 3000 and 100,000 kg ha–1 in the upper 0.2 m of the soil profile. Of this total K content, 98% is bound in the mineral form, whereas 2% is in soil solution and exchangeable phases (Schroeder, 1979; Bertsch and Thomas, 1985).

4

Potassium, among mineral cations required by plants, is the largest in nonhydrated size (r = 0.133 nm) and the number of oxygen atoms surrounding it in mineral structures is high (8 or 12), which suggests that the strength of each K-O bond is relatively weak (Sparks and Huang, 1985). Potassium has a polarizability equal to 0.088 nm3, which is higher than for Ca2+, Li+, Mg2+, and Na+, but lower than for Ba2+, Cs+, NH+, and Rb+ ions (Rich, 1968, 1972; Sparks and Huang, 1985). Ions with higher polarizability are preferred in ion exchange reactions. Potassium has hydration energy of 142.5 kJ g–1 ion–1, which indicates little ability to cause soil swelling (Helfferich, 1962).

 

Forms of soil K

Soil K exists in four forms in soils; solution, exchangeable, fixed or non exchangeable, and structural or mineral (Fig. 1). Quantities of exchangeable, non exchangeable and total K in the surface layer (0-20 cm) of a variety of soils are shown in Table 1. Exchangeable K and non exchangeable K levels comprise a small portion of the total K. The bulk of total soil K is in the mineral fraction (Sparks and Huang, 1985). There are equilibrium and kinetic reactions between the four forms of soil K that affect the level of soil solution K at any particular time, and thus, the amount of readily available K for plants. The forms of soil K in the order of their availability to plants and microbes are solution > exchangeable > fixed (non exchangeable) > mineral (Sparks and Huang, 1985; Sparks, 1987; Sparks, 2000).

Fig. 1: Interrelationships of various forms of soil K (Sparks and Huang, 1985).

 

Fig. 2: Exchangeable K Dynamics in soil

 

Table 1: Potassium status of some selected soils

Origin of Soil a

Exchangeable K   Non exchangeable K              Total K

--------------------- (cmol kg–1) ---------------------

Source

 

Alfisols Nebraska, USA West Africa

 

 

0.40

0.46

 

 

--

--

 

 

--

3.07

 

Soil Taxonomy (1975) Juo

 

 

 

 

 (1981)

 

Inceptisols California, USA Maryland, USA

 

 

0.40

0.20

 

 

--

--

 

 

--

--

 

Soil Taxonomy (1975)

Soil Taxonomy (1975)

 

Mollisols Iowa,  USA Nebraska,  USA

 

 

0.27

0.40

 

 

--

--

 

 

--

--

 

Soil Taxonomy (1975)

 

Ultisols Delaware, USA Florida, USA Virginia, USA West Africa

 

 

0.33

0.14

0.11

0.24

 

 

0.49

0.25

0.17

--

 

 

22.5

2.71

6.5

8.06

Parker et al. (1989) Yuan et al. (1976) Sparks et al. (1980) Juo (1981)

aData is from surface soils (0-20 cm depth).

 

 

Table 2: Potassium status of Delaware soils and sand fractionsa

 

 

Horizon        Depth

Soils

Sand fractions

CaCl2

Extractable

HNO3

Extractable

Mineral

Kb

Total K

Total Kc

K

feldsparsd

cm      -------------------------- cmol kg–1   ---------------------------                    Frequency %

 

Kenansville loamy sand

Ap               0-23             0.25

Bt2            85-118           0.25

 

 

0.42

0.49

 

 

35.02

45.30

 

 

35.69

46.04

 

 

30.88

33.86

 

 

9.5

12.0

 

Rumford loamy sand

Ap               0-25             0.33

BC            89-109           0.21

 

 

0.49

0.54

 

 

21.67

23.39

 

 

22.51

23.96

 

 

18.62

16.76

 

 

6.7

8.2

 

Sassafras fine loamy sand

Ap               0-20             0.35

Cl              84-99            0.13

 

 

0.56

0.36

 

 

43.54

45.99

 

 

44.45

46.68

 

 

28.95

36.69

 

 

16.0

24.0

aFrom Sandusky et al. (1987). bMineral K =  [(total K) - (CaCl2  ext. K +  HNO3   ext. K)]. cThese data represent the amount of total K in the sand based on a whole soil basis.

Determined through petrographic analyses of the whole sand fractions and represents the percentage  of  the total point counts  in  a  given  sample  that  were  K  feldspars.  The remaining minerals in the sand fractions were quartz, plagioclase, and Opaques.

 

     Soil solution K is the form of K that is directly taken up by plants and microbes and also is the form most subject to leaching in soils. Levels of soil solution K are generally low, unless recent amendments of K have been made to the soil.  The  quantity  of  K  in  the  soil  solution  varies  from  2  to  5  mg K L–1 for normal agricultural soils of humid regions and is an order of magnitude higher in arid region soils (Haby et al., 1990). Levels of solution K are affected by the equilibrium and kinetic reactions that occur between the forms of soil K, the soil moisture content, and the concentrations of divalent cations in solution and on the exchange phase (Sparks and Huang, 1985; Sparks, 2000).

Fig. 3: Potassium adsorption versus time in pure systems (Sparks and   Jardine, 1984)

 

Exchangeable K

 

Exchangeable K is the portion of the soil K that is electrostatically bound as an outer-sphere complex to the surfaces of clay minerals and humic substances. It  is  readily  exchanged  with  other  cations  and  also  is  readily  available  to plants.

 

Non exchangeable K

 

Non exchangeable or fixed K differs from mineral K in that it is not bonded within the crystal structures of soil mineral particles. It is held between adjacent tetrahedral layers of dioctahedral and trioctahedral micas, vermiculites, and integrated clay minerals such as chloritized vermiculite. Potassium becomes fixed because the binding forces between K and the clay surfaces are greater than the hydration forces between individual K ions. This results in a partial collapse of the crystal structures and the K+   ions are physically trapped to varying degrees, making K release a slow, diffusion controlled process (Sparks, 1987). Nonexchangeable K also can be found in wedge zones of weathered micas and vermiculites. Only ions with a size similar to K, such as NH4 and H3O, can exchange K from wedge zones. Large hydrated cation,

4

such  as  Ca2+ and  Mg2+,  cannot  fit  into  the  wedge  zones.  Release of non exchangeable  K to  the   exchangeable   form occurs when levels of  exchangeable and soil solution K are  decreased by crop removal and/or leaching and perhaps by large increases in microbial activity (Sparks, 1980; Sparks, 2000).

     Non exchangeable K is moderated to sparingly available to plants Piper, 1950; Mengel, 1985; Sparks and Huang, 1985; Sparks, 1987). Mortland et al. (1956) showed that biotite could be altered to vermiculite by plant removal of K. Schroeder and  Dummler  (1966)  showed  that  the  non exchangeable  K  associated  with some German soil illites was an important source of K to crops. The ability of plants to take up non exchangeable K appears to be related to the plant species. Steffens and Mengel (1979) found that rye-grass (Lolium perenne) could take up non exchangeable K longer without yield reductions, while red clover (Trifolium pratense) could not. This was attributed to the rye-grass having a longer root length, which would allow it to grow at a relatively low K concentration. A similar concentration would result in a K deficiency in red clover. It may be that the difference in root mass, root length, and root morphology between monocots and dicots explains why monocots feed better from non exchangeable K than dicots (Mengel, 1985).

 

Mineral K

 

As noted earlier, most of the total K in soils is in the mineral form, mainly as K-bearing primary minerals such as Muscovite, biotite, and feldspars. For example, in some Delaware soils, Sadusky et al. (1987) found that mineral K comprised about 98% of the total K (Table 2). Most of the mineral K was present as K feldspars in the sand fractions.

 

Table 3: Potassium release from soils using a H-resin and oxalic acida,b

Horizon

Depth cm

H-Resin                   Oxalic Acid

---------------- cmol kg–1   ----------------

Kenansville loamy sand

Ap

Bt2

 

 

0-23

89-118

 

 

0.199

0.251

 

 

1.97 x 10–3

2.97 x 10–3

 

Rumford loamy sand

Ap

BC

 

 

0-25

89-109

 

 

0.172

0.231

 

 

1.41 x 10–3

1.97 x 10–3

 

Sassafras fine loamy sand

Ap

C1

 

 

0-20

84-99

 

 

0.235

0.246

 

 

5.64 x 10–4

1.69 x 10–3

aFrom Sadusky et al. (1987).

bThese values represent amounts of K released at 30 d.

 

 

Table 4: Potassium  released  after  30  d  from  Kenansville  soil  and  sand       fractions  using H-resina

Whole soil

Coarse sand

Medium sand

Fine sand

Horizon                K

mg kg–1

% of whole soil

K released mg kg–1

% of whole soil

K released mg kg–1

% of whole soil

K released mg kg–1

Ap                  77.5

43.0

53.6

10.0

65.3

20.5

71.6

Bt2                 98.0

24.1

76.2

11.1

69.6

26.7

99.7

aAdapted from Sadusky et al. (1987)

 

Potassium Uptake

 

Potassium uptake by plants is affected by several factors.

 

Soil moisture

 

Higher soil moisture usually means greater availability of K. Increasing soil moisture increases movement of K to plant roots and enhances availability. Research has generally shown more responses to K fertilization in dry years.

 

Soil aeration and oxygen level

 

Air is necessary for root respiration and K uptake. Root activity and subsequent K uptake decrease as soil moisture content increases to saturation. Levels of oxygen are very low in saturated soils.

 

Soil temperature

 

Root activity, plant functions, and physiological processes all increase as soil temperature increases. This increase in physiological activity leads to increased K uptake. Optimum soil temperature for uptake is 60-80°F. Potassium uptake is reduced at low soil temperatures.

Tillage system

 

Availability of soil K is reduced in no-till and ridge-till planting systems. The exact cause of this reduction is not known. Results of research point to restrictions on root growth combined with a restricted distribution of roots in the soil.

 

Solution-exchangeable K dynamics

 

The rate and direction of reactions between the solution and exchangeable forms of K determine whether applied K will be leached into lower horizons, taken up by plants, converted into unavailable forms, or released into available forms (Fig. 2).

The reaction rate between soil solution and exchangeable phases of K is strongly dependent on the type of clay minerals present (Sivasubramaniam and  Talibudeen,  1972;  Sparks  et  al.,  1980;  Sparks  and  Jardine,  1981,  1984; Jardine and Sparks, 1984) and the method employed to measure kinetics of K exchange (Sparks, 1989, 1995; Amacher, 1991; Sparks et al., 1996). Vermiculite, montmorillonite, kaolinite, and hydrous mica vary drastically in their ionic preferences, ion binding affinities, and types of ion exchange reactions. Such fundamental differences in these clay minerals account for the varying kinetics of K exchange. Kinetics of K exchange on kaolinite and montmorillonite are usually quite rapid (Malcolm and Kennedy, 1969; Sparks and Jardine, 1984). An illustration of this is shown in Fig. 3.  

In the case of kaolin clays, the tetrahedral layers of adjacent clay layers are held tightly by H bonds; thus, only planar external surface and edge sites are available for ionic exchange. With montmorillonite, the inner peripheral space is not held together by H bonds, but instead is able to swell with adequate hydration, and thus allow for rapid passage of ions into the interlayer space. Malcolm and Kennedy (1969) found that the rate of Ba exchange on kaolinite and montmorillonite was rapid with 75% of the total exchange occurring in three seconds.

Kinetics of K exchange on vermiculites and micaceous minerals tend to be extremely slow. Both are 2:1 phyllosilicates with peripheral spaces that impede many ion exchange reactions. Micaceous minerals typically have a more restrictive interlayer space than vermiculite since the area between layer silicates of the former is selective for certain types of cations (e.g., K, Cs+). Bolt et al. (1963) theorized the existence of three types of binding sites for K exchange on hydrous mica. The authors hypothesized that slow kinetics were due to internal exchange sites, rapid kinetics to external planar sites, and intermediate kinetics to edge sites.

 

Rates of K exchange in soils

 

A number of studies have appeared in the literature on the kinetics of K exchange in  soils  (see  references  in  Sparks  and  Huang,  1985;  Sparks,  1987;  Sparks, 2000).  Sparks  and  Jardine  (1981)  studied  K  adsorption/desorption   on  a Matapeake  soil  of  Delaware.  Potassium desorption was slower than adsorption. This was ascribed to the difficulty in describing K from partially collapsed interlayer sites of the vermiculites clay minerals. Jardine and Sparks (1984) described the kinetics of K adsorption/desorption on an Evesboro soil with two simultaneous first-order equations at 283 and 298K. These reactions were ascribed to sites of varying K reactivity. Reaction 1, the more rapid one that terminated in 10 to 12 minutes was ascribed to external surface sites of the organic and inorganic phases of soil that are readily accessible for exchange. Reaction 2, that continued for many hours, was attributed to interlayer sites of the 2:1 clay minerals that predominated in the <  2 mm clay fraction.

 

Potassium fixation

 

The phenomenon of K fixation or retention significantly affects K availability. The fact that fixation processes are limited to interlayer ions such as K has been explained in terms of best fit of K ions (the crystalline radius and coordination number are ideal) in an area created by holes and adjacent oxygen layers (Barshad, 1951). The important forces involved in interlayer reactions in clays are electrostatic attractions between the negatively charged layers and the positive interplay ions, and expansive forces due to ion hydration (Kittrick, 1966).

The degree of K fixation in clays and soils depends on the type of clay mineral and its charge density, the degree of interlayering, the moisture content, the concentration of K  ions as well as the concentration of competing cations, and the pH of the ambient solution bathing the clay or soil (Rich, 1968; Sparks and Huang, 1985). The major clay minerals responsible for K fixation are montmorillonite, vermiculite, and weathered micas. In acid soils, the principal clay mineral responsible for K fixation is dioctahedral vermiculite. Weathered micas fix K under moist as well as dry conditions, whereas same month-morillonites fix K only under dry conditions (Rich, 1968). The degree of K fixation is strongly influenced by the charge density on the layer silicate. Those with high charge density fix more K than those with low charge density (Walker, 1957). Weir (1965) noted that K fixation by montmorillonites is limited unless the charge density of the clays is high. Low charge montmorillonite (Wyoming) stays at 1.5 nm when K saturated unless it is heated (Laffer et al., 1966). Schwertmann (1962ab) noted that soil montmorillonites have a greater capacity to fix K than do many specimen montmorillonites. Soil montmorillonites  have  higher  charge  density  and  a  greater  probability  of having wedge positions near mica-like zones where the selectivity for K is high (Rich, 1968).

The importance of interplay hydroxy Al and hydroxy Fe3+ material on K fixation were first noted in the classic work of Rich and Obenshain (1955). They theorized that hydroxy Al and hydroxy Fe3+ interlayer groups acted as props to decrease K fixation. This theory was later corroborated in the work of  Rich  and  Black  (1964)  who  found  that  the  introduction  of  hydroxy Al groups into Libby vermiculite increased the Gapon selectivity coefficient (kG) from 5.7 to 11.1 x 10–2  L mol–1/2.

Wetting and drying and freezing and thawing can significantly affect K fixation (Hanway and Scott, 1957; McLean and Simon, 1958; Cook and Hutcheson, 1960). The degree of K fixation or release on wetting or drying is dependent on the type of colloid present and the level of K+ ions in the soil solution. Potassium fixation by 2:1 clay minerals may be strongly influenced by the kind of adsorbed cations or the anions within the system. In  studies  with  the  silicate  ion,  Mortland  and  Gieseking  (1951)  found that  montmorillonite  clays  dried  with  K2SiO3   were   altered  in  their swelling  properties  and  fixed  K  in  large  amounts.  Hydrous  mica, clays also  fixed  large  amounts  of  K  that  could  not  be  removed  with  boiling HNO3.

Volk (1934) observed a marked increase in K fixation in soils where the pH was raised to about 9 or 10 with Na2CO3. Martin et al. (1946) showed at pH values up to 2.5 there was no fixation; between pH 2.5 and 5.5, the amount of K fixation increased very rapidly. Above pH 5.5, fixation increased more slowly. These differences in K fixation in pH were discussed by Thomas and Hipp  (1968). At  pH  values  >  5.5, Al3+ cations  precipitate  as  hydroxy polycations,  which  increase  in  the  number  of  OH  groups  as  pH  increases until they have a form like gibbsite (Thomas, 1960). At this pH (~8), Al3+ does not neutralize the charge on the clay and cannot prevent K fixation. Below pH 5.5, Al3+ and Al(OH)x species dominate. Below pH 3.5, H3O+ predominates (Coleman and Harward, 1953; Thomas and Hipp, 1968).

Fig. 4: Release of non exchangeable K+ form sand silt (n) and clay fractions (Rahmatullah and Mengel, 2000)

 

The increase in K fixation between pH 5.5 and 7.0 can be ascribed to the decreased numbers of Al(OH)x  species which decrease K fixation (Rich and Obenshain, 1955; Rich, 1960, 1964; Rich and Black, 1964). At low pH, the lack of K fixation is probably due to large numbers of H3O+ and their ability to replace K as well (Rich, 1964; Rich and Black, 1964).

Potassium release from soil minerals

 

The release of K from micas proceeds by two processes:  (1) the transformation of K-bearing micas to expansible 2:1 layer silicates by exchanging the K with hydrated cations, and (2) the dissolution of the micas followed by the formation of weathering products. The relative importance of these two mechanisms depends on the stability of micas and the nature of soil environments (Montagne et al., 1982; Sparks and Huang, 1985; Sparks, 2000).

Release of K from feldspars appears to involve a rapid exchange with H, which creates a thin layer of hydrolyzed aluminosilicates. This residual layer ranges in thickness from several to a few tens of nm and seems to cause the initial  nonstoichiometric  release  of  alkali  and  alkaline  earths  relative  to  Si and Al. Following this step, there is continued dissolution, which removes hyperfine particles. After these are removed, further dissolution breaks down the outer surface of the residual layer at the same rate that alkalis are replaced by H at the interface between fresh mineral surfaces and the residual layer. This releases all constituents to the solution. The release is now stoichiometries. Thus, the weathering of feldspars appears to be a surface-controlled reaction (Sparks, 1989).

A number of physiochemical and mineralogical factors govern the release of K from micas by both cation exchange reactions and dissolution processes. These include tetrahedral rotation and cell dimensions, degree of tetrahedral tilting, hydroxyl orientation, chemical composition, particle size, structural imperfections, degree of K depletion, layer charge alterations and associated reactions, hydronium ions, biological activity, inorganic cations, wetting and drying, and other factors (Sparks and Huang, 1985). This review will focus on the latter four factors.

Biological activity promotes K release from micas (Mortland et al., 1956; Boyle et al., 1967; Weed et al., 1969; Sawhney and Voight, 1969). The organisms deplete the K in the soil solution, and their action may be analogous to that of tetraphenylboron (TPB) in artificial weathering of micas. Furthermore, the overall action of organisms is more complex when organic acids are produced (Boyle et al., 1967; Spyridakis et al., 1967; Sawhney and Voight, 1969; McLean and Watson, 1985).

The importance of organic acids in weathering of rock-forming minerals has been recognized for a long time (Sprengel, 1826; Bolton, 1882; Huang and Keller, 1970). All soils contain small but measurable amounts of biochemical compounds such as organic acids. Furthermore, since the time required for soil formation can extend over a period of centuries, the cumulative effect, in a soil of even very small quantities of chelating agents will be considerable. The influence of oxalic and citric acids on the dynamics of K release from micas and feldspars was studied by Song and Huang (1988). They found that the sequence of K release from K-bearing minerals by oxalic and citric acids is biotite >  microcline >  orthoclase >  muscovite.

The activity of K+ ions in the soil solution around mica particles greatly influences the release of K from micas by cation exchange. When the K level is  less  than  the  critical  value,  K  is  replaced  from  the  interlude  by  other cations from the solution. On the contrary, when the K level is greater than the critical value, the mica expansible 2:1 mineral takes K from the solution. The critical K level is highly mineral dependent, being much higher in the trioctahedral minerals (Scott and Smith, 1967; Newman, 1969; von Reichenbach, 1973; Henderson et al., 1976). The critical levels for muscovite are so low that even the K impurities in laboratory chemicals or dissolved from glassware are often sufficient to prevent any K release (Scott and Smith,

1967).

The nature and concentration of the replacing cations also influence the critical K level of the cations tested in Cl– solutions.  Rausell-Colom  et  al. (1965) found the critical K levels decreased in the order Ba2+   >  Mg2+  >  Ca2+ =  Sr2+   for  the  same  concentration  of  these  ions  and  with  a  constant  mica particle size. The activity of all of these replacing ions in the solution phase must be much greater than that of the K for significant K release to occur. The activity of various cations in the soil solution is governed by other minerals in the soil systems, by pedogenic processes, and by anthropogenic activities.

The release of K upon drying a soil is related to the clay fraction. When a soil is dried, the degree of rotation of weathered soil minerals, such as micas, may be changed. Thus, the K-O bond may be modified. Dehydration of interlayer cations may permit a redistribution of interlayer cations, because Ca could now compete with K for wedge sites. This seems to account for the release of K from soils upon drying. Rich (1972) found that Virginia soils, which contain hydroxy Al interlayers and appreciable amounts of K, did not release K upon drying. The presence of the hydroxy Al interlayers may block or retard the interlayer diffusion of K ions and may change the b dimension of micas, the degree of tetrahedral rotation, and the length and strength of the K-O bond.

Other factors that can affect K release from soils are leached, redox potential (Eh), and temperature. Leaching promotes the K release from K- bearing minerals by carrying away the reaction products. Therefore, leaching accelerates  the  transformation  of  minerals,  e.g.,  micas,  to  expansible  2:1 layer  silicates  and  other  weathering  products  if  the  chemistry  of  leaching water favors the reaction. Redox potential of soils could influence K release from micas since it has been pointed out that the tenacity with which K is held by biotite is greater after oxidation of its structural Fe. It appears that, other factors being equal, the extent of the K release from biotite should be less in soil environments that oxidize Fe than in soil environments that reduce it. Major elements in K-bearing feldspars do not exist in more than one valence state, thus the prevailing Eh of a soil may not be of direct concern to chemical weathering of the feldspars. However, the weatherability of feldspars can be affected by complexing with organic acids that are vulnerable to oxidation. Therefore, the stability of feldspars may be indirectly related to the prevailing properties of soil.

Increasing temperature has been shown to increase the rate of K release from biotite (Rausell-Colom et al., 1965; Sangakkara, 2000) and K feldspars (Rasmussen, 1972). Under conditions of leaching of biotite with 0.1 mol NaCl L–1, the rate of K release appears directly proportional to temperature in the range of 293 to 323  K  (Mortland,  1958).  Under similar leaching conditions, Mortland and Ellis (1959) observed that the log of the rate constant for K release from fixed K in vermiculite was directly proportional to the inverse of the absolute temperature. Preheating of micas to high temperatures (1273 K) prior to TPB extraction (Scott et al., 1973) was found to enhance the rate of K extraction from Muscovite, to decrease the rate for better, and to have little effect on phlogopite, except at

 

Table 5: Effect  of  K  applications  on  corn  grain  yields  at  four  sites  over  three  growing seasons. Yield was not significantly (p = 0.05) affected by treatment for any year- site combinationa

Annual K

Applicationb

kg K ha–1

 

Soil

 

Rumford

-------------------

Kenansville           Matapeake

---------- yield, Mg ha–1   ----------

Sassafras

-------------------

 

 

0

94

94S

282

282S

SEMc

 

 

13.1

13.4

14.0

13.5

13.8

0.3

1982

 

 

13.1

12.9

13.0

13.1

12.2

0.4

12.4

12.3

12.3

11.4

11.4

0.5

12.1

11.3

11.7

11.1

11.2

0.4

 

 

0

94

94S

282

282S SEM

 

 

8.9

9.4

8.5

11.5

11.5

0.9

1983

 

 

9.2

9.1

7.6

6.9

6.9

1.2

–

–

–

–

–

–

12.7

12.5

12.1

11.8

12.2

0.4

 

 

0

94

94S

282

282S

SEM

 

 

9.8

10.1

10.3

10.0

10.5

0.2

1984

 

 

–

–

–

–

–

–

–

–

–

–

–

–

8.7

9.2

9.8

9.4

9.3

0.4

aFrom Parker et al. (1989b).

bS indicates K was applied in three equal portions to give the total rate indicated.

cStandard error of the mean.

 

very high temperatures. The decrease in K release from biotite by preheating is presumably because of oxidation of Fe at high temperatures. The more rapid rate with muscovite following heating remains unexplained.

The importance of K release from soil feldspars cannot be ignored. In the past, researchers have thought that only small amounts of K from feldspars were released over a growing season to plants. However, a number of reports have appeared that show a remarkable lack of response by crops such as corn (Zea mays L.) to K applications on sandy soils (Hest, 1969 Liebhardt et al., 1976; Sparks et al., 1980; Woodruff and Parks, 1980; Olsen and Sommers, 1982; Munso, 1985; Parker et al., 1989b; Khondakar et al., 1983; Wang and Harrell, 2005) of the Atlantic Coastal Plain Region of the USA. The lack of crop response is anomalous, since these sandy soils have low levels of the solution, exchangeable and nonexchangeable K (Maclean and Brydon, 1963; Barber, 1984 Sparks, 1987; Parker et al., 1989a). However, they often contain large quantities of mineral K, particularly as feldspars.

Much of the total K in soils from Delaware is contained in the sand fractions. For example, Sadusky et al. (1987) found that 87 and 74% of the total soil K in the Ap and B2t horizons, respectively, of a Kenansville loamy sand was contained in the sand fractions. Much of this total K in the sand fractions could be directly attributable to the large quantities of K-feldspars. To assess the release rates of K from sandy soils and in particular, to determine nonexchangeable K release kinetics from feldspars, Sadusky et al. (1987) conducted a study on several soils from Delaware. A H-saturated resin and oxalic acid were used to affect K release. Over a 30 day period, large quantities of K were released from the soils (Table  3).  Initially, K release increased rapidly and then began to level off as equilibrium was approached. More K was released from the subsoil horizons than from the Ap horizons of each soil which was directly attributable to the high clay contents in the subsoil horizons that were high in vermiculites minerals.

Significant quantities of K were released from the course, medium, and fine sand fractions of the Kenansville soil to H-resin (Table 4) with the fine fraction releasing the most. This finding was important because the fine fraction comprised the bulk of the sand in the soils.  If one averages the quality of K release from the three sand fractions, multiplies by the percentage of sand in each soil horizon, and then divides this quantity by the total K released from the whole soil, some interesting data are obtained. In the Kenansville Ap  and  B2t  horizons,  68  and  63%,  respectively  of  the  total  K released came from the sand fractions. Thus, the importance of the sand fraction in these soils in supplying K is immense. Recent research by Rahmatullah and Mengel (2000) also clearly indicates the role that sand and silt fractions of soils play in K release (Fig. 5).

 

Leaching of K in soil

 

Soil solution K is either leached or sorbed by plants or soils. A number of factors influence the movement of K in soils, including the CEC, soil pH and liming, method and rate of K application, and K absorption by plants (Richards, 1965; MacKay et al., 1966; Terry and McCants, 1968; Sparks, 1980; Al-Kanani et al., 1984; Sparks, 2000; Damon and Rengel, 2007). The ability of a soil to retain applied K is very dependent on the CEC of the soil. Thus, the amount of clay and SOM in the soil strongly influences the degree of K leaching. Soils with a higher CEC have a greater ability to retain added K, whereas leaching of K is often a problem in sandy soils (Reitemeier, 1950; Weir, 1965; Luo and Jackson, 1985; Sparks and Huang, 1985; Tisdale, 1985).

Movement of applied K has been related to the method of application. Nolan  and  Pritchett  (1960)  compared  banded  and  broadcast  placement  of KCl applied at several rates to an Arredondo fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudult) in lysimeters under winter and summer crops. For the lowest rate of application, cumulative K removal for both placements was only about 5.0 kg ha–1.

A number of  investigations have been conducted to determine the relationship of crop uptake and rate of K application to leaching of K. Jackson and Thomas (1960; Ward and Johnston, 1960; Lutrick, 1963; Malavolta, 1985; Cakmak, 2005) applied up to 524 kg K ha–1 prior to planting sweet potatoes (Ipomoea batatas L.) on a Norfolk sandy loam (fine loamy, siliceous, thermic Typic Paleudult). At harvest time, soil and plant K exceeded applied K at the 131 and 262 kg K ha–1 rates. However, at the 524 kg K ha–1 rate, 38 kg K was unaccounted for by soil and plant K.

 

CONCLUSION

 

Potassium plays significant roles in enhancing crop quality. High levels of available K improve the physical quality and disease resistance in plants and crops since it actively take part in enzyme activation, stomatal activity, photosynthesis, sugars transport, starch synthesis, protein synthesis as well as transport of water and nutrient. The effects of K deficiency can cause reduced yield potential and quality of grain and fruits. Under normal conditions, the uptake of K by plant may affect because it depends upon the availability in soil. Since most of K is bounded with minerals and is not available to plants. Therefore, the application of K in the form of fertilizer is suggested that again depends upon soil properties because soil properties significantly affect the availability of K to plant and crops.   

 

 

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