FORMS OF SOIL POTASSIUM-A REVIEW

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1 Agri. Reviews, 35 (1) : 64-68, 2014 DOI /j AGRICULTURAL RESEARCH COMMUNICATION CENTRE FORMS OF SOIL POTASSIUM-A REVIEW M. Lalitha and M. Dhakshinamoorthy 1 National Bureau of Soil Survey and Land Use Planning, Bangalore , India Received: Accepted: ABSTRACT The knowledge about different forms and availability of potassium is must while studying the response of crops to K. Because Potassium supply to crop plants is a complex phenomenon involving relationships among various K fractions in soil. Potassium availability to plants in general is governed by different forms of K viz water soluble K, exchangeable K, fixed K and mineral K. Plants utilize not only the readily available K but also the non-exchangeable and mineral K during the crop growth. The potassium availability to plants is determined by the rate of change in the dynamic equilibrium between different forms of K in the soil which in turn is controlled by the mineral make up, rate of weathering and exchange properties of the soil. Key words: Exchangeable K, Forms of K, Non- exchangeable K, Total K and Water soluble K Potassium is the major nutrient and also a most abundant element in soils but the K content of the soil varies from place to place based on physicochemical properties of soil. Potassium exist in soil in different forms viz., water soluble, exchangeable, non-exchangeable (fixed), mineral K, lattice K and total K. But these forms are not homogeneously distributed in soils. Its amount in soil depends on the parent material, degree of weathering, K gains through manures and fertilizers and losses due to crop removal, erosion and leaching. Usually the amounts of non-exchangeable and total K present in the soil are high compared to water soluble and exchangeable K. The dynamics of potassium in soil depends on the magnitude of equilibrium among various forms and mainly governed by the physicochemical properties of soil. The bulk of soil potassium (about 98% of total K) usually exists in unavailable form in primary (micas and feldspars) and secondary (illite group) clay minerals. The available K and exchangeable K in general are readily available to plants. Different forms of Soil K: Potassium exists mainly in four farms viz., water soluble, exchangeable, nonexchangeable (fixed) and mineral K. These fractions exist in dynamics equilibrium among themselves and these forms in turn govern the K nutrition in crops. 1 Directorate of Research, Tamil Nadu Agricultural University, Coimbatore The nature of the equilibrium is variable and depends upon the soil type and nature of the clay minerals. The readily available K constitutes only 1 to 2 per cent of total K and exists in soil in two forms, viz., solution and exchangeable K adsorbed on soil colloidal surface (Brady and Weil, 2002). These forms remain in a dynamic equilibrium with one another. The readily available or water soluble K has been reported to be a dominant fraction in the initial stage while exchangeable and non-exchangeable K contribute more in the later stages of crop growth (Subehia et al. 2003). According to increasing order of plant availability, soil K exists in four forms i.e. mineral ( ppm), non-exchangeable ( ppm), exchangeable ( ppm), and solution (1-10 ppm). K cycling or transformations among the K forms in soils are dynamic. Soils that are rich in vermiculite and micas can have large amounts of non-exchangeable K, whereas soils containing kaolinite, quartz and other siliceous minerals contain less available and exchangeable K (Martin and Sparks, 1985). The distribution of K forms in the soil and the equilibrium between them determine the K status of the soil and the potential of K supply to plants (Rubio and Gil-Sotres, 1997; Pavlov, 2007, Srinivasarao et al., 2000). There are physical,

2 chemical, biological and climatic factors affecting K forms and equilibrium of K in soil, which could be related to clay mineralogy (Srinivasa et al., 2000; Barre et al., 2008), texture (Pal et al., 2001), moisture (Zeng and Brown, 2000), cation exchange capacity (Sharpley, 1990; Sardi and Csitari,1998), ph (Sahu and Gupta, 1987; Uribe and Cox, 1988), and concentrations of other ions (Zawartka et al., 1999) of the soil. Besides these soil properties, fertilization and cropping are the most important management factors that influence K equilibrium in soils (Singh et al., 2002; Simonson et al., 2007). Water soluble K: Potassium present in soil solution as soluble cation is termed as water soluble K which is readily absorbed by the plants and relatively unbound by cation exchange forces and invariably subject to leaching losses in relation to soil properties (Ramamoorthy and Velayutham, 1976). Appreciable quantities of potassium is likely to occur when applying water soluble K fertilizers and from irrigation water of high K content or soils contain high mixed soluble salts. The amount of water soluble K fraction in soil followed the order of Alluvial soil < Black soil < Red soil< Laterite soil (Ramanathan, 1978). In intensively cultivated soils of India, the water soluble K content is 0.2 per cent of the total K (Tandon and Sekhon, 1998) and it ranged from 4 to mg kg -1 in the soils of India. The water soluble K fraction is roughly 0.11 per cent of the total K in soils of Tamil Nadu (Ramanathan et al., 1981). The water soluble K is positively correlated with clay and silt and negatively correlated with sand (Basumatary and Bordoloi, 1992). Hence soils contain high amount of clay content has more the water soluble K. In addition Srinivasarao and Takkar, (1997) also stated that soils with larger amounts of clay showed greater amounts of water soluble and ammonium acetate extractable K in both the rhizosphere as well as nonrhizosphere. Water soluble K constituted 0.12 per cent of total K in surface soils indicating almost negligible contribution to the total potassium of soils. Generally surface soils had relatively high water soluble K than the subsurface soils. The possible reason for this could be an upward translocation of K by capillary rise (Anil Sharma et al., 2009) and also could be due to vegetation, release of labile K from organic Vol. 35, No. 1, residues and addition of farmyard manure (Ranganathan and Satyanarayana 1980). Exchangeable K: The exchangeable K is the form of K present in the soil matrix and can be replaced by cations of neutral salts present in the soil solution. Exchangeable K constitutes approximately 90 per cent of the available K. Usually the percentage of exchangeable K to total K is below 2 per cent (Schroeder, 1974). The exchangeable K was found to be closely correlated with clay content, ph, OC, CaCO 3 and CEC (Baruah et al., 1991). The highest exchangeable K was obtained in clay plus silt fraction of black soil followed by red, alluvial and laterite soils (Jayaraj and Mosi, 1973). In the same line Singh et al., 1985 also found that ammonium acetate K was positively correlated with per cent silt and clay and negatively with sand, indicating that the finer particles contained higher amount of potassium as compared with coarse fractions. But Srinivasarao et al., 2007 pointed that despite greater content of K-rich mica in Inceptisols and Aridisols, lower levels of exchangeable K were found to be due to lesser mobility of K from illite clay structure to the exchange complex because these minerals typically have a restrictive interlayer space which is selective for K ions, resulting in its low desorption (Sparks and Huang 1985; Sparks 1987). In slightly alkaline and neutral soils, the Ca 2+ ion is the dominating ion which promotes the opening of clay mineral packets and consequent release of lattice K. Besides the ph and clay content, the submergence of soil also affected this form. Murthy and Singh (1975) observed a reduction in exchangeable K during the first week of submergence. The exchangeable K content was higher in the surface soils than the subsurface soils. The higher concentration of exchangeable K in surface soils could be attributed to the addition of K through plant residues, manures and fertilizers (Anil Sharma et al., 2009). In addition exchangeable K could give a better indication of the potential K supplying power of a soil and used for making fertilizer recommendation to the crops (Sharpley, 1989). Non-exchangeable K: Non-exchangeable K is the forms of K barring the water soluble K and readily exchangeable form of K. This form of K is not readily available to plants. However it is in equilibrium with

3 66 AGRICULTURAL REVIEWS available forms and consequently acts as an important reservoir of slowly available K (Perkins, 1973). Generally, the K held at inter-lattice positions is non-exchangeable K and this form is not exchangeable by NH 4 OAc (Ramamoorthy and Velayutham, 1976). Inherent K status of a soil depends upon the rate and amount of nonexchangeable K. The content of non-exchangeable K to total K was 3.58 per cent in the soils of Tamil Nadu (Ramanathan et al., 1981). Ekambaram and Kothandaraman (1983) noticed the highest amount of non-exchangeable K in black soils followed by alluvial and red soils. Dhillon et al. (1985) revealed that the pattern of non-exchangeable K at different depths and it was higher in sub-surface soils compared to the surface soils. This might be due to release of fixed K to compensate the removal of water-soluble K and exchangeable K by plants. Further, Anil Sharma et al. (2009) opined higher values of non-exchangeable K fraction in sub-surface layers related to silt and clay content which could easily fix the potassium particularly in the soils rich in illitic clay minerals. In the soils of north-west India, fixed K is the principal source for supplying K to plants (Pasricha 2002). The per cent utilization of fixed K decreased as the level of added K increased to rice crop (Ramanathan, 1978 and Nagarajan, 1980). The low K concentration favours the net release of non-exchangeable K which is mainly interlayer K of clay mineral (Martin and Sparks, 1983). The contribution of non-exchangeable K to crops was relatively more in untreated plots than those receiving fertilizers K and there was close relati onship between K in crops and nonexchangeable K released from the soil (Ganeshamurthy and Biswas, 1985). Many studies explained the fact of substantial contribution of nonexchangeable K in plant K nutrition and soil K fertility management especially under continuous cropping in the absence of K inputs (Srinivasarao et al., 1999, 2001). The amount of fixed K in the soil depends on the particle-size distribution, types and quantities of clay minerals and removal of K from minerals. It is significantly correlated with silt and clay fraction of soil (Das et al., 1993). Fixed K level in soils, rich in K-bearing minerals, being critical for a steady supply of K to plants, sufficient fixed-k level and absence of K response, indicate that these soils supply K to plants and they would be degraded through rapid irreversible weathering of micas, illites and other K minerals. Mukhopadhyay and Datta (2001) warned of such a situation at different places in the Indo-Gangetic plain, and suggested K dressing to compensate K loss from soils. Lattice K: It is fraction of K that gets fixed in lattice space of the 2:1 clay minerals. This form of K is distinct from mineral K in that, it is not bonded covalently within the crystal structure of soil mineral particle but held between adjacent tetrahedral layers of dioctahedral and trioctahedral wedge zones of weathered micas and vermiculite (Sparks, 1987). The lattice K constitutes from to per cent of the total K in different soils. The large presence of lattice K could be because the soils are rich in K-bearing minerals (Mukhopadhyay and Datta, 2001). The lattice K is made available to plants by weathering and the amount released depends upon in the soil texture and environmental conditions (Grewal and Kanwar, 1973). The release and fixation of the lattice K is mainly governed by the type of clay minerals, soil reaction, type of cation etc. Mineral K: Major portion of soil K exists as part of mineral structure and in a fixed or non-exchangeable form. In general more than 90 per cent of the total K in the soils is found in mineral form as structural K (Pasricha, 2002). Anil Sharma et al. (2009) also observed the per cent contribution of mineral K to total K in soils was > 90%, thus indicating the dominance of this form over the other forms of K. They further revealed that the mineral K was found to be less in surface soils than the sub-surface soils. This may be because of the intense weathering of K minerals at the surface than the subsurface. Sharpley, 1989 found that mineral K was a function of clay content of soil (r 2 of 0.66 to 0.90). The important minerals which contain K are Illite (4 to 7%), orthoclase (9 to 15%), albite (upto 4%), muscovite (8 to 11%), biotite (6 to 9%), hornblende (1 to 3%) and montmorillonite (upto 0.5%) (Bear, 1958). Feldspar and mica are very resistant to weathering hence hold potassium very tightly. According to Sidhu and Dhillon (1985), muscovite, biotite, microcline and orthoclase were found to be the K bearing minerals present in sand fractions. Micas, orthoclase and microcline occurred in silt, while illite was found in clay fractions.

4 Total potassium: A large portion of the total potassium in soil occurs as structural component of soil minerals and is unavailable to plants. As per Sekhon 1999, the total K varies as illitic alluvial soils > smectitic Vertisols > vertic intergrades > kaolinitic red > laterite soils > kaolinitic acidic alluvial soils. Ahmed and Walia (1999) observed higher amounts of total K in sub-surface soils than the surface soils. Anil Sharma et al. (2009) also observed that the depth-wise distribution of total K was more in subsurface soils than the surface soils. The same time Srinivasarao et al., 2007, opined that total K was generally higher in Inceptisols followed by Aridisols, Vol. 35, No. 1, Vertisols, and Vertic sub-groups, while it was low in Alfisols and Oxisols. Whereas the total K was high in surface soils and it decreased with depth in all the Inceptisols and unlike Inceptisols, total K decreased with depth in most of the Vertisols. Ghosh and Mukhopadhyay (1999) claimed that the total K showed significant positive correlation with silt and clay fraction of soil indicating that substantial quantities of K bearing minerals are present in silt and clay fractions of the soils under investigation. Sharma et al. (2006) found that total potassium was high in clay soil which shows that among the various particle-size fractions, clay is a principal host of K in these soils. REFERENCE Ahmed, N. and Walia C.S. (1999). Profile distribution of various forms of potassium in some landforms of Bundelkhand region. J. Potassium Res., 15: 1-4. Aide, M. T, Cwick G. J. and M. F. Cumming. (1999). Clay mineralogy and potassium status of selected soils in the glacial Lake Agassiz region of central Manitoba. Can. J. Soil Sci, 79: Anil Sharma, Jalali V.K., Arya V.M. and Pradeep Rai. (2009). Distribution of Various Forms of Potassium in Soils Representing Intermediate Zone of Jammu Region. J. Indian Soc. Soil Sci., 57(2): Barre, P., Velde B., Fontaine C., Catel N. and Abbadie L.. (2008). Which 2 : 1 clay minerals are involved in the soil potassium reservoir? Insights from potassium addition or removal experiments on three temperate grassland soil clay assemblages, Geoderma. 146(1-2): Baruah, H.C., Bora D.K., Baruah T.C. and Nath A.K. (1991). Fixation of potassium in three major soil orders of Assam. J. Pot. Res., 7(3): Basumatary, A and Bordoloi R.K. (1992). Forms of K in some soils of Assam in relation to soil properties. J.Indian.Soc.Soil.Sci, 40; Bear, F.E. (1958). Chemistry of the Soil. Reinhold Publishing Corporation, New York. Brady, N.C and Weil R.R. (2002). The Nature and Properties of Soils, 13th Ed. Pearson,Delhi. Das, K. Sign S.K. and Shyampura R.L. (1993). Forms of K in relation to land form and soil properties of Basaltic terrian. J. Indian Soc. Soil Sci., 41: Dhillon, S. K., P. S. Sidhu, K. S. Dhillon and Y. P Sharma. (1985). Distribution of various potassium forms in some benchmark soils of North-West India. J. Pot. Res., 1: Ekambaram, S. and Kothandaraman G.V. (1983). Influence of potassium application on the progressive changes in the different forms of K in soils during crop growth. Indian Potash J., 8: Ganeshamurthy, A.N. and Biswas C.R. (1985). Contribution of potassium from non-exchangeable sources in soil to crops. J. Indian Soc. Soil Sci., 33: Ghosh, B.N and Mukhopadhyay A.K. (1999). Prediction of K supplying power of a soil from its releasing power. Int. J. Trop. Agric. 17(1-4): Grewal, J.S. and Kanwar J. S.. (1973). Potassium fixation in some soils of Punjab, Haryana and Himachal. J. Ind. Soc. Soil Sci. 15: Jayaraj, G. and Dhanapalan Mosi A. (1973). Studies on different forms of potassium in the mechanical fractions of typical soils of Tamil nadu. Madras Agric. J., 60: Karmarkar, S.V., Dudley L.M., Jurinak J.J. and James D.W. (1991). Chloride and perchlorate influence on calciumpotassium and magnesium-potassium exchange. Soil Sci. Soc. Am. J. 55: Martin, H.W and Sparks D.L. (1985). On the behavior of Nonexchangeable potassium in soils. Commun. Soil Sci. Plant Anal. 16: Martins, H.W and D.L. Sparks. (1983). Kinetics of non exchangeable K release from two coastal plain soils. Soil Sci. Soc. Am. J. 47: Mukhopadhyay, S.S and Datta S.C. (2001). Potassium availability in relation to soil mineralogy in the Indo-Gangetic Plains. In: Proceedings of Potassium in Indian agriculture. International Potash Institute. Gurgaon.pp Murthy, G.K. and Singh T.A. (1975). Potassium, phosphorous and calcium changes in submerged calcareous soils as affected by reduction condition II RISO. 25(1):21-29.

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