Release pattern of non-exchangeable potassium reserves in Alfisols, Inceptisols and Entisols of West Bengal, India

Geoderma 207–208 (2013) 8–14

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Release pattern of non-exchangeable potassium reserves in Alfisols, Inceptisols and Entisols of West Bengal, India Gautam Kumar Sarkar a,⁎, Asoke Prasun Chattopadhyay b, Saroj Kumar Sanyal c a b c

Fertiliser Control Laboratory, Department of Agriculture, Government of West Bengal, Berhampore, Murshidabad, West Bengal, 742101, India Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal, 741235, India Bidhan Chandra Krishi Viswavidyalaya, Kalyani, Nadia, West Bengal, 741235, India

a r t i c l e

i n f o

Article history: Received 28 August 2012 Received in revised form 17 April 2013 Accepted 29 April 2013 Available online 2 June 2013 Keywords: Non-exchangeable potassium Step-K Constant rate K Threshold value of K release for non-exchangeable K Clay mineralogy

a b s t r a c t Eight surface soil samples representing three soil orders viz., Inceptisols, Alfisols and Entisols were analyzed to characterize the non-exchangeable potassium (K) reserves. The mineralogical composition of the experimental soils varied widely. The reserves of Step-K and Constant rate K were computed by repeated extraction of soils with boiling1 M HNO3. The cumulative release of non-exchangeable K by such repeated extraction followed a semi-logarithmic behavior with number of extractions, suggesting that the release of non-exchangeable K decreased with successive extractions. The threshold levels of K in soil solution below which the release of K from the initially non-exchangeable K reserves starts were also evaluated for the selected soils in terms of K activity ratio, K concentration and exchangeable K in 0.01 M and 0.002 M CaCl2 solution. Higher threshold value of Entisols and Inceptisols compared to Alfisols indicates less tenacity with which K is held in wedge zones of micaceous minerals. These threshold values changed considerably for all the soils as the electrolyte concentration decreased from 0.01 M to 0.002 M. Specifically held K, determined as exchangeable K below which the Gapon constant (KG) showed a sharp rise, varied from soil to soil in almost the same manner as noted for threshold K levels in these soils. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The importance of non-exchangeable potassium in both the K cycle and plant nutrition is widely recognized (Beckett, 1971; Goulding, 1981; Sparks and Huang, 1985) and it is the major source of K for the cropping systems being followed. Hence characterizations of soil reserve K and its release pattern is important to determine the K supplying capacity of soils. Non-exchangeable K+ ions in soil are bound coulombically to the negatively charged clay interlayer surface sites and this binding force exceeds the hydration forces between individual K+ ions, resulting in a partial collapse of the crystal structure (Kittrick, 1966). As a result, the K+ ions are physically trapped to varying degrees, making diffusion the rate-limiting step for K release (Martin and Sparks, 1983). Release of non-exchangeable K to the exchangeable form occurs when levels of exchangeable and soil solution K are decreased (Doll and Lucas, 1973) by crop removal and/or by leaching (Sparks et al., 1980), and perhaps by large increase in microbial activity. Non-exchangeable K is moderately to sparsely available to plants, depending on various soil parameters (Goulding and Talibudeen, 1979).

⁎ Corresponding author. Tel.: +91 8900137650; fax: +91 3482254753. E-mail address: [email protected] (G.K. Sarkar). 0016-7061/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2013.04.029

Beckett (1971) introduced the concept of ‘intermediate K’ and defined as it is a fraction of non-exchangeable K which is held around the edges and wedge zones of micaceous clay minerals. This intermediate K gets released when the K concentration in soil solution upon depletion approaches a certain critical low value, known as ‘threshold’ concentration (Scott and Smith, 1966). This threshold value is independent of the amount of K reserve, but depends rather on the clay structure and degree of expansion (Datta and Sastry, 1988). Further K fixation involves realignment of aluminosilicate layers of the expanded (2:1) mineral in the c-axis direction (Goulding, 1984). Therefore, it is likely that there will be a gap between release threshold level and fixation threshold level for soil K. The appropriate management practice has to evolve ways and means to maintain the exchangeable K level in soil somewhere at an optimum intermediate between these two threshold levels. The threshold K level may not be a fixed value for soils subjected to longterm, high intensity cultivation, without extraneous application of K. This is because of intensive release of K from interlayer positions and the fact that the extent of wedge zones may change the threshold value (Datta and Sastry, 1989). Higher threshold value indicates less tenacity with which K is held in wedge zones of micaceous minerals (Datta and Sastry, 1988). The present study was conducted on eight soils of West Bengal, India belonging to Inceptisols, Alfisols and Entisols orders, to examine the

G.K. Sarkar et al. / Geoderma 207–208 (2013) 8–14

reserves of non-exchangeable K and threshold levels of such K release, and also the specifically held K, by determining the Gapon constant (KG) for K-(Ca + Mg) exchange in the experimental soils.

9

The activity coefficient (AR K) values were computed from the measured concentrations of Ca 2+, Mg 2+ and K + using the formula, K

AR ¼

aK CK fK ¼ x √aðCaþMgÞ √CðCaþMgÞ √f ðCaþMgÞ

2. Materials and methods The eight surface soil samples from 0 to 0.15 m depth were collected from different agroclimatic regions of the state of West Bengal. The soils of Ranaghat (Aeric Endoaquepts) and Polba (Aeric Endoaqualfs) are from Gangetic alluvial plain (districts Nadia and Hooghly), Ranibundh soil (Typic Haplustalfs) and Mohammad (Md.) Bazar soil (Typic Haplustalfs) are from the lateritic plateau regions of western part of West Bengal (districts Bankura and Birbhum), Sonakhali soil (Typic Endoaquepts) and Gosaba soil (Typic Endoaquepts) are from the Sunderban deltaic region (district South 24 Parganas) and the soils of Garubathan (Typic Udorthents) and Hatighisa (Fluventic Dystrochrepts) are from the foothills of the Himalayas (districts Darjeeling and Jalpaiguri). Each soil sample was gently broken up by hand, being careful not to unnecessarily tear the plant roots, then air-dried at 35 °C to constant weight. Coarse litter and plant material were removed, being careful to minimize soil losses in the process. The soil sample was then passed through a 2-mm sieve to produce the fine earth (b 2 mm). Important physicochemical properties of these soils were determined using standard procedures (Jackson, 1967). The soil clay was fractionated by the method described by Jackson (1967). Soil pH and electrical conductivity (EC) were determined using 1:2.5 (w/v) soil to water suspension (Jackson, 1967). Organic carbon (Organic C) was estimated by the wet oxidation method of Walkley and Black, as outlined by Jackson (1967). Determination of sizefractions, namely the distribution of the sand, silt and clay fractions in soils, was carried out following the hydrometer method (Bouyoucos, 1962). The textural class of the soils was determined with the help of the triangular textural diagram (Brady, 1990). The CEC of the soils was determined using the method given by Schollenberger and Simon (1945). The Ca-saturated and K-saturated homoionic clays were used for the XRD analysis in a Philips Holland X-ray Diffractometer using Ni-filter, Cu-Kα radiation obtained at 40 kV generator tension and 20 mA generator current with a scanning speed of 1° (one degree) 2θ per minute, and a time constant of 4. The methodology proposed by Gjems (1967) was adopted for the semi-quantitative estimation of the clay minerals present in the experimental soils. Water soluble K was determined by the method adopted by Grewal and Kanwar (1966) by using the soil:water ratio of 1:5. Exchangeable K was determined by neutral normal NH4OAc extractant in the ratio of 1:10 (soil:extractant) as outlined by Pratt (1965). Non-exchangeable K (NEK) reserve in the experimental soils was determined by one time extraction of soils with boiling 1 M nitric acid (HNO3) as outlined by Pratt (1965). Cumulative extraction of non-exchangeable K was done by following a seven-times extraction schedules of soils, using boiling 1 M HNO3 in the ratio of 1:10 (soil:HNO3) for periods of 10 min for each extraction, according to the method of Haylock (1956), as modified by MacLean (1961). This method involves removal of exchangeable K by soaking the soil overnight with 0.1 M HNO3 before the first extraction of the soil with boiling 1 M HNO3, and then successive extraction of the soil with boiling 1 M HNO3 to a stage where the release of K from soil continues at a more or less constant rate [this amount was described as Constant rate K (CR-K), by Haylock, 1956]. By subtracting the amount of CR-K from the amount of K released in each step of successive extraction and then summing up, the amount of relatively easily extractable (and hence available) form of nonexchangeable K can be computed. This latter form is known as Step-K (Haylock, 1956). Threshold K levels for the release of intermediate K were obtained by following the method adopted by Datta and Sastry (1988), which is based on equilibrium of soil with 0.01 M and 0.002 M CaCl2 solutions for different soil:solution ratios.

where ‘C’ is the molar concentration and ‘f’ is the activity coefficient of the respective ions. The activity coefficients in the equilibrium solution were obtained using the Davies equation (Amacher, 1984) namely, log f i ¼ −

0:502Z2 √I
þ 0:2 I 1 þ √I

where ‘Z’ is the valency of an ion and ‘I’ is the ionic strength of the soil solution. The ionic strength (I) was calculated from the electrolyte conductivity (ECe) according to a relation proposed by Griffin and Jurinak (1973), namely I ¼ 0:0127 ECe By plotting KT (i.e., total K = solution K + exchangeable K) against (a) activity ratio, (b) exchangeable K, and (c) K concentrations, curves were drawn for all the soil:solution ratios. The Gapon constant (KG) for [K-(Ca + Mg)] exchange was also obtained, corresponding to each equilibrium soil:solution ratio, by using the relation, KG ¼

√aðCaþMgÞ ½Ex K x ½ExðCa þ MgÞ aK

The observed values of KG were plotted against the exchangeable K at equilibrium in order to ascertain the specifically held K in the experimental soils corresponding to different levels of exchangeable K in soils. 3. Results and discussion The important physicochemical properties of the experimental soils are presented in Table 1. The collected soil samples are of three orders namely, Alfisols (Polba, Ranibundh and Md. Bazar soil), Entisols (Garubathan soil) and Inceptisols (Ranaghat, Sonakhali, Gosaba and Hatighisa soil). The soils of Ranaghat, Polba, Sonakhali and Gosaba have aquic moisture regimes. The soil samples were collected from different agroclimatic regions of West Bengal as reflected by their variation of color. The results show that the pH of the soils ranged from as low as 4.37 of Gosaba to as high as 7.14 of Ranaghat. The soils were thus highly acidic to neutral in reaction. The acidity in Sonakhali and Gosaba soils is due to presence of acid forming parent material below 1 m depth from the soil surface, but in Garubathan and Hatighisa soils is due to slow decomposition of organic matter. The Sonakhali and Gosaba soils were high in salt content as shown by their electrical conductivity, but acidic in reaction as shown by their pH, while the Hatighisa soil exhibited the lowest electrical conductivity. The texture of the soils is from sandy loam (less clay i.e., low nutrient holding capacity) to clay loam (more clay i.e., medium nutrient holding capacity). The soil samples from Garubathan and Hatighisa exhibited high organic carbon content. The soil samples from Sonakhali and Gosaba exhibited high clay content and hence CEC and also exchangeable cations content relative to other soils. Among the soil samples, Sonakhali and Gosaba showed high base saturation percentage in comparison with others. The clay fractions of Entisols (Garubathan soil) and Inceptisols (Ranaghat, Sonakhali, Gosaba and Hatighisa soil) were rich in the content of illite followed by kaolinite. Kaolinite is the dominant clay mineral in the soils of Polba and Md. Bazar (both soils are under the order of Alfisols) followed by illite but the soil of Ranibundh (Alfisols) contained illite as the dominant clay mineral followed by kaolinite (Table 2). The

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G.K. Sarkar et al. / Geoderma 207–208 (2013) 8–14

Table 1 Important physicochemical properties of soils. Location of soils with order

Taxonomic Classification of soils

Ranaghat (Inceptisols)

Aeric Endoaquepts

Color

10YR 5/4 yellowish brown Polba (Alfisols) Aeric 10YR 5/6 Endoaqualfs yellowish brown Ranibundh Typic 7.5YR 5/6 (Alfisols) Haplustalfs strong brown Md. Bazar Typic 10YR 6/6 (Alfisols) Haplustalfs brownish yellow Sonakhali Typic 10YR 6/3 (Inceptisols) Endoaquepts pale brown Gosaba Typic 10YR 6/3 (Inceptisols) Endoaquepts pale brown Garubathan Typic 10YR 6/3 (Entisols) Udorthents pale brown Hatighisa Fluventic 10YR 6/2 (Inceptisols) Dystrochrepts light brownish grey

pH

EC Sand Silt (dS m−1) (%) (%)

CEC [cmol Exchangeable cations Organic Water (p+) kg−1] [cmol (p+) kg−1] holding carbon (g kg−1) capacity (%) Ca+2 + Mg+2 Na+

Clay Texture (%)

7.14 0.112

28.1

24.7 47.2 Clay loam

6.55

48.0

20.8

6.38 0.167

36.3

28.3 35.4 Clay loam

6.70

45.2

12.3

4.75

0.730 0.160 45.9

6.10 0.112

36.8

28.0 25.2 Sandy clay loam

4.04

34.0

8.9

3.30

0.460 0.210 44.6

5.98 0.090

44.3

26.3 29.4 Sandy clay loam

5.83

34.9

8.3

2.30

0.580 0.120 35.9

5.34 5.49

8.8

35.4 55.8 Clay loam

13.5

59.3

19.9

8.05

3.12

1.33

4.37 1.76

14.8

30.0 53.2 Clay loam

12.6

57.9

17.0

5.80

2.74

0.961 55.9

5.48 0.044

71.8

14.4 13.8 Sandy loam

18.1

58.4

7.6

1.75

0.240 0.140 27.8

5.46 0.035

62.8

25.0 12.2 Sandy loam

17.0

56.4

5.9

1.00

0.320 0.260 26.8

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). By repeated extraction with boiling 1 M HNO3 after removal of available K with 0.1 M HNO3 two categories of non-exchangeable K were distinguished, namely (1) Step-K which decreased with successive extraction to zero by the third or fourth extraction and (2) Constant rate K (CR-K) which occurred in similar amounts in each extraction for each soil (Fig. 1, Tables 3 and 4). The more the amount of ‘Step-K’ the more will be the plant utilizable non-exchangeable K. The latter may be plant available under K stress situation (De et al., 1993). Sonakhali soil showed much higher amounts of non-exchangeable K and Step-K, followed by Gosaba soil and Hatighisa soil, whereas Md. Bazar contained the least amount among the eight experimental soil samples (Table 3). The non-exchangeable K (NEK) or reserve K being an index of K availability under stress situation, the soils from Sonakhali, Gosaba and Hatighisa were expected to release more K under stress condition effectively (Table 3). Similarly the soils from Garubathan, Ranibundh and Ranaghat were expected to release K under long-term cropping effectively (Table 3). The NEK of the soils from Md. Bazar and Polba reveals that these soils might not support K nutrition to crops, without fertilization, under long-term cropping. The study of clay mineralogy

11.3

K+

Base saturation (/%)

0.790 0.290 59.3

62.8

of these soils (Table 2) revealed that among the soils, Md. Bazar and Polba soils contained lesser amounts of mica, and possibly this was the reason of lower non-exchangeable K in these two soils as compared to the remaining soils. Like non-exchangeable K, the Step-K content was directly correlated to the dominance of micaceous minerals in the clay fraction of the experimental soils. The ratio of Step-K to non-exchangeable K may better reflect the mobilizable NEK reserves in different soils (Subba Rao et al., 1993). Table 3 shows that all the soils except Md. Bazar had more mobilizable NEK. The contents of CR-K, which represent the interlayer K (Quéméner, 1979) were subjected to less variations than was Step-K among the selected soils except the soils of Md. Bazar and Polba. Simple linear equations for describing best the cumulative NEK release in all the soils studied are presented in Table 4. The release of cumulative NEK followed a semi-logarithmic behavior with increasing number of extractions with a highly significant correlation coefficient. This suggests that the release of NEK decreased with increasing number of extractions (Grewal et al., 1998). In particular, Polba and Md. Bazar soils followed almost the same pattern of K release which may be reflection of the predominance of kaolinite in the clay fraction (Table 2) of these Alfisols studied (Debnath and Sanyal, 1996). It may be mentioned here that non-exchangeable K also can be found in wedge zones of weathered micas and vermiculites (Rich, 1964). Only ions with a size similar to K+, such as NH4+ and

Table 2 Semi-quantitative (percent) abundance of clay minerals in clay fraction of soils. Location of soils

Kaolinite

Mica/illite

Smectite

Vermiculite

Chlorite

Mixed-layer

Feldspars

Quartz

Ranaghat Polba Ranibundh Md. Bazar Sonakhali Gosaba Garubathan Hatighisa Sd

15 36 28 40 15 17 17 18 10.05

30 23 38 24 44 40 35 37 7.55

15 – – 15 7 12 3 12 4.76

13 – – – 7 6 5 – 3.59

11 4 2 – 14 13 17 9 5.41

8 30 25 14 8 6 16 16 8.50

5 4 4 5 3 4 5 5 0.744

3 3 3 2 2 2 2 3 0.535

G.K. Sarkar et al. / Geoderma 207–208 (2013) 8–14

11

Constant rate K (CR-K)

Step potassium (Step-K) 1.5

6 5

Ranaghat 1

4

Polba

3 0.5

2 1

0

0 1

2

3

4

5

6

2

3

4

5

6

7

6

7

6

7

0.5

8

Potassium extracted /cmol(p+)kg-1

1

7

0.4

6

Ranibundh

Md. Bazar

0.3

4

0.2

2

0.1 0

0 1

2

3

4

5

6

7

1

10

2

3

4

5

10

Sonakhali

8

8

6

6

4

4

2

2

Gosaba

0

0 1

2

3

4

5

6

1

7

7

Garubathan

4 3 2 1 0 1

2

3

4

5

3

8 7 6 5 4 3 2 1 0

6 5

2

6

7

4

5

Hatighisa

1

2

3

4

5

6

7

Extraction numbers Fig. 1. Successive extraction of K from eight soils by boiling M HNO3 (after removal of exchangeable K).

H3O+, can exchange K from wedge zones. Large hydrated cations, such as Ca2+ and Mg2+, cannot fit into the wedge zones. Release of nonexchangeable 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, 2000).

Simple correlation coefficients of Step-K and CR-K with different forms of K and other soils parameters are shown in Table 5. The results (as presented in Table 5) indicate that both the Step-K and CR-K were significantly and positively correlated with NEK as observed earlier by some other workers as well (Das et al., 1997; Debnath, 1995; Subba Rao et al., 1993) indicating significant contribution of NEK to Step-K.

Table 3 Different forms of potassium in soils. Location of soils

Ranaghat Polba Ranibundh Md. Bazar Sonakhali Gosaba Garubathan Hatighisa

Forms of potassium [cmol (p+) kg−1]

Step-K: NEK

Water soluble K

Exchangeable K

Non-exchangeable K (NEK)

Total K

Step-K

Constant rate K (CR-K)

0.029 0.015 0.051 0.010 0.564 0.218 0.025 0.030

0.289 0.164 0.205 0.123 1.33 0.961 0.143 0.264

6.57 1.44 6.76 0.540 10.5 10.1 7.76 8.59

60.3 48.7 33.1 26.4 71.8 70.5 48.7 73.3

6.79 2.11 7.76 0.520 11.02 10.27 8.03 9.98

0.40 0.14 0.32 0.06 0.36 0.28 0.31 0.34

1.033 1.465 1.147 0.963 1.048 1.015 1.034 1.215

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G.K. Sarkar et al. / Geoderma 207–208 (2013) 8–14

Table 4 Release pattern of K by successive extraction of soils with boiling IM HNO3 and the best-fit equations for cumulative release of non-exchangeable K with number of extractions. Location of soils

K extracted [cmol (p+) kg−1] extraction numbers I

II

III

IV

V

VI

VII

Ranaghat

5.53

1.51

0.78

0.57

0.40

0.40

0.40

Polba

1.30

0.80

0.38

0.19

0.14

0.14

0.14

Ranibundh

5.89

1.98

0.60

0.57

0.32

0.32

0.32

Md. Bazar

0.44

0.16

0.10

0.06

0.06

0.06

0.06

Sonakhali

9.05

2.00

0.81

0.60

0.36

0.36

0.36

Gosaba

8.23

1.88

0.69

0.59

0.28

0.28

0.28

Garubathan

5.79

2.16

0.75

0.57

0.31

0.31

0.31

Hatighisa

6.76

1.80

1.64

1.14

0.34

0.34

0.34

Best-fit equation # Y=

r

5.56 + log x 1.40 + log x 6.14 + log x 0.43 + log x 9.27 + log x 8.47 + log x 6.11 + log x 6.75 + log x

4.70

0.991⁎⁎

2.06

0.993⁎⁎

4.67

0.991⁎⁎

0.58

0.997⁎⁎

5.16

0.995⁎⁎

4.55

0.991⁎⁎

5.04

0.989⁎⁎

6.92

0.993⁎⁎

# Y = cumulative release of non-exchangeable K. x = number of extractions. r = correlation coefficient. ⁎⁎ Denotes level of significance at 1%.

Table 5 Coefficients of simple correlation of Step-K, Constant rate K contents with various soil parameters and forms of K. Parameters

Step-K

Constant rate K

pH CEC Organic carbon Sand Silt Clay Water soluble K Exchangeable K Non-exchangeable K Constant rate K

−0.539 0.273 0.622 −0.199 0.152 0.172 0.566 0.633 0.993⁎⁎ 0.846⁎⁎

−0.023 0.386 0.381 −0.126 −0.022 0.136 0.334 0.359 0.836⁎⁎

⁎⁎ Denotes level of significance at 1%.

The Step-K was also found to be positively related with the silt and clay fraction of the given soils, while the CR-K was negatively related with the silt fractions and positively with the clay fraction. The relationship of total K extracted with activity ratio (AR K), K concentration and exchangeable K at two different concentrations of background electrolyte (CaCl2) was determined in this investigation (Table 6) and for Ranaghat soil (Aeric Endoaquepts) it is presented in Figs. 2 and 3 as representative curves. The curves for Ranaghat soil (Figs. 2 and 3) showed that below a certain level of K, called the

threshold level, there was a rapid rise in total K extracted, indicating the release of intermediate K from the wedge zones. The behavior of other soils was similar i.e., below a certain level of K, a rapid rise in total K extracted. The results (as presented in Table 6) showed the threshold K levels for release of intermediate potassium of all the experimental soils. Higher threshold value of Entisols and Inceptisols as compared to Alfisols indicates less tenacity with which K is held in wedge zones of micaceous minerals. The threshold values in terms of activity ratio, exchangeable K and K concentration changed for all the soils as the electrolyte (CaCl2) concentration decreased from 0.01 M to 0.002 M, i.e., with the change in ionic strength of the medium (Table 6). These observations are at variance with the results obtained by Datta and Sastry (1988) and Adhikari and Ghosh (1993), but substantiate the results of Debnath and Sanyal (1996), where the possible reason of such a change was explained by postulating the release of intermediate K in the soils via two steps. The postulated steps were (i) release of K + ions within the negatively charged clay matrix related to electrolyte concentration, followed by (ii) diffusion of the released K+ ions to the exchangeable positions on the colloidal surface. The variations in the threshold values for the release of K + ions with ionic strength of the medium, as stated above, would suggest that the ionic reaction of the chemical step [i.e., step (i)], was the dominating one, controlling the aforesaid release of intermediate K (Debnath and Sanyal, 1996; Laidler, 1973; Sparks, 1986). The given soil samples were found to release intermediate K at a low activity ratio, except (to some extent) for Sonakhali soil and Gosaba soil which were rich in micaceous clay minerals (Table 2). This is in agreement with the observation of Rausell-Colom et al. (1965) viz. that the release of K from non-exchangeable sites takes place whenever the K activity in the solution phase becomes low relative to the activity of Ca and Mg. At such a low activity ratio, the specific sites in the wedge zones and edges of micaceous minerals tended to become more important in governing the equilibrium of soil K. The soils of Sonakhali and Gosaba contained higher amounts of water soluble, exchangeable and non-exchangeable potassium compared to other soils (Table 3), which explains the release of intermediate K at a comparatively higher activity ratio, K concentration in soil solution and exchangeable K, to maintain the equilibrium dynamics involving various forms of soil K. Threshold values in terms of activity ratio and K concentration varied widely for all the soils as the electrolyte concentration of CaCl2 decreased from 0.01 M to 0.002 M (Table 6). This is in agreement with the presence of micaceous minerals present in the soils, as in the vicinity of the threshold values, K concentration is governed mainly by specific sites corresponding to edge and wedge zones of micaceous minerals which are occupied by K ions only. Potassium ions get released from the micaceous minerals when hydrated cations expand the structure in a direction normal to the basal plane without breaking Al\O and Si\O bonds or they completely disrupt the structure. Mukhopadhyay (1995) suggested another school of thought that the degree of ionic hydration increases as the electrolyte concentration decreases, but the

Table 6 Threshold K levels for release of intermediate potassium in terms of activity ratio, K concentration and exchangeable K in 0.01 M CaCl2 (A) and 0.02 M CaCl2 (B) solution. Location of soils

Ranaghat Polba Ranibundh Md. Bazar Sonakhali Gosaba Garubathan Hatighisa

ARK × 103

K concentration × 104 (mol L−1)

Exchangeable K [cmol (p+) kg−1]

A

B

A

B

A

B

0.55 0.42 0.48 0.37 3.00 2.46 0.60 0.76

0.40 0.35 0.45 0.30 3.20 2.67 0.48 0.57

0.40 0.35 0.40 0.28 2.00 1.90 0.34 0.50

0.20 0.17 0.18 0.12 2.20 1.50 0.15 0.21

0.165 0.090 0.103 0.084 0.390 0.260 0.075 0.116

0.152 0.075 0.094 0.080 0.370 0.210 0.058 0.088

G.K. Sarkar et al. / Geoderma 207–208 (2013) 8–14

13

12 10

4 0.01M CaCl2

8 3

6

8

0.002M CaCl2

7 6 5

4

2

4

2

3 1

2

0 0

0.5

1

1.5

Activity ration x 12

12

10

10

8

8

6

6

4

4

2

2

0

0.2

0.4

0

0 0

0.1

0.2

0.3

0

0.1

0.2

0.3

Fig. 4. Estimation of threshold level for release of intermediate K and critical value\of KG for Ranaghat soil in equilibrated solution of 0.01 M CaCl2 and 0.002 M CaCl2 as a representative case.

0

0

1

2

103

0

0.5

1

1.5

Fig. 2. Relationship of total K extracted (KT) with activity ratio, exchangeable K and K concentration in equilibrated solution of 0.01 M CaCl2 for Ranaghat soil as a representative case.

Arrow indicates threshold value

4.5 4 3.5 3 2.5 2 1.5

number of hydrated cations available for expanding the clay structure decreases, and K + ions on the edges and wedge zones get released at lower K concentrations and activity ratios. The Gapon constant for K–Ca exchange (KG) gives the measure of relative tenacity of K with which it is held in planer sites and the wedge zones of clay with the latter leading to a relatively higher value of KG (De et al., 1993). A high value of KG would imply high amount of exchangeable K or less strongly bound K or high K release. Fig. 4 showed the relationship of KG with the exchangeable K at 0.01 M and 0.002 M CaCl2 as a representative case for Ranaghat soil. The curve for the soil ran almost in straight line, inclined to the X-axis, upto a certain value with decreasing value of exchangeable K content (shown by an arrow), the KG value rose sharply. This sudden rise in KG was attributed to the release of specifically held K from the wedge zones. The behavior of other soils was similar. Table 7 records such critical values for the present soils, as well as the amounts of exchangeable K which were specifically held in the wedge zones leading to such sharp rise in KG. This amount of exchangeable K was found to be of the same order of magnitude as that of the threshold values presented in Table 6. These findings led to the earlier convention that the K ions were more tightly held in the specific sites than in the planar sites.

1

4. Conclusions

0.5 0 0

0.2

0.4

0.6

0.8

1

1.2

4.5

4.5 4

4

3.5

3.5

3

3

2.5

2.5

2

2

1.5

1.5

1

1

0.5

0.5

0

0 0

0.1

0.2

0.3

The present investigation suggests that as the solution and exchangeable K in soils decrease, i.e., under K stress situation, the non-exchangeable K (NEK) reserves (especially the Step-K fraction of NEK) release K to the soil solution which may be utilized by the plants, especially on a long-term basis (e.g. for a cropping sequence). The results indicate that Alfisols rich in kaolinite (Polba and Md. Bazar Table 7 Gapon characteristics for release of intermediate K in soils. Location of soils

0

0.5

1

Fig. 3. Relationship of total K extracted (KT) with activity ratio, exchangeable K and K concentration in equilibrated solution of 0.002 M CaCl2 for Ranaghat soil as a representative case.

Ranaghat Polba Ranibundh Md. Bazar Sonakhali Gosaba Garubathan Hatighisa

0.01 M CaCl2 solution

0.002 M CaCl2 solution

Specifically held K Critical value Specifically held K Critical value [cmol (p+) kg−1] of KG [cmol (p+) kg−1] of KG 0.165 0.095 0.105 0.083 0.380 0.240 0.080 0.115

23.5 21.0 34.5 44.0 19.0 19.0 29.0 22.0

0.155 0.074 0.095 0.080 0.290 0.205 0.060 0.095

31.0 34.5 32.5 38.0 16.0 17.0 34.0 33.0

14

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soils) require frequent K fertilization under long-term cropping than the illite dominant soils of Entisols (Garubathan soil) and Inceptisols (Ranaghat, Sonakhali, Gosaba and Hatighisa soil) expected to release more K under long-term cropping effectively. Higher threshold value of Entisols and Inceptisols as compared to Alfisols indicates less tenacity with which K is held in wedge zones of micaceous minerals. Acknowledgments This investigation was supported by the Department of Agriculture, Government of West Bengal, West Bengal, India. References Adhikari, M., Ghosh, T.K., 1993. Threshold levels of potassium release in relation to clay mineralogy and specifically held potassium of soils. Journal of the Indian Society of Soil Science 41, 663–666. Amacher, M.C., 1984. Determination of ionic activities in soil solutions and suspension: principal limitations. Proceedings of the Soil Science Society of America 48, 519–524. Beckett, P.H.T., 1971. Potassium potential — a review. Potash review subject 5. Suite 30, 1–41. Bouyoucos, G.J., 1962. Hydrometer method improved for making particle size analyses of soils. Journal of Agronomy 54, 464–465. Brady, N.C., 1990. The Nature and Properties of Soils, Tenth edition. Macmillan Publishing Co., New York. Das, P.K., Acharya, N., Das, H.K., Sahu, G.C., 1997. Potassium release characteristics of some soils in the watershed areas of Phulbani district, Orissa. Journal of the Indian Society of Soil Science 45, 724–728. Datta, S.C., Sastry, T.G., 1988. Determination of threshold levels for potassium release in three soils. Journal of the Indian Society of Soil Science 36, 676–681. Datta, S.C., Sastry, T.G., 1989. K-threshold levels in relation to K-reserve, release and specificity. Journal of the Indian Society of Soil Science 37, 268–275. De, S., Mani, P.K., Sanyal, S.K., 1993. Release pattern of non-labile potassium in some Entisols, Inceptisols, Mollisol and Alfisols of West Bengal. Journal of the Indian Society of Soil Science 41, 658–662. Debnath, A., 1995. Some aspects of potassium relationship with reference to plant availability. (Ph. D. Thesis) B.C.K.V., West Bengal. Debnath, A., Sanyal, S.K., 1996. Release potential of non-labile potassium in some Entisols, Alfisols and Inceptisol. Journal of the Indian Society of Soil Science 44, 49–54. Doll, E.C., Lucas, R.E., 1973. Testing soils for potassium, calcium and magnesium. In: Walsh, L.M., Beaton, J.D. (Eds.), Soil Testing and Plant Analysis. Soil Science Society of America, Madison, Wisconsin, U.S.A. Gjems, O., 1967. Studies on clay minerals and clay mineral formations in soil profiles in Scandinavia. Meddelelser fra De Norsek Skogforsoksvesen 303. Goulding, K.W.T., 1981. Potassium retention and release in Rothamsted and Saxmundham soils. Journal of the Science of Food and Agriculture 32, 667–670. Goulding, K.W.T., 1984. Thermodynamics and potassium exchange in soils and clay minerals. Advances in Agronomy 36, 202–262.

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