Scientific registration n o : 2611 Symposium n o : 13B Presentation : Poster. PAL Yash (1), WONG Mike (2), GILKES Bob (1)

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1 Scientific registration n o : 2611 Symposium n o : 13B Presentation : Poster Forms of potassium and potassium adsorption behavior of south-west Australian soils Formes et modalités d adsorption du potassium dans les sols du sud-ouest de l Australie PAL Yash (1), WONG Mike (2), GILKES Bob (1) (1) Dept. of Soil Science and Plant Nutrition, The University of Western Australia, NEDLANDS, WA 6009 (2) CSIRO Land and Water, FLOREAT PARK, WA 6014 Introduction The distribution of soil K among water soluble, exchangeable and non-exchangeable forms, is related to many soil properties including surface area, mineralogy, surface charge density and degree of interlayering of clay minerals present (Mengel and Kirkby, 1980; Shaviv et al., 1985). Similarly the fate of K added as fertilizer differs between soils so that micaceous clay minerals with an interlayered structure, high surface area and high lattice charge density (e.g. illites, vermiculites) fix K to a greater extent than clays of low charge density (such as kaolinite). The capacity of soil to maintain a given K level in solution has been defined as the K buffer capacity (KBC) of a soil. It is calculated as the slope of the K adsorption/desorption isotherm at zero adsorption (Beckett, 1964; LeRoux and Sumner, 1968). It has been suggested that a high KBC value for a soil is indicative of constant availability of K over long period and that a low KBC indicates a need for frequent K fertilization (Koch et al., 1970; Sparks and Liebhardt, 1981). Carrol (1961) reported potassium deficiency in the medium to low rainfall zone of the wheat belt region of western Australia. Pastures in high rainfall zone of south western Australia are commonly K deficient and K application is necessary for maintaining the clover component in pastures (Cox, 1973). McArthur (1991) reported on the exchangeable and extractable K status of south-west Australian soils but these forms are unsatisfactory measures of nutrient availability since their concentration in the soil at any time is small in relation to long term losses by crop removal and leaching and give little indication of reserves of non-exchangeable but potentially available K. In particular there is little information on the total reserves and adsorption behavior of these mostly kaolinite dominated soils o south-western Australia. The ability to predict yield responses to K fertilizers from K soil tests is limited and requires further investigation. This study examines the K status and adsorption/desorption behavior of 227 samples and relates this behavior to soil properties. 1

2 Materials and methods Soil samples were collected from 41 profiles located in the 350 to 650 mm rainfall zone of south-western Australia. Samples were collected by digging pits of 1.50 X 0.75 m to 100 cm deep after rains during the months of August and September Bulk horizon samples were air dried and passed through a 2 mm sieve for use in this study. Soil ph was measured in H 2 O and 0.01M CaCl 2 using a 1:5 soil to solution ratio, after the soil suspension had been equilibrated at 25±1 o C for 1 h on an end-over-end shaker. The particle size distribution of the soils was determined by the pipette method, after treating with H 2 O 2 ( 30 %) to remove organic matter (Gee and Bauder, 1986). Organic carbon (OC) was determined with a high frequency induction furnace (LECO) and cation exchange capacity (CEC) by the method of Blackmore et al. (1987). Water soluble K was extracted by end-over-end shaking of 4 g soil with 20 ml deionised water for 1 h. Exchangeable K was determined in 0.01M (AgTH) + using the method of Blackmore et al. (1987). One molar HNO 3 extractable K was determined by boiling two gram of soil in 20 ml 1M HNO 3 at 113 C for 25 minutes and making the final volume to 100 ml (Pratt, 1965). Non-exchangeable K was obtained by the difference between 1M HNO 3 extractable K and exchangeable K. Plant available K was estimated by shaking 1 g soil with 100 ml 0.5M NaHCO 3 (ph 8.5) for 16 h at 25±1 o C (Rayment and Higginson, 1992). Total K was determined using X-ray fluorescence (Philips PW 1400/00); for XRF analysis ~10 g soil pellets were prepared using alvenol as a soil binder and applying a pressure of ~6 tonne. K sorption isotherms were obtained for each soil by equilibrating 2 g soil with 20 ml of millimolar CaCl 2 to which five levels of K, ranging from 0 to 4.1 mmol K L -1, were added. Two drops of toluene were added to inhibit microbial activity. Soils were equilibrated for a period of 1 h by shaking on a reciprocal shaker at a speed of 200 cycles min -1 at 25±1 o C. After centrifugation, the K concentration in the equilibration solution was measured by flame photometry. The amount of K sorbed by the soil was calculated by subtracting the equilibrium K concentration from the amount of K initially added. The K buffering power was interpolated from the K sorption isotherms by determining the slope of the lower linear portion of the curve. Freundlich isotherms were obtained by adding a value equal to the amount of K desorbed in a solution containing an initial K concentration of 0 mmol K L -1 to the amounts of K adsorbed/desorbed in solutions containing initial K concentrations of 0.5, 1.0, 2.0 and 4.1 mmol K L -1. The data obtained were log transformed. The value corresponding to log of zero was omitted so isotherms were constructed with four points. 2

3 Table 1. Median and range values of some properties for the 227 soil samples Properties Median Range ph (H 2 O) EC (ms m -1 ) OC (%) Clay (%) CEC cmol (+) Kg H 2 O-K cmol K Kg NaHCO 3 -K cmol K Kg Exchangeable-K cmol K Kg Nonexchangeable-K cmol K Kg HNO 3 -K cmol K Kg Total K cmol K kg Buffer power cmol kg -1 /mmol L Equilibrium-K mmol K L Released-K (CaCl 2 -K) mmol K L Selectivity-k K/(Ca+Mg) (litre mole) 1/ 'b' (Freundlich slope) 'a' (Freundlich intercept) The K selectivity coefficient based on the Gapon equation was determined by the method of Rich and Black (1964). The K selectivity coefficient k K/Ca+Mg was calculated as follows: k K/Ca +Mg = [K sorbed/(ca + Mg) sorbed] [K solution/(ca + Mg) 1/2 solution] Sorbed K concentration was expressed as cmol K kg -1 soil and K in solution as moles litre -1. Results and discussion Characteristics of the soil samples: Soils are predominantly duplex with sand, loamy sand or sandy loam surface horizons. The clay content ranged from 0.16 to 65 % (Table 1). Soil ph in H 2 O ranged from 3.9 to 9.7, except for subsoils of some soils from low rainfall sites all the soils were acidic or neutral in reaction. Organic carbon (OC) ranges from 0.02 to 7.7 %, low values are associated with all subsoils and top horizons of sandy soils. Plant available K (i.e. NaHCO 3 extractable K) ranged from 0 to 2.7 cmol K kg -1 soil with a median value of cmol K kg -1 soil. For 27 soil profiles, NaHCO 3 extractable K content decreased down the profile and for 14 profiles it increased down in the profile. On the basis of the suggested critical value of 40 mg K kg -1 soil (NaHCO 3 extractable K)(Edwards, 1996), 54 % of the samples are deficient in available K. Most of these samples are from subsurface horizons. Water soluble K: Amounts of K in soil solution ranged from 0 to 0.12 cmol K kg -1 soil (median, cmol K kg -1 soil). The proportion of total K as water soluble K is very low and ranges from 0.01 to 1.8 % of the total K and in general too small to supply crop needs. There are significant (p < 0.01) positive relationships between water soluble K and both exchangeable K and non-exchangeable K (Table 2). 3

4 Table 2 Correlation coefficients between different forms of K and soil properties for 227 samples. ph EC OC CS FS Silt Clay CEC H 2O-K Exch.-K Non- HNO 3-K exch.-k ph 1.00 EC OC CS FS Silt Clay CEC H2O-K Exch.-K Non-exch.-K HNO 3-K Total K* * correlation coefficients for 41 surface horizon samples only bold coefficients are significant at the p<0.01 level Exchangeable K: The soils contain little exchangeable K. Values ranged from 0.03 to 1.8 cmol K kg -1 soil (median, 0.12 cmol K kg -1 soil) in the surface horizons and from 0 to 2.8 cmol K kg -1 soil in subsurface horizons (median, 0.13 cmol K kg -1 soil). For soils of low to medium CEC, the exchangeable K occupancy supporting optimal K nutrition has been reported to be close to 3 to 4 % of the exchange sites (Schneider and Villemin, 1993). Except for siliceous sandy soils (SCP 11, MRA 5, MER 1 and KTG 4), all the soils have K occupancy well above this limit; 85 % of the surface and 57 % of subsurface soils have more than 3 % K occupancy of the exchange sites. Non-exchangeable K: The values of non-exchangeable K varied from 0 to 4.8 cmol K kg -1 soil (median, 0.18 cmol K kg -1 soil) in the surface horizons and 0 to 5.9 cmol K kg -1 soil (median, 0.13 cmol K kg -1 soil) in subsurface horizons. Non-exchangeable K has a significant positive relationships with ph, EC, silt and CEC of the soils (Table 2). These observations are consistent with those of Moss and Coultar (1964) and Singh et al. (1983). The positive relationship between non-exchangeable K and CEC (r=+0.79) indicates that some of K may be present in the lattice of clay minerals and which could not be exchanged with (AgTH) + but could be extracted with HNO 3, presumably by partial dissolution of clay minerals. Total K: Amounts of total K in 82 surface and subsurface samples varied from 0.51 to 70.8 cmol K kg -1 soil (median, 14.9 cmol K kg -1 soil). Total K has significant positive relationships with silt (r=+0.54), HNO 3 extractable K (r=+0.50), exchangeable K (r= +0.49) and non-exchangeable K (r=+0.48). X-ray diffraction analysis of powders of whole soil revealed that many soils contain significant amounts of K-containing feldspars, with orthoclase predominating in less weathered soils of the Kellerberrin area and microcline dominating in highly weathered soils of the Swan Coastal Plain. The mica to kaolinite peak intensity ratio for XRD patterns of clay fraction has no statistical relationship with any form of K in these soils but the feldspar to quartz peak intensity ratio for whole soil XRD patterns has a significant positive relationship with total K (r=+0.57). 4

5 K Sorption Isotherms The sorption isotherms were obtained by plotting the amounts of K sorbed as cmol K kg -1 soil against equilibrium solution concentration as mmol K L -1 in 1 mmol CaCl 2. The convex shape of these isotherms indicated a progressive reduction in the increment of K sorbed for each increment in concentration of K in solution. Soils differed widely in their K adsorption behavior. For the highest initial solution concentration of K (4.1 mmol K L -1 ), K adsorbed ranged from 5 to 67 percent of added K. The release of K from soils (not K treated) in millimolar CaCl 2 ranged from 0 to 0.57 cmol K kg -1 soil, the high values are associated with the soils that contain high amount of the fine fractions (silt and clay). Some light textured soils released all of their exchangeable K to the CaCl 2 solution. Equilibrium K concentration ranged from 0 to 1.40 mmol K L -1 soil with a median of 0.24 mmol K L -1. Equilibrium K concentration is linearly related to the initial K saturation of the exchange complex (r=+0.76). The capacity of soils to resist change in the K concentration in soil solution is called the buffer power (BP) and ranges from 0.02 to 7.0 cmol K kg -1 /mmol K L -1 with a median of 0.28 cmol K kg -1 /mmol K L -1. In general, higher buffering power values are for fine textured soils. High values are indicative of the availability of adequate K over a long period, whereas a low buffering power indicates the need for frequent fertilization (LeRoux and Sumner, 1968). Buffering power is linearly related to non-exchangeable K (r=+0.75), exchangeable-k (r=+0.76), HNO 3 -K (r=+0.76) and CEC (r=+0.83). When these relationships are examined for surface horizon soils only, values of the coefficient (r) increased (r>+0.87) and silt also had a significant positive relationship (r=+0.83) with K buffering capacity (BP). The equation selected by stepwise regression is: BP = (OC) (CEC) (NaHCO 3 -K) (r=0.87) Table 3. Relationships of Freundlich constants 'a' and 'b' with the most predictive soil properties. Index Equation r 'a' Y= (CEC) r = 0.82 Y= (Clay) r = 0.65 Y= (Exch.-K) r = 0.60 'b' Y= (Exch.-K) r = 0.71 Y= (CEC) r = 0.64 The sorption data were tested for their conformation to the Langmuir equation using the linear form C/X=1/bXm + C/Xm (where X is the amount of K adsorbed per unit soil, C is the K concentration in soil solution, b is a coefficient which reflect the relative rates of adsorption and desorption at equilibrium and is thus an affinity term, and Xm is the adsorption maximum) but did not conform adequately. The inability of the Langmuir equation to describe the data may indicate that there are several discrete types of K adsorption sites in these soils. However, all log transformed data conformed well to Freundlich adsorption isotherms (Fig. 1). Singh et al. (1987) also reported that K adsorption fitted well to the Freundlich equation but Langmuir plots provided an adequate description of data when they were split into two linear parts (i.e. the so-called 2-surface Langmuir plot). 5

6 Log X MRA 5 (35-60 cm) Log C y = 0.75 x R 2 = 0.99 KELL 6 (0-5 cm) Log X y = 0.97 x R 2 = 0.96 Log C Fig. 1. Freundlich adsorption isotherms for MRA 5 and KELL 6 soils The Freundlich constants a and b which represent intercept and slope of the log transformed adsorption isotherm may be taken as measures of the extent of adsorption and the energy of adsorption respectively. These constants are linearly related to some soil properties (Table 3). The Freundlich constant a increases with fineness of texture (% clay) of the samples which is a measure of surface area and the abundance of sorption sites. It thus has significant positive linear relationships with CEC, % silt and % clay and a negative relationship with % sand, all significant at the p< 0.01 level. The Freundlich constant b is an indicator of the energy of adsorption and is not closely related to most of the measured soil properties. This absence of clear relationships probably reflects the diversity of mineral species present which is the subject of ongoing research. Its values range from 0.20 to 1.62 with a median value of For 17 profiles where there is a trend of increasing b value with depth, values of silt, CEC, non-exchangeable K and exchangeable K are significantly related (r=+0.70, r=+0.77, r=+0.79, r=+0.84 respectively) to the b value. Conclusions From the results obtained it is clear that soils are poorly provided with the various forms of K. Most of light textured soils contain little non-exchangeable K. The near absence of micaceous minerals due to kaolinite dominance is the cause of a considerable proportion of total K being in soil solution and on exchange sites which might be lost to leaching. Soils have low adsorption capacity and K fertilization will be more effective if K applied in periodic small doses during the growing season to reduce leaching losses. Acknowledgments We gratefully acknowledge the financial support of the Grain Research & Development Corporation. References Beckett, P.H.T. (1964). Studies on soil potassium: 2 The immediate Q/I relations of labile potassium in the soil. J. Soil Sci. 15, Blackmore, L.C., Searle, P.L. and Daly, D.K. (1987) A method for chemical analysis of soils. N.Z. Soil Bur. Sci. Rep

7 Carrol, M.D. (1961) Potassium deficiency in the wheat belt. Journal of Agriculture, Western Australia 2, Cox, W.J. (1973) Potassium for pastures. Journal of Agriculture, Western Australia 14, Edwards, N.K. (1996) Potassium fertilizer improves wheat yield and grain quality on duplex soils. Proc. first workshop on potassium in Australian agriculture pp Gee, G.W. and Bauder, J.W. (1986). Particle size analysis. In Arnold Klute (ed) Methods of Soil Analysis. Am. Soc. Agron., Soil Sci. Soc. Am., Madison pp Koch, J.T., Orchard, E.R. and Sumner, M.E. (1970). Leaf composition and yield response of corn in relation to quantity-intensity parameters for potassium. Soil Sci. Soc. Am. Proc. 34, LeRoux, J. and Sumner, M.E. (1968). Labile potassium in soils: 1. Factors affecting the quantity intensity (Q/I) parameters. Soil Sci. 106, McArthur, W.M. (1991). Reference Soils of South-Western Australia. Aust. Soc. Soil Sci. (W.A. Branch), Dept. of Agri., West. Aust., Perth, pp Mengel, K and Kirkby, E.A. (1980). Potassium in crop production. Adv. Agron. 33, Moss, P. and Coultar, J.K. (1964) The potassium status of west Indian volcanic soils. J. Soil Sci. 15, Pratt, P.F. (1965) Potassium. In C.A. Black (ed.) Methods of Soil Analysis Part 2: Chemical and Microbiological Properties. Am. Soc. Agron., Madison, Wisc. pp Rayment, G.E. and Higginson, F.R. (1992). Australian Laboratory Handbook of Soil and Water Chemical Methods. Inkata Press. Melbourne, pp Rich, C.I. and Black, W.R. (1964) Potassium exchange as affected by cation size, ph and mineral structure. Soil Sci. 97, Schneider, A. and Villemin, P. (1993) Importance of texture and CEC in K fertilization advice. Potassium symposium, pp Intern. Potash Institute, Basel, Switzerland.. Shiviv, A., Mohsin, M., Pratt, P.F. and Mattigod, S.V. (1985). Potassium fixation characteristics of five southern California soils. Soil Sci. Soc. Am. J. 49, Singh, A.P., Mehta, S.C. and Mittal, S.B. (1987). Potassium adsorption in some potassium depleted soils. J. Indian Soc. Soil Sci. 35, Singh, B.P., Singh, M. and Shukla, U.C. (1983) Forms of potassium in some soils of different agroclimatic regions of eastern Haryana. J. Indian Soc. Soil Sci. 31, Sparks, D.L. and Liebhardt, W.C. (1981). Effect of long-term lime and potassium application on quantity-intensity (Q/I) relationships in sandy soil. Soil Sci. Soc. Am. J. 45, Key words: feldspar, illite, K forms, adsorption isotherms Mots clés : feldspath, illite, formes du otassium, isothermes d adsorption 7