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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Plant-Available Potassium Assessment with a Modified Sodium Tetraphenylboron Method

^a CH2M HILL, 2525 Airpark Drive, Redding, CA 96001 USA

bjoern{at}purdue.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The conventional 1 M NH₄OAc-exchangeable potassium (K⁺) soil test is inadequate in soils where nonexchangeable K⁺ contributes significantly to crop nutrition. Studies were conducted (i) to compare the abilities of the 1 M NH₄OAc method with a modified NaBPh₄ method to estimate critical soil K⁺ levels, (ii) to estimate the contribution of nonexchangeable K⁺ to plant-available K⁺, (iii) to compare the abilities of the 1 M NH₄OAc method and the modified NaBPh₄ method to estimate plant dry matter yield and plant-available K⁺, and (iv) to compare the abilities of both methods to measure soil K⁺ balance. Winter wheat (Triticum aestivum L. `Abe') was grown in eleven Midwestern soils in a greenhouse using consecutive 28-d defoliation and regrowth cycles. Soils also were incubated for 6 mo with five K⁺ rates (0–809 mg K⁺ kg^-1). Ammonium acetate- and NaBPh₄-extractable K⁺ (5-min extraction period) were determined in soil samples taken after every three defoliation cycles and after incubation. Critical soil K⁺ levels could not be determined by either method alone but could be predicted by including cation-exchange capacity (CEC) and illitic K⁺ content in regression models. Nonexchangeable K⁺ represented a significant portion of plant-available K⁺. Plant-available K⁺ and dry matter (DM) yield were well related to NH₄OAc-extractable K⁺ only in soils with low nonexchangeable K contribution (r² = 0.889 and 0.915, respectively), but they were well related to NaBPh₄-extractable K⁺ in all soils (r² = 0.984 and 0.874, respectively). Slopes for NH₄OAc-extractable K⁺ vs. soil K⁺ balance varied widely among soils (0.16–0.68) depending on NH₄OAc-extractable K⁺, illitic K⁺, and clay content, but for NaBPh₄-extractable K⁺ slopes were near unity. These studies suggest that the modified NaBPh₄ method may be a superior K⁺ soil test compared to the NH₄OAc method. Illite content and CEC data may help in developing better soil K⁺ management guidelines.

Abbreviations: CEC, cation-exchange capacity

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
SOIL POTASSIUM (K⁺) exists in solution, exchangeable, and nonexchangeable forms that are in dynamic equilibrium with each other. Solution and exchangeable K⁺ are replenished by nonexchangeable K⁺ when they are depleted by plant removal or leaching. Some nonexchangeable K⁺ held in the interlayers of expandable 2:1–type clay minerals such as illite and vermiculite can be released relatively easily to provide a substantial portion of the K⁺ removed by crops during the growing season (Barber and Mathews, 1962; Richards et al., 1988; Mengel and Uhlenbecker, 1993; Rahmatullah et al., 1994). This interlayer K⁺ is also the major source controlling the long-term K⁺-supplying potential of soils. Therefore, a K⁺ soil test should measure a proportional amount of the nonexchangeable K⁺ that can become available during the growing season, or it must show the relationship between readily available levels and the potential for release of nonexchangeable K⁺ over a wide range of soils.

The neutral 1 M ammonium acetate (NH₄OAc) method, which extracts both solution and exchangeable K⁺, is the most common soil test method used to develop K⁺ fertilizer recommendations. This method does not measure plant-available nonexchangeable K⁺ nor the relationships among different pools of soil K⁺. When exchangeable K⁺ is depleted to its critical level, further plant K⁺ uptake is regulated by the rate of K⁺ release from the nonexchangeable pool (McLean and Watson, 1985). The inadequacy of the NH₄OAc soil test has been clearly demonstrated in illitic (McLean, 1976; Portela, 1993; Eckert and Watson, 1996) and vermiculitic (Cassman et al., 1990) soils. Nair et al. (1997) showed that NH₄OAc-extractable K⁺ alone was a poor indicator of K⁺ availability to cardamom [Elettaria cardamomum (L.) Maton] in kaolinitic soils, unless it is integrated with soil K⁺ buffer power. Obviously, NH₄OAc-extractable K⁺ alone is not always a reliable estimate of plant-available K⁺.

Richards and Bates (1988) found that StepK (loosely bound nonexchangeable K⁺ released by repeated extraction with 1 M HNO₃) was a more reliable index of the K⁺-supplying power of Canadian soils than is NH₄OAc-extractable K⁺. They concluded that a simple method of measuring StepK would greatly improve soil tests for plant available K⁺. Earlier work by Metson (1960) also showed that the release rate of a critical portion of nonexchangeable K⁺ (measured similarly to StepK) should supplement exchangeable K⁺ values in order to develop more reliable fertilizer recommendations.

Fixation and release of nonexchangeable K⁺ affects the abundance and binding strength of exchangeable K⁺, as well as quantity–intensity factors (McLean, 1976). Based on more than forty years of extensive field and laboratory studies (Bray, 1944, 1945; Woodruff, 1955; Woodruff and McIntosh, 1960; McLean, 1976; McLean et al., 1979; McLean et al., 1982; Vitosh et al., 1995) K⁺ fertilizer recommendations were developed using NH₄OAc soil test values that were adjusted based on CEC. This approach works well in some soils but is not reliable for soils with appreciable amounts of nonexchangeable interlayer K⁺. McLean et al. (1979, 1982) suggested the use of a correction factor (K_f) to account for the K⁺ fixation potential of soils. This correction factor is determined from studies on fixation and recovery (exchangeability) of added K⁺ in representative groups of soil. However, McLean and Watson (1985) suggested that this approach is not suited for routine soil test purposes because of the extra equipment, bench space, and time required to estimate K_f.

The sodium tetraphenylboron (NaBPh₄) procedure was developed by Smith and Scott (1966) to release interlayer K⁺ in soil micas. The BPh^-₄ anion combines with K⁺ in solution and precipitates as potassium tetraphenylboron (KBPh₄). Therefore, NaBPh₄ can mimic the action of plant roots by depletion of soil-solution K⁺ as KBPh₄ and cause further release of exchangeable and nonexchangeable K⁺. The NaBPh₄ method allows flexibility as a soil test because the extraction (incubation) time can be varied to alter the amount of K⁺ released. Wentworth and Rossi (1972) showed that although the extractability of K⁺ by NaBPh₄ in some layer-silicate minerals was in the order vermiculite > illite > biotite > phlogopite > muscovite; extractable K⁺ in all minerals was well related to K⁺ released to barley (Hordeum vulgare L.) in the greenhouse (r² = 0.96). Jackson (1985) used a modification of this method (0.03 M NaBPh₄, 16-h extraction) to measure reserve K⁺ in New Zealand soils and found that it was superior to his standard procedure (1 M NH₄OAc) for estimating K⁺ availability to ryegrass (Lolium perenne L.). The author suggested that the approach was not suited for routine use because it is relatively tedious.

Cox et al. (1996) modified the original NaBPh₄ procedure developed by Smith and Scott (1966) to make it more adaptable for routine work. Since then, we have refined the method even further by decreasing extraction time and cost to extend its potential as a soil test method for K⁺. The objectives of this study were (i) to compare the abilities of the 1 M NH₄OAc method and the modified NaBPh₄ method to estimate critical soil K⁺ levels, (ii) to estimate the contribution of nonexchangeable K⁺ to plant-available K⁺, (iii) to compare the abilities of the 1 M NH₄OAc method and the modified NaBPh₄ method to estimate plant dry matter yield and plant-available K⁺, and (iv) to compare the abilities of both methods to measure soil K⁺ balance.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils
Eleven surface soils (0–30 cm) representing a wide range of textural classes were collected from five Midwestern states (Indiana, Iowa, Kentucky, Ohio, and Wisconsin). Selected soil properties are presented in Table 1 . Other soil properties are presented in Cox et al. (1996). The textural classes represented were sand (CEC = 4 cmol_c kg^-1), silt loam (CEC = 8 to 12 cmol_c kg^-1), and clay loam (CEC = 32 to 36 cmol_c kg^-1). Soil K⁺ status was determined by the 1 M NH₄OAc method and by three NaBPh₄ extraction times (1, 5, and 15 min).

Plant Availability and Depletion of Soil Potassium
A bulk sample of the Chalmers soil was limed (3 g CaCO₃ kg^-1) and incubated for 2 wk before use in this study because of its low initial pH (4.7). All soils were sieved through a 1-cm screen and 1-kg portions were blended with a minus-K⁺ nutrient mixture (Allen et al., 1976) then placed into perforated 2-L pots. The soil in the pots was moistened to a matric potential of approximately -0.033 MPa, covered, and left to equilibrate on the greenhouse bench for 1 wk. Winter wheat (Triticum aestivum cv. Abe) was sown at fifteen seeds per pot, then thinned to twelve plants per pot after emergence. The greenhouse was naturally lit at 27 ± 5°C with the photoperiod extended to 16 h with high pressure Na lamps (130 mmol m^-2 s^-1). Plants were watered daily with deionized water and all drainage was collected in a base container and returned to the pots to eliminate nutrient loss by leaching. Nitrogen was supplied weekly at 40 mg N kg^-1 soil as Ca(NO₃)₂. Plants were defoliated every 28 d by removing herbage at 2.5 cm above the soil surface, and the stubble in the pots was left to regrow. After the third defoliation, the soil in the pots was sieved moist, most of the roots recovered, and a soil sample was taken for analysis. The oven-dried weight of the soil sample was obtained to determine the mass of soil remaining in the pots. The remaining soil was blended with the minus-K⁺ nutrient mixture and placed in the pots for another cropping sequence of three defoliation and regrowth cycles. Roots were not recovered during the remainder of the experiment because root contribution was <10% of cumulative K⁺ uptake in the first cropping sequence. The cropping and soil sampling sequence was repeated three times (nine crops total) for soils that became K⁺-deficient relatively quickly and up to six times for other soils. The Pewamo soil did not become K⁺-deficient after twenty crops; therefore, the study was terminated. Nutrient addition and plant population were reduced for each additional cropping sequence throughout the experiment to minimize variation in plant density (based on soil mass) and soil fertility with respect to other nutrients.

All foliage plus the roots collected after the first cropping sequence were dried and weighed to determine DM yield. Plant tissue was digested in a mixture of concentrated H₂SO₄ and 340 g kg^-1 (w/w) H₂O₂ for K⁺ determination.

Potassium in the soil samples collected at the end of each cropping sequence was determined using the 1 M NH₄OAc and the 5-min NaBPh₄ extraction methods.

Determination of Critical Potassium Concentration in Foliage
The critical level for K⁺ in the foliage (K⁺ status below which response to K⁺ fertilization is expected) was determined by growing wheat for 28 d in 1-kg portions of the Bloomfield and K⁺-deficient Chalmers soils at five K⁺ fertilization rates (0, 50, 100, 200, and 400 mg K⁺ kg^-1 as KCl; 3 replicates per treatment). These two soils were selected based on their relatively low NH₄OAc-extractable K⁺ and clear difference in texture. All other conditions and foliage analyses were similar to those outlined previously.

The data from this study were plotted as relative DM yields vs. foliage K⁺ concentration. The plot was evaluated using a linear–plateau model to determine the critical foliage K⁺ concentration required to obtain 90% maximum yield (Leigh, 1989).

Extractability of Added Potassium
To establish different levels of positive soil K⁺ balance (relative to initial soil K⁺ status), five K⁺ rates (0, 101, 202, 404, and 809 mg K⁺ kg^-1) were applied by sprinkling a K₂SO₄ solution (200 mg K⁺ L^-1) on 100-g samples of each soil. The K⁺-amended soils were thoroughly mixed then put into perforated polyethylene containers (5 cm deep x 9 cm diam.). Soil matric potential was adjusted to -0.033 MPa by adding deionized water, then the containers were placed in an incubator at 25°C for 6 mo. The moisture lost by evaporation was replaced twice weekly.

After incubation, the K⁺-amended soils were air-dried and sieved through a 2-mm screen. Extractable K⁺ was determined in triplicate 0.5-g samples using the 1 M NH₄OAc and NaBPh₄ (5 min) methods.

Statistical Analysis
All statistical analyses were conducted using SAS (1990). Simple linear regression analysis was used to evaluate the ability of the NH₄OAc and NaBPh₄ methods to estimate the dynamics of soil K⁺ supply (plant available K⁺, critical soil K⁺ levels, and soil K⁺ balance). Where either extraction procedure was inadequate as a single parameter to estimate the dynamics of K⁺ supply, stepwise multiple regression analysis was used to identify soil properties (CEC, exchangeable K⁺, illite K⁺, organic C, pH, clay) that controlled respective response variables. Simple linear regression analysis was also used to determine relationships between extractable K⁺ and soil K⁺ balance. For each soil, regressions of cumulative K⁺ removed by cropping (negative K⁺ balance) vs. extractable K⁺ levels at the end of each cropping sequence and K⁺ added in the incubation study (positive K⁺ balance) vs. extractable K⁺ were evaluated. The slopes of these relationships were compared using t tests (Steele and Torrie, 1980).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Critical Foliage Potassium Concentration
The critical level for K⁺ in the wheat foliage was estimated as 19 g K⁺ kg^-1 (DM basis) (Fig. 1) . This estimate of critical K⁺ concentration for wheat is based on the assumptions that no other element is limiting (Munson and Nelson, 1990) and that the highest growth attained in this experiment coincides with the highest yield potential. When foliage K⁺ declined to <19 g K⁺ kg^-1, the soils were considered K⁺-deficient. The estimated critical K⁺ concentration was lower than the sufficiency range (32–40 g K⁺ kg^-1) determined for field-grown wheat at 3 to 4 Feekes stage (Westfall et al., 1990). One probable cause for our lower critical foliage K⁺ was the use of NO^-₃ as the only N source. Dibb and Thompson (1985) showed that foliage K⁺ and DM yield of 36-d-old corn with adequate K⁺ supply in the greenhouse were lower in NO₃-fed plants than in NH⁺₄-fed plants. The critical foliage K⁺ determined in our study might not be an accurate estimate for wheat under field conditions; however, this critical level provided the plant K⁺ status that we used to measure the relative K⁺-supplying potential of soils and to compare the abilities of the extraction methods to estimate this potential.

View larger version (25K): [in this window] [in a new window]   Fig. 1 Determination of critical foliage K+ concentration in 28-d-old wheat in the greenhouse using a linear–plateau model. Plants were grown in soils at five K+ fertilization rates (0, 50, 100, 200, and 400 mg K+ kg-1 as KCl)

View larger version (19K): [in this window] [in a new window]   Fig. 2 Plot of (a) NH4OAc- and (b) NaBPh4-extractable K+ vs. foliage K+ concentration in 28-d-old wheat grown immediately after soil was sampled

Measured critical NH₄OAc-extractable K⁺ varied widely among groups of soils (Table 2) . The critical levels were lower than those predicted by using the algorithm developed by Vitosh et al. (1995) (Table 2), but the estimates were closely related (measured = 1.19 x predicted - 56.8; r² = 0.94). The data indicate that the soils can be placed into broad groups that are based on textural classes; sand (37 mg K⁺ kg^-1) silt loams (range is 59–77 and mean = 67 mg K⁺ kg^-1), and clay loams (range is 111–151 and mean = 131 mg K⁺ kg^-1). Soil properties (CEC, texture, vermiculite, illite K⁺, and organic C) were used in stepwise multiple regression analyses to evaluate their effects on critical levels of NH₄OAc-extractable K⁺. This analysis showed that critical levels could be well predicted using the model

(1)

where illite K⁺ is nonexchangeable K⁺ released by 7-d incubation in NaBPh₄ (g K⁺ kg^-1) and CEC is determined by Ca²⁺ saturation followed by Mg²⁺ displacement (cmol_c kg^-1) (Jackson, 1958). The Chalmers and Milford soils were not used to generate this model because the first wheat crop in these soils was K⁺-deficient [<19 g (kg DM)^-1]. For the Chalmers and Milford soils, predicted critical values generated using Eq. [1] (67 mg K⁺ kg^-1 for the Chalmers soil and 68 mg K⁺ kg^-1 for the Milford soil) indicate that NH₄OAc-extractable K⁺ at the start of the greenhouse study (Table 1) was at or below the critical level.

(2)

where illite K⁺ = nonexchangeable K⁺ released by 7-d incubation in NaBPh₄ (g K⁺ kg^-1) and CEC is determined by Ca²⁺ saturation followed by Mg²⁺ displacement (cmol_c kg^-1) (Jackson, 1958). For the Chalmers and Milford soils, predicted critical values generated using Eq. [2] (168 mg K⁺ kg^-1 for the Chalmers soil and 158 mg K⁺ kg^-1 for the Milford soil) indicate that NaBPh₄-extractable K⁺ at the start of the greenhouse study (Table 1) was at or below the critical level.

Relationship Between Extractable Potassium and Dry Matter Yield
The cumulative DM yields before soils became K⁺-deficient are presented in Table 3 . Since the first foliage harvests in the Chalmers and Milford soils were K⁺-deficient, data for these soils were not included. Some variation in DM yield also may have been attributable to differences in soil physical characteristics, which affected drainage and aeration in the pots. The fine-textured soils (especially the Pewamo) were not well drained compared to the coarse-textured soils during the first two cropping sequences. Therefore, more perforations were made in the pots to increase drainage and aeration in subsequent crops.

View this table: [in this window] [in a new window]   Table 3 Cumulative dry matter (DM) yield and plant-available K+ during consecutive defoliation and regrowth of 28-d-old wheat in the greenhouse up to the defoliation where soils became K+-deficient.

View larger version (19K): [in this window] [in a new window]   Fig. 3 Relationship between wheat dry matter yield (DM) and (a) NH4OAc-extractable K+ (regression does not include Hoytville and Pewamo soils) and (b) NaBPh4-extractable K+ (regression does not include Pewamo soil)

Contribution of Nonexchangeable Potassium to Plant-Available Potassium
Plant-available K⁺ was estimated as the cumulative K⁺ removed by wheat up to the defoliation cycle in which foliage K⁺ concentration was below 19 g K⁺ kg^-1. Plant-available K⁺ (Table 3) was greater than the initial NH₄OAc-extractable K⁺ (Table 1) in all soils except the K⁺-deficient Chalmers and Milford. Plant-available nonexchangeable K⁺ (total plant available minus initial exchangeable) represents the net contribution from nonexchangeable pools. Nonexchangeable K⁺ released (total plant available minus initial exchangeable plus final exchangeable) represents the gross contribution from nonexchangeable pools. Nonexchangeable K⁺ accounted for 16 to 76% of plant-available K⁺ (Table 3). The contribution of nonexchangeable K⁺ to total plant-available K⁺ was most significant in the Bloomfield sand and the Hoytville and Pewamo clay loams. The large contribution of nonexchangeable K⁺ in sandy soils has been attributed to K⁺ release from silt-size minerals (Abed and Drew, 1966; Rehm et al., 1984; Niebes et al., 1993; Rahmatullah et al., 1994). In a related study, we found relatively high release rates of NaBPh₄-extractable K⁺ in the fine silt fraction of the Bloomfield sand (Cox and Joern, 1997). The large contribution of nonexchangeable K⁺ in the Hoytville and Pewamo soils coincide with their relatively high illitic K⁺ content (Table 1) and nonexchangeable K⁺ release rates (Cox and Joern, 1997); and, therefore, this may be due partly to release of illitic K⁺. Among the other soils there was no relationship between nonexchangeable K⁺ release and illitic K⁺. Portela (1993) showed that the amount of nonexchangeable K⁺ released before ryegrass became K⁺-deficient was well correlated to illite content of the clay fraction in some Portuguese soils. McLean (1976) indicated that the Hoytville soil is known to release large amounts of nonexchangeable K⁺ and long-term cropping would be required to show crop response to added K⁺.

Relationship between Extractable and Plant-Available Potassium
Ammonium Acetate–Extractable Potassium
Ammonium acetate–extractable K⁺ was related to plant-available K⁺ in all soils except the Hoytville and Pewamo soils (Fig. 4a) . The slope of 1.19 indicates that plant-available K⁺ is 20% greater than NH₄OAc-extractable K⁺. The inability of the NH₄OAc method to estimate plant-available K⁺ in the Hoytville and Pewamo soils and the 20% underestimation in the other soils are both attributed to the contribution of nonexchangeable K⁺ to total plant-available K⁺.

View larger version (31K): [in this window] [in a new window]   Fig. 4 Relationship between plant-available K+, NH4OAc-extractable K+, and NaBPh4-extractable K+

Predictive ability and adaptability to routine work are important considerations in selecting a procedure and an extraction time for routine soil testing. The 1-min NaBPh₄-extractable K⁺ is best related to plant-available K⁺ (Fig. 4b). Although only 80% of the 5-min NaBPh₄-extractable K⁺ is plant-available (Fig. 4c), we adopted this extraction period for most of this study because it was more convenient than shorter periods. In automated operations such as commercial laboratories, extraction periods shorter than 5 min might have better predictive ability and be more suited for routine use. Longer extraction periods might be disadvantageous because of excessive extraction of unavailable K⁺, and, therefore, poor predictive ability.

Extractable Potassium in Relation to Potassium Balance
Soil K⁺ balance is the adjustment in soil K⁺ status that results from crop removal or fertilizer addition. For most of the soils, t test comparisons (data not shown) showed that slopes of negative K⁺ balance vs. NH₄OAc- or NaBPh₄-extractable K⁺ were similar (P > 0.05) to slopes of positive K⁺ balance vs. NH₄OAc- or NaBPh₄-extractable K⁺. Therefore, for each soil, the data for both negative and positive K⁺ balance were further evaluated as a single plot between extractable K⁺ and soil K⁺ balance (Fig. 5) .

View larger version (40K): [in this window] [in a new window]   Fig. 5 Ammonium acetate- (a and b) and NaBPh4-extractable (c and d) K+ in relation to soil K+ balance. Negative soil K+ balance = cumulative K+ removed by wheat in the greenhouse. Positive soil K+ balance = K+ added as K2SO4

The slope of the relationship between NH₄OAc-extractable K⁺ and soil K⁺ balance varied widely (0.16–0.68) among soils (Table 4) . These differences in slope are due mainly to the K⁺ fixation capacity of the soils. As fixation capacity increases, the slope decreases and the K⁺ addition or removal required to produce a unit change in NH₄OAc-extractable K⁺ increases. The slopes were lowest in the Hoytville and Pewamo soils (0.16 and 0.36, respectively). Previous studies have shown that the K⁺ fixation capacity (relative fixation of added K⁺) of these two soils were much greater than the other soils used in this study (Cox et al., 1996). McLean et al. (1982) found that the slope of exchangeable K⁺ vs. added K⁺ in 7-wk moist incubation studies was 0.20 in a Hoytville soil compared with up to 0.75 in silt loam soils. In our study, t test results showed that the slope for each soil was significantly different (P < 0.05) from the slope for most of the other soils (32 out of 45 comparisons) (data not shown). This indicates that the ability of the NH₄OAc method to measure changes in soil K⁺ status resulting from crop removal or K⁺ fertilization depends largely on soil properties.

View this table: [in this window] [in a new window]   Table 4 Simple linear regression equations describing the relationship of soil K+ balance vs. NH4OAc- and NaBPh4-extractable K+ (mg K+ kg-1).

(3)

where NH₄OAc K⁺ = initial NH₄OAc-extractable K⁺ (mg kg^-1), illite K⁺ = nonexchangeable K⁺ released by 7-d incubation in NaBPh₄ (g kg^-1), and clay (g kg^-1) is determined according to Jackson (1958). The application of Eq. [3] to the Crider (2) and Pewamo soils (Table 4) showed that the slope for the Pewamo soil (0.32) was well predicted, but the slope for the Crider (2) soil (0.51) was underestimated.

Sodium Tetraphenylboron–Extractable Potassium
Slopes of regressions relating NaBPh₄-extractable K⁺ to K⁺ balance were similar and near unity (Fig. 4c and 6d; Table 4). The greatest deviation from unit slope was in the Bloomfield soil (0.88). The t test comparisons showed that the slope for each soil was not significantly different (P > 0.05) from the slope for most other soils in the study (30 out of 45 comparisons) (data not shown). These results show that the NaBPh₄ method can measure unit changes in soil K⁺ status resulting from crop removal or K⁺ fertilization, irrespective of soil type. Quémener (1974) indicated that measurement of soil K⁺ status using NaBPh₄ has the potential to estimate variations in soil K⁺ reserves.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
In this study, plant-available K⁺ in soils was estimated as the quantity of K⁺ available to wheat in the greenhouse while still maintaining a predetermined critical foliage K⁺ concentration (19 g K⁺ kg^-1). Although it may be difficult to use absolute response in pot trials to assess availability of soil K⁺ in the field, this approach can provide an adequate benchmark to compare the relative ability of the conventional 1 M NH₄OAc and modified NaBPh₄ soil test methods to measure K⁺ supply. Critical extractable levels for each soil could not be determined by either method alone, but soils can be broadly grouped based on textural classes. For both methods, critical extractable K⁺ levels specific to each soil can be better predicted by including CEC and illite K⁺ in multiple regression models. The effect of CEC and illite K⁺ in predicting critical extractable K⁺ is noteworthy. As K⁺ depletion causes soil-solution K⁺ to approach critical levels, CEC and content of K⁺-bearing minerals like illite are the major properties controlling the K⁺-buffering capacity of soils (Mengel and Busch, 1982). The inability of the NH₄OAc method to estimate critical levels, plant available K⁺, and soil K⁺ balance was due mainly to contribution of nonexchangeable pools, especially in the illitic Hoytville and Pewamo soils. Critical NH₄OAc-extractable levels coincided with minimum or steady state exchangeable K⁺. At this soil K⁺ level, a significant amount of nonexchangeable K⁺ is available to plants. Therefore, at critical NH₄OAc-extractable K⁺ levels, the K⁺ fertilization required to achieve desired sufficiency levels is unpredictable if the cropping history is unknown. In K⁺-deficient soils, a percentage of applied K⁺ (dependent on K⁺ fixation capacity and extent of K⁺ depletion) will enter fixation sites and make it nonextractable by the NH₄OAc method. Accurate K⁺ fertilization guidelines based on the NH₄OAc method will require soil-specific information on K⁺ fixation behavior to manage soil K⁺ at below or just above critical levels.

In our study the NaBPh₄ method gave more reliable estimates of plant-available K⁺ and soil K⁺ balance than the NH₄OAc method because it extracts both exchangeable and nonexchangeable K⁺. The interaction between soil properties and environmental conditions, such as wet–dry and freeze–thaw cycles that influence K⁺ fixation and release, affects the reliability of the NH₄OAc soil test method. The NaBPh₄ method, however, may provide reliable estimates of K⁺ supply for a wide range of soil types and environmental conditions because it is less sensitive to K⁺ fixation. These studies show that the NaBPh₄ method should provide a more reliable K⁺ soil test compared with the NH₄OAc method. Currently, studies are being conducted to further evaluate the former method under field conditions and to determine its utility as a multi-element extractant.SAS Institute 1990

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Journal paper no. 15627 of the Indiana Agric. Res. Program, West Lafayette.

Received for publication January 16, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

• Abed F.M.A.H., Drew J.V. The importance of illite as a source of potassium in Nebraska soils. Soil Sci. Am. Proc. 1966;30:242-245.
• Allen S.E., Terman G.L., Clements L.B. Greenhouse techniques for soil–plant–fertilizer research. Muscle Shoals, AL: National Fertilizer Development Center, 1976.
• Badraoui M., Bloom P.R., Delmaki A. Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica. Plant Soil 1992;140:55-63.
• Barber T.E., Mathews B.C. Release of non-exchangeable soil potassium by resin-equilibration and its significance for crop growth. Can. J. Soil Sci. 1962;42:266-272.
• Bray R.H. Soil–plant relations: I. The quantitative relation of exchangeable potassium to crop yield and to crop response to potash additions. Soil Sci. 1944;58:305-324.
• Bray R.H. Soil–plant relations: II. Balanced fertilizer use through soil tests for potassium and phosphorus. Soil Sci. 1945;60:463-473.
• Breland H.L., Bertramson B.R., Borland J.W. Potassium-supplying power of several Indiana soils. Soil Sci. 1950;65:237-247.
• Cassman K.G., Bryant D.C., Roberts B.A. Comparison of soil test methods for predicting cotton response to soil and fertilizer potassium on potassium fixing soils. Commun. Soil Sci. Plant Anal. 1990;21:1727-1743.
• Coffman C.B., Fanning D.S. Vermiculite determination on whole soils by cation-exchange capacity methods. Clays Clay Miner. 1974;22:271-283.
• Cox A.E., Joern B.C. Release kinetics of nonexchangeable potassium by sodium tetraphenylboron in midwestern soils. Soil Sci. 1997;162:588-598.
• Cox A.E., Joern B.C., Roth C.B. Nonexchangeable ammonium and potassium in soils with a modified sodium tetraphenylboron method. Soil Sci. Soc. Am. J. 1996;60:114-120.[Abstract/Free Full Text]
• Dibb D.W., Thompson W.R., Jr. Interaction of potassium with other nutrients. In: Munson R.D., ed. Potassium in agriculture. Madison, WI: ASA, CSSA, and SSSA, 1985:515-533.
• Eckert D.J., Watson M.E. Integrating the Mehlich extractant into existing soil test interpretation schemes. Commun. Soil Sci. Plant Anal. 1996;27:1237-1249.
• Jackson B.L.J. A modified sodium tetraphenylboron method for the routine determination of reserve-potassium status of soil. N.Z.J. Exp. Agric. 1985;13:253-262.
• Jackson M.L. Soil chemical analysis. Englewood Cliffs, NJ: Prentice-Hall, 1958.
• Leigh, R.A. 1989. Potassium concentrations in whole plants and cells in relation to growth. p. 117–126. In Methods of K-research in plants. 21st Colloq. Int. Potash Inst., Louvain-la-Neuve, Belgium. 19–21 June 1989. Int. Potash Inst., Bern, Switzerland.
• Leigh R.A., Wyn Jones R.G. A hypothesis relating critical potassium concentrations for growth to the distribution and function of this ion in plant cell. New Phytol. 1984;97:1-13.
• Mallarino A.P., Blackmer A.M. Comparison of methods for determining critical concentrations of soil test phosphorus for corn. Agron. J. 1992;84:850-856.[Abstract/Free Full Text]
• McLean E.O. Exchangeable K levels for maximum crop yields on soils of different cation exchange capacities. Commun. Soil Sci. Plant Anal. 1976;7:823-838.
• McLean E.O., Adams J.L., Hartwig R.C. Improved corrective fertilizer recommendations based on a two-step alternative usage of soil tests: Recovery of soil-equilibrated potassium. Soil Sci. Soc. Am. J. 1982;46:1198-1201.[Abstract/Free Full Text]
• McLean E.O., Oloya T.O., Adams J.L. Soil tests to inventory the initially available levels and to assess the fates of added P and K as bases for improved fertilizer recommendations. Commun. Soil Sci. Plant. Anal. 1979;10:623-630.
• McLean E.O., Watson M.E. Soil measurements of plant-available potassium. In: Munson R.D., ed. Potassium in agriculture. CSSA, and SSSA, Madison, WI: ASA, 1985:277-308.
• Mengel K., Busch R. The importance of potassium buffer power on the critical potassium level in soils. Soil Sci. 1982;133:27-32.
• Mengel K., Uhlenbecker K. Determination of available interlayer potassium and its uptake by ryegrass. Soil Sci. Soc. Am. J. 1993;57:561-566.
• Metson, A.J. 1960. The long-term potassium-supplying power of New Zealand soils. p. 621–630. Trans. Int. Congr. 9th Soil Sci. Adelaide, Australia.
• Munson R.D., Nelson W.L. Principles and practices in plant analysis. In: Westerman R.L., ed. Soil testing and plant analysis, 3rd ed Madison, WI: SSSA Book Ser. 3. SSSA, 1990:359-387.
• Nair K.P.P., Sadanandan A.K., Hamza S., Abraham J. The importance of potassium buffer power in the growth and yield of cardamom. J. Plant Nutr. 1997;20:987-997.
• Niebes J., Dufey J.E., Jaillard B., Hinsinger P. Release of nonexchangeable potassium from different size fractions of two highly K-fertilized soils in the rhizosphere of rape (Brassica napus cv Drakkar). Plant Soil 1993;155(156):403-406.
• Portela E.A.C. Potassium supplying capacity of northeastern Portugese soils. Plant Soil 1993;154:13-20.
• Pratt P.F. Potassium removal from Iowa soils by greenhouse and laboratory procedures. Soil Sci. 1951;71:107-117.
• Quémener, J. 1974. The use of reagents containing sodium tetraphenylboron for the extraction of soil potassium: Applications to the study of fertility problems. p. 65–72. In Potassium research and agricultural production. Proc. 10th Congr. Int. Potash Inst., Budapest, Hungary. June 1974. Int. Potash Inst., Bern, Switzerland.
• Rahmatullah B.Z., Shaikh M.A., Gill, Salin M. Bioavailable potassium in river-bed sediments and release of interlayer potassium in irrigated arid soils. Soil Use Manage. 1994;10:43-46.
• Rehm G.W., Sorensen R.C., Wiese R.A. Soil test values for phosphorus, potassium, and zinc as affected by rate applied to corn. Soil Sci. Soc. Am. J. 1984;48:814-818.[Abstract/Free Full Text]
• Richards J.E., Bates T.E. Studies on the potassium-supplying capacities of southern Ontario soils: II. Nitric acid extraction of nonexchangeable K and its availability to crops. Can. J. Soil Sci. 1988;68:199-208.
• Richards J.E., Bates T.E., Sheppard S.C. Studies on the potassium-supplying capacities of southern Ontario soils: I. Field and greenhouse experiments. Can. J. Soil Sci. 1988;68:183-197.
• SAS Institute. Procedures guide for personal computers. Version 6, 4th ed Cary, NC: SAS Inst, 1990.
• Smith S.J., Scott A.D. Extractable potassium in grundite illite: Method of extraction. Soil Sci. 1966;102:115-122.
• Steele R.G.D., Torrie J.H. Principles and procedures of statistics. New York: McGraw-Hill, 1980.
• Tabatabai M.A., Hanway J.J. Potassium supplying power of Iowa soils at their minimum levels of exchangeable potassium. Soil Sci. Soc. Am. Proc. 1969;33:105-109.
• Vitosh M.L., Johnson J.W., Mengel D.B. Tri-state fertilizer recommendations for corn soybeans, wheat, and alfalfa. Extension Bulletin E2567. East Lansing, MI: Michigan State Univ, 1995.
• Wentworth S.A., Rossi N. Release of potassium from layer silicates by plant growth and by NaTPB extraction. Soil Sci. 1972;113:410-416.
• Westfall D.G., Whitney D.A., Brandon D.M. Plant analysis as an aid in fertilizing small grains. In: Westerman R.L., ed. Soil testing and plant analysis. 3rd ed. SSSA Book Ser. 3. Madison, WI: SSSA, 1990:495-519.
• Woodruff C.M. The energies of replacement of calcium by potassium in soils. Soil Sci. Soc. Am. Proc. 1955;19:167-171.
• Woodruff, C.M., and J.L. McIntosh. 1960. Testing soil for potassium. p. 80–85. Trans. Int. Congr. Soil Sci., 7th. Madison, WI.

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