A Modification to the Adams-Evans Soil Buffer Determination Solution

Transcription

1 Communications in Soil Science and Plant Analysis, 36: , 2005 Copyright # Taylor & Francis, Inc. ISSN print/ online DOI: / A Modification to the Adams-Evans Soil Buffer Determination Solution Gobena Huluka Department of Agronomy and Soils, Auburn University, Auburn, Alabama, USA Abstract: Many soil analysis labs routinely determine lime requirement of acidic soils using different buffer solutions for optimum plant growth. The Adams-Evans lime determination solution was introduced more than 40 years ago and has been used by many soil analysis labs. Even though many buffer solutions have been developed since then, very little attention has been paid to address the toxic nature of chemicals involved in buffer solutions. The most commonly used buffer solutions, such as the Adams-Evans, Shoemaker-McLean-Pratt (SMP), Woodruff, and others, contain p-nitrophenol, which is toxic to humans and the environment. Use of p-nitrophenol requires prescribed containment and disposal procedures, that creates extra burden on soil analysis labs that provide their invaluable service at low cost. Replacing p-nitrophenol with monobasic potassium phosphate (KH 2 PO 4 ), which has similar buffering capacity but with no known toxicity, is beneficial to soil testing labs and the environment. The original Adams-Evans buffer solution was compared with the modified Adams-Evans buffer solution with soils of different ph, cation exchange capacity and lime requirement. The linear regression between the buffer ph values and lime recommendations made by Adams-Evans and the modified Adams-Evans solutions were highly significant. Thus, the modified Adams-Evans buffer solution can be used without loss of established recommendation criteria as the original buffer solution. Keywords: Soil buffer solution, lime determination, soil acidity, soil analysis Received July 2004, Accepted May 2005 Address correspondence to Gobena Huluka, Department of Agronomy and Soils, Auburn University, 226 Funchess Hall, Auburn, AL 36849, USA. hulukgo@ auburn.edu 2005

2 2006 G. Huluka INTRODUCTION Soil acidity was one of the acute problems facing agriculture before application of liming materials became a routine remedial practice. From the 1930s to 1960s, many soil acidity correction methods that involved buffer solution for rapid determination of lime requirement were developed (Pierre and Worley 1928; Morgan 1930; Mehlich 1938; Woodruff 1948; Adams and Evans 1962; Shoemaker et al. 1961; Peech 1965). van Leirop (1983) and Ssali and Nuwamanya (1981) have studied application of buffer solutions for liming determination of acidic organic and tropical soils, respectively. More recently, Liu et al. (2004) determined lime requirement of 17 Georgian soils by direct titration with Ca(OH) 2. Soil testing labs have been making recommendations of limestone or other specific liming materials required to raise soil ph to a desired value under field conditions for their respective clients. A comprehensive review of the common soil lime requirement determination methods for different regions in the United States and a detailed procedure used in each region is given by Sims (1996). Buffer solutions resist abrupt change in ph and have been used to determine the amount of liming materials needed to raise soil ph to a specific value. Buffer solutions are expected to measure both the capacity (fraction of reserve, exchangeable, and soluble acidity) and the intensity (soil solution acidity) factors of soil acidity. The ability of buffer solutions to measure the capacity factor of soil acidity is the main reason for their extensive use. Soil parent material, texture, organic matter content, and exchangeable phytotoxic elements [e.g., aluminum (Al), iron (Fe), manganese (Mn)] content affect the amount of lime required to adjust soil ph to a targeted value (1996). Acidic soils are prevalent in the southeastern United States due to humid subtropical climate and acid-causing anthropogenic activities. The Adams- Evans buffer was developed in the early 1960s for soils that predominate in 1 : 1 and low-activity clays, Fe and Al oxides and have relatively low cation exchange capacity (CEC) (5). Many soil-testing labs in this region have adopted the Adams-Evans buffer solution to make lime recommendations for different crops to correct soil acidity problems (Sims 1996; SRIEG ). Most buffer procedures determine the lime requirement of acid soils by relating the change in ph of the buffer reacting with the soil and the corresponding soil water ph. Adams and Evans (1962) developed a quadratic relationship between soil water ph and degree of base unsaturation or acid saturation (Hsat 1 ) by applying the least-square statistical method to determine the best-fit equation. A solution of the quadratic equation for soil water ph and desired ph (Hsat 2 ) was used to determine the amount of liming material needed. The equations are given as follows: Measured soil ph ¼ 7:79 5:55Hsat 1 þ 2:227ðHsat 1 Þ 2 ; ð1þ

3 Modified Adams-Evans Buffer Solution 2007 ð8000ð8-buffer phþþ CaCO 3 ðmg=haþ ¼ ðhsat 1 Þ ðhsat 1 Hsat 2 Þð1:5Þ2:24; ð2þ where Hsat 1 and Hsat 2 are base unsaturation of soil at measured and desired ph, respectively. The solution of Eq. (1) was incorporated into Eq. (2) to determine the amount of lime required to raise ph of a given soil to a targeted ph at base unsaturation Hsat 2. The relationship between ph and base unsaturation may not be the same for all soils, and that creates inherent uncertainty in the method as cautioned by Adams and Evans (1962). It is possible to develop a different Eq. (1) above for different soils, which may result in different base unsaturation calculations and lime recommendations. But the accuracy of a given method needs to be evaluated by considering both the statistical goodness and field calibration. The Adams-Evans (1962), Woodruff (1948), Schofield (1933), and Shoemaker et al. (1961) buffer solutions contain p-nitrophenol in addition to other chemicals. Para-nitrophenol provides a good buffering capacity between ph 7.0 and 8.0 with the other chemicals because its dissociation constant (pka) is around But p-nitrophenol has also been shown to be toxic to the environment and has been shown to cause abnormal cell growth in fish at 1 mg/l (Braunbeck et al. 1989). Human exposure by inhalation or ingestion of p-nitrophenol may cause headache, drowsiness, nausea, and blue color in lips, ears, and fingernails (Solutia Inc. 2004). Laboratory preparation of p-nitrophenol demands special precautions. Thus, many labs have resorted to buying a prepared solution that has increased their expenses. Routine soil testing labs process thousands of lime requirement analyses annually and generate high volumes of p-nitrophenol waste solution. Because draining p-nitrophenol down the sink may contaminate water resources including groundwater, many labs regulate its disposal either by photodecomposition or store it securely for off-site treatment. Developing a buffer solution that serves the special purpose of making lime requirement determination for low CEC and coarse textured soil is time-consuming and requires field calibration. But replacing p-nitrophenol with nontoxic and readily available chemical will ensure the continuous use of the Adams-Evans buffer solution without a need for extensive field calibration since the use of buffers is convenient and invaluable. PROCEDURES Four hundred seven soil samples from different counties of Alabama were randomly selected from samples sent to the Auburn University Soil Testing lab for routine analysis. The buffer ph was measured by the original

4 2008 G. Huluka Adams-Evans buffer solution as routinely done in the lab (Hue and Evans 1986) and by the modified Adams-Evans buffer solution. The original Adams-Evans buffer solution consists of 20 g p-nitrophenol, 15 g H 3 BO 3, 74 g KCl, and 10.5 g KOH dissolved in 1000 ml of H 2 O (Adams and Evans 1962). The modified Adam-Evans buffer solution contains 20 g KH 2 PO 4, 17 g H 3 BO 3, 74 g KCl, and 12.5 g KOH in 1000 ml H 2 O. Both solutions were adjusted to ph by using either dilute KOH or HCl. Soil water ph was measured by scooping 20 ml of soil into a 50-mL cup and by adding 20 ml of deionized water. The samples were mechanically stirred for 30 sec and allowed to stand for 30 min. Just before reading the ph, each sample was stirred, and the ph reading was taken to the nearest 0.1 unit while stirring with an automatic stirrer. After soil-water ph was taken, 20 ml of either the Adams-Evans or the modified Adam-Evans buffer solution was added to each sample. The solution was mechanically stirred for 4 min. The ph of the control solution (1 : 1 H 2 O to buffer) was adjusted to ph units, and then the soil buffer ph was read to the nearest 0.01 ph units. The base unsaturation and lime requirement were calculated by using Eqs. (1) and (2), respectively, as given above. Least-square statistical analysis method was used to develop a quadratic equation relating soil base unsaturation and soil water ph similar to Eq. (1) above, and regression analysis was performed on the Adams-Evans and the modified Adams-Evans data to establish linear dependency using Statistical Analysis Systems (SAS) procedures (SAS/STAT 1990). RESULTS AND DISCUSSION The ph of the soil samples used in this study ranged from 4.10 to 7.60 with a mean value of About 14, 80, and 6% of the samples had the following ph values: 5.0, , and ph.6.5, respectively. These ph values are normal for Alabama soils. Liming is not usually recommended for soils with ph6.5 for common crops. Soil samples with high and low ph values were included in this study to examine the applicability range of the modified Adams-Evans buffer solution. The mean values of P, K, Mg, and Ca were , , , and cmol C /kg, respectively. The quadratic relationship between soil water ph and base unsaturation was highly significant (Figure 1). The equation represented the best fit of the data with the least-square errors for any given data set. The higher the water ph of the soil, the lower the base unsaturation and vice versa. The quadratic equation for the relationship was very similar to the Adams-Evans original Eq. (1) given above. Depending on the relationship between soil water ph and base unsaturation, different quadratic equations can be formulated for different soils that will also affect lime recommendations.

5 Modified Adams-Evans Buffer Solution 2009 Figure 1. The relationship between soil ph and base unsaturation. The regression between the Adams-Evans and the modified Adams-Evans solutions was highly significant (Figure 2). The K in KCl replaces exchangeable acidic cations, whereas KOH rapidly reacts with soil solution H þ and KH 2 PO 4 buffers the solution. The H 3 BO 3 increases the buffering capacity of the solution and also preserves the buffer solution for extended storage time. According to the regression equation, the buffer ph measured by the modified Adams-Evans buffer is linearly related to the original Figure 2. The relationship between Adams-Evans buffer ph and the modified Adams-Evans buffer ph.

6 2010 G. Huluka Adams-Evans buffer. Lime recommendation could be made using this relationship for the analysis determined with use of the modified Adams- Evans buffer solution. The linear regression relationships to raise soil ph to 6.5 and 7.0 by the two buffer solutions for all samples were highly significant (Figures 3 and 4). According to the regression equation given in Figure 3, slightly less agricultural limestone (lbs/a) is recommended by the modified Adams-Evans buffer than the original Adams-Evans buffer method to adjust soil to ph 6.5. In addition, slightly less lime is recommended by the modified Adams- Evans method to adjust soil ph to 7.0. These differences are insignificant when recommendations are made in tons/a. The average amount of agricultural lime (ton/a) needed to adjust the soil ph to either 6.5 or 7.0 by the Adams-Evans and the modified Adams-Evans buffer methods is given in Table 1. The buffer values were sorted by water ph. The number of samples (n) that had the same water ph but that had a range of closely related buffer ph values were indicated in the table with the amount of lime needed rounded to whole numbers for both methods at ph 6.5 and 7.0. It should be noted that soils that have the same water ph and similar buffer ph values can have the same lime recommendations due to the rounding of recommendations to tons/a (Sims 1996; Sims and Dennis 1989; Council on Soil Testing and Plant Analysis 1980). Samples with water ph greater than 7.0 and less than 4.4 were not included. As expected, raising ph of an acid soil to ph 7.0 needs more liming material than liming just to ph 6.5. In general, soils with lower water ph needed more liming material to raise their ph than soils with relatively higher Figure 3. The relationship between CaCO 3 required by the Adams-Evans and the modified Adams-Evans buffer solutions to adjust the soil ph to 6.5.

7 Modified Adams-Evans Buffer Solution 2011 Figure 4. The relationship between CaCO 3 required by the Adams-Evans and the modified Adams-Evans buffer solutions to adjust the soil ph to 7.0. water ph. This was expected because most of the soils had similar CEC and mineral content. No lime was recommended for soils that had water ph of 6.8 or above by either methods. The buffer ph readings were made 30 min after the modified Adams- Evans buffer solution was mixed with the soil samples, and no significant change was observed within 48 h. But stirring the soil suspension with an automatic stirrer while reading the buffer ph affected the values to some degree. Because stirring is a standard protocol in the Adams-Evans buffer procedure, it is recommended in the modified buffer as well. The modified buffer soil suspension was relatively more stable, which indicates rapid completion of the acid displacement and neutralization reactions. The modified Adams-Evans buffer solution is very stable for an extended period of storage time and is unaffected by extended light exposure. There was not any sign of fungal growth in the modified Adams-Evans solution when stored at room temperature for extended period (more than 6 months). In addition, the modified Adams-Evans solution is easy to prepare because KH 2 PO 4 easily dissolves in water. It is a colorless solution and does not stain clothing and other lab wares as opposed to the p-nitrophenol solution, which is yellowish/orange. Monopotassium phosphate is much

8 2012 G. Huluka Table 1. Ag-lime needed to raise soil ph to 6.5 and 7.0 for Adams-Evans and the modified Adams-Evans buffer solutions Buffer ph Ag lime (ton/a) Water ph AE1 AE2 ph ¼ 6.5 ph ¼ 7.0 Mean Range n Mean Range AE1 AE2 AE1 AE AE1, Adams-Evans buffer; AE2, Modified Adams-Evans buffer;, 1000 lb/a. cheaper, easily available, and environment-friendly compared with the p-nitrophenol it replaces in the Adams-Evans buffer solution. CONCLUSIONS Four hundred seven soil samples were used to establish whether the modified Adams-Evans buffer solution can be used to determine lime requirement estimation of different Alabama soils as the original Adams-Evans buffer solution. The results of this study indicated that the modified Adams-Evans

9 Modified Adams-Evans Buffer Solution 2013 buffer solution can be as effective as the original solution for lime recommendation and is more suitable to work with in the lab and is also environmentfriendly. Nontoxic or less toxic buffer solutions will continue to play a pivotal role for lime determination. Since organic matter, Al, Fe, and clay contents can significantly affect the acid buffering capacity of a soil, and the amount of lime needed is affected by the targeted ph and other factors. Thus, cautions need to be exercised when making lime recommendation. ACKNOWLEDGMENT I would like to thank all folks at the Auburn University Soil Testing Lab for their help with sample analysis and professionalism. REFERENCES Adams, F. and Evans, C.E. (1962) A Rapid Method for Measuring Lime Requirement of Red-Yellow Podzolic Soils. Soil Science Society of America Proceedings, 26: Braunbeck, T., Storch, V., and Nagel, R. (1989) Sex-Specific Reaction of Liver Ultrastructure in Zebra Fish (Brachydanio rerio) After Prolonged Sublethal Exposure to 4-Nitrophenol. Aquatic Toxicology, 14: Council on Soil Testing and Plant Analysis. (1980) Handbook on Reference Methods for Soil Testing, Revised edition, Athens, Georgia: University of Georgia, 130. Hue, N.V. and Evans, C.E. (1986) Procedures Used for Soil and Plant Analysis by the Auburn University Soil Testing Laboratory; Agric. Exp. Stn. Dep. Ser. 16, 31, Auburn University: Auburn, Alabama. Liu, M., Kissel, D.E., Vendrell, P.F., and Cabrera, M.L. (2004) Soil Lime Requirement by Direct Titration with Calcium Hydroxide. Soil Science Society of America Journal, 68: Mehlich, A. (1938) Use of TEA-OAc-Ba(OH) 2 Buffer for the Determination of Some Base Exchange Properties and Lime Requirement of Soil. Soil Science Society of America Proceedings, 3: Morgan, M.F. (1930) Factors Affecting the Estimation of Lime Requirement from ph Values. Soil Science, 29: Peech, M. (1965) Lime Requirement. In Methods of Soil Analysis; Black, C.A., ed.; Part 2, Agronomy Monogr. 9, Agronomy Society of America: Madison, Wisconsin, Pierre, W.H. and Worley, S.L. (1928) The Buffer Method and the Determination of Exchangeable Hydrogen for Estimating the Amounts Lime Required to Bring Soils to Definite ph Values. Soil Science, 26: SAS/STAT. (1990) SAS/STAT User s Guide, Version 6. Cary, North Carolina: SAS Institute. Schofield, R.K. (1933) Rapid Method of Examining Soils II. Journal of Agricultural Science, 23: Shoemaker, H.E., McLean, E.O., and Pratt, P.F. (1961) Buffer Methods for the Determination of Lime Requirement of Soils with Appreciable Amount of Exchangeable Aluminum. Soil Science Society of America Proceedings, 25:

10 2014 G. Huluka Sims, J.T. (1996) Lime Requirement. In Methods of Soil Analysis; Sparks, D.L., ed.; Part 3, SSSA Book Series 5,, Soil Science Society of America: Madison, Wisconsin, Sims, J.T. and Dennis, L. (1989) Evaluation of Lime Requirement Method for Delaware Soils. Communications of Soil Science and Plant Analysis, 20: Solutia, Inc. (2004) Para Nitrophenol: Material Safety Data Sheet. domino.solutia.com/solutia/smsdslib.nsf/; Accessed October 9, SRIEG-18. (1992) Reference Soil and Media Diagnostic Procedures for the Southern Region of the United States, Bull. 374, Blacksburg, Virginia: Virginia Agricultural Experiment Station, Virginia Polytechnic State University. Ssali, H. and Nuwamanya, J.K. (1981) Buffer ph Methods for Estimation of Lime Requirement of Tropical Acid Soils. Communications in Soil Science and Plant Analysis, 12: van Lierop, W. (1983) Lime Determination of Acid Organic Soils Using Buffer-pH Methods. Canadian Journal of Soil Science, 63: Woodruff, C.M. (1948) Testing Soils for Lime Requirement by Means of a Buffered Solution and the Glass Electrode. Soil Science, 66: