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© 2010 Plant Management Network.
Accepted for publication 2 August 2010. Published 31 August 2010.

Special Approaches Are Needed When Testing Calcareous Sands

Rodney A. St. John, Assistant Professor, Department of Horticulture, Forestry and Recreation Resources, Kansas State University, Manhattan, KS 66506; and Nick E. Christians, Professor, Department of Horticulture, Iowa State University, Ames, IA 50010

Corresponding author: Rodney A. St. John. rstjohn@ksu.edu

St. John, R. A., and, Christians, N. E. 2010. Special approaches are needed when testing calcareous sands. Online. Applied Turfgrass Science doi:10.1094/ATS-2010-0831-01-RS.

Abstract

Some soil testing procedures dissolve calcium carbonate (CaCO₃), which will cause an increase in the extractable calcium concentration and an increase in the estimated cation exchange capacity (CEC). On the high-sand, low-organic matter, calcareous rootzones used for construction of some putting greens and sports fields, the dissolution of calcium carbonate can greatly influence the results. The objectives of this research were to measure the effects of CaCO₃ on different soil testing procedures and to make recommendations to help turf managers interpret the results from different tests. Raising the pH of the standard 0.5M ammonium acetate (NH₄OAc) solution from pH 7.0 to pH 8.1 reduced the dissolution of CaCO₃ by 33%. The Mehlich III solution is very acidic and dissolved almost five times the amount of lab-grade CaCO₃ as NH₄OAc pH 7.0 and therefore is not recommended for calcareous sands. The higher the concentration of CaCO₃ in the sand, the more dissolution, and therefore, the exchangeable Ca, the CEC, the cation ratios, and the cation saturation percentages will all be more misleading. Send your samples to labs that are experienced in dealing with calcareous sands, ask which tests they are using, and be aware of any dissolution problems when interpreting the results.

Introduction

Many golf course putting greens and sports fields utilize sand-based rootzones based upon United States Golf Association (USGA) specifications (15). Sand-based rootzones are chosen for their beneficial physical properties, like improved water and air infiltration rates and reduced capacity for compaction. Unfortunately, sand-based rootzones can have some less-than-desirable chemical properties. These sand rootzones have very low cation exchange capacities (CEC), ranging from 1 to 6 cmolc/kg (2). Low CEC sands have a limited ability for holding nutrients; therefore, care must be taken when applying fertilizers to limit the possibility for leaching while maintaining adequate nutrition for growth and aesthetics.

Additionally, many sands used for construction of putting greens and sports fields are calcareous. Calcareous soils are created from the weathering of rocks, like limestone (CaCO₃), or shells that contain different forms of carbonates. The solubility of CaCO₃ is affected by pH and carbon dioxide (CO₂) concentrations. Due to the relationship between CaCO₃, CO₂, CO₃^2- (carbonates), and HCO_3- (bicarbonates), the soil solution of calcareous sands and soils is usually buffered at approximately pH 8.2. This alkaline pH can limit the availability of several nutrients like phosphorus (P), iron (Fe), and manganese (Mn).

For these reasons, frequent soil testing to assess the nutritional status of sand-based growing media is a necessity. Unfortunately, the soil testing procedures generally used to measure exchangeable cations and CEC may not be providing accurate results. Ammonium acetate (NH₄OAc) pH 7.0 method is one that is frequently used for measuring exchangeable cations and estimating the CEC by the summation of the exchangeable cations (18). Because the NH₄OAc pH 7.0 extracting solution has a neutral pH, is strongly buffered, and has a relatively high ionic strength, carbonates and gypsiferous particles will dissolve, and it is not recommended for calcareous soils (9,12,17,19). It is important to note that the exchangeable cations measured from the NH₄OAc pH 7.0 procedure are commonly added together to estimate the cation exchange capacity. But, if the procedure is dissolving CaCO₃, MgCO3, or other particles containing cations, the CEC will be overestimated. This is especially important when dealing with sand-based greens. On soils with large proportions of clay and organic matter, this dissolution problem may have a smaller effect on the results, but in the high-sand, low-organic-matter, calcareous rootzones used for turf, the dissolution of calcium carbonate can greatly influence the results. Any errors due to dissolution of carbonates are going to greatly change the CEC, exchangeable cations, cation ratios, and the interpretation of the results needed for developing a fertility program.

Currently, there are two main ways to approach this problem of dissolution, either limit the dissolution, or make corrections for it. To limit or prevent dissolution, the pH of the extracting solution can be raised or the ionic strength can be lowered. One correction-type procedure, the NH₄Cl method (18), measures the carbonate and sulphate concentration in the soil extracts and relates those concentrations to dissolved CaCO₃ and CaSO4, respectively (18). These amounts can then be subtracted from the total exchangeable Ca measured to more accurately report the exchangeable cations and CEC.

The objectives of this research were to measure the variability of exchangeable cation and CEC measurements of standard soil testing procedures and help turfgrass managers understand different soil testing methods so they can correctly interpret the results. To meet these objectives, we initially compared the results of different soil testing procedures on a variety of locally collected sand/soil samples. We then constructed a set of manufactured sand samples that had increasing levels of CaCO₃ from different sources and performed tests for CEC and exchangeable cations on these manufactured samples.

Initial Comparison of Field Samples

An initial comparison of methods for exchangeable cations was performed on a collection of 19 different sand/soil samples collected from Ames, IA. The samples were taken from pure sand-based greens, from a USGA specification green, and from a rootzone containing 1/3 peat, 1/3 sand, and 1/3 Nicollet soil. The samples were analyzed for exchangeable calcium concentration using 0.5M ammonium acetate (NH₄OAc) at pH 7.0 and 8.1, 0.5M ammonium chloride (NH₄Cl), and Mehlich 3 according to the procedures listed below.

The average exchangeable Ca concentration varied greatly depending upon the method used. Using the Mehlich 3 extracting solution produced the highest average exchangeable Ca concentrations (Table 1). Raising the pH of the ammonium acetate solution from 7.0 to 8.1 reduced the Ca concentration of the soil extracts an average of 33%. The NH₄Cl procedure measured an average 16% less Ca than the NH₄OAc pH 7.0 extraction procedure.

Table 1. Average extractable Ca concentration from 19 soil and sand
samples collected from Ames, IA, according to four different soil extracting
techniques. Each value is the average concentration from all soils from
which three sub-samples were taken from each soil sample for analysis.

 ^x Means with the same letter are not significantly different according to
Tukey’s multiple comparison error rate (P ≤ 0.05).

Creating Sands with Increasing Calcium Carbonate Content

To further measure the dissolution effect, a set of ‘manufactured’ sand samples was created for quantifying the effect of CaCO₃ on different analysis techniques for measuring exchangeable cations and CEC. Twenty-four sand samples were created in the laboratory using a silica sand base (Unimin Corp., Portage, WI) and adding increasing percentages by volume of either a reagent grade CaCO₃ (C64-500, CAS# 471-34-1, Fisher Scientific, Pittsburgh, PA) or a local calcareous sand (Table 2). The calcareous sand had 11% CaCO₃ as determined by gravimetric procedures (5,18). All sands and amendments were air-dried before mixing and were stored in plastic zippered bags. There were 24 bags of silica sand mixed with either reagent grade CaCO₃ or calcareous sand from which sub-samples were taken for each analysis. Three sub-samples were taken from each of the 24 bags of silica sand mixed with either reagent grade CaCO₃ or calcareous sand and analyzed separately by each of the seven methods listed below for a total of 504 samples analyzed. The nonlinear (NLIN) procedure of Statistical Analysis Software (SAS Institute Inc., Cary, NC) was used to model monomolecular regression equations.

Table 2. List of ‘manufactured sand’ samples created in the laboratory to measure the effects of CaCO₃ on different soil testing procedures for measuring exchangeable cations, CEC, and ECEC. The amendments were either reagent grade CaCO₃ (lab-grade) or a local calcareous sand and were added by a percent volume basis. The 24 bags of air dried sand mixed with amendment were sub-sampled for soil analysis.

 ^x Reagent-grade calcium carbonate CaCO₃ (C64-500, CAS# 471-34-1, Fisher Scientific, Pittsburgh, PA).

 ^y Local calcareous sand with a CaCO₃ percentage of 11% determined
gravimetrically (5,15).

Procedures Measuring Exchangeable Calcium Concentration

Five different extraction techniques for measuring exchangeable calcium listed in Table 3 were used to measure the exchangeable Ca concentration from the 24 different manufactured sand samples described above. The procedures were performed as follows. The ammonium acetate (NH₄OAc) pH 7.0 and 8.1 used similar procedures. The extraction solutions for each were prepared according to referenced methods by Suarez (18). A 5-g silica sand/amendment sample was weighed into a 50-ml polypropylene centrifuge tube to which 33 ml of extracting solution was added. The tubes were then shaken for 30 min, centrifuged at 2000 × g for 10 min, and then the supernatant was filtered through a Whatman 42 paper into a 100-ml volumetric flask. The process was repeated two more times, and the extractant was brought to 100-ml volume. Exchangeable cations were determined by using inductively coupled argon plasma emission (ICAP) techniques (4,8) on an IRIS/AP Duo (Thermo Jarell-Ash, Franklin, MA) with a charged injection device (3).

Table 3. List of methods used to determine exchangeable cations of 24 sand mixed with amendment samples.

The NH₄Cl solution was produced according to Suarez (18). Then, extraction of the exchangeable cations followed the same procedure as the NH₄OAc method, above. Five grams of silica sand/amendment was equilibrated three times with 33 ml of NH₄Cl solution, centrifuged, and filtered for a final volume of 100 ml. Extractable cations were measured from the collected extractant by the ICAP techniques listed above. After which, sub-samples of the extractant were analyzed for alkalinity and sulfate concentrations. The sum of the alkalinity and sulfate was related to the Ca dissolved from CaCO₃ and CaSO4, respectively, and is subtracted from the exchangeable Ca concentration.

The Mehlich 3 solutions were produced according to the procedure detailed by Mehlich (7). A 2.5-g silica sand/amendment sample was weighed into a 50-ml polypropylene centrifuge tube to which 25 ml of Mehlich 3 extracting solution was added. The tubes were shaken for 15 min, centrifuged at 2000 × g for 5 min, and filtered through a Whatman 42 paper. Exchangeable cations were determined by the ICAP analysis techniques described above.

The water extraction procedure was followed according to the method published in the Soil Analysis Handbook of Reference Methods (16). A 5-g silica sand/amendment sample was weighed into a 50-ml polypropylene tube to which 25 ml of DI water was added. The tube was then shaken for 30 min, allowed to rest for 10 min, and then filtered through a Whatman 42 paper. Soluble cations were analyzed by using the ICAP methods described above.

Effects of Increasing Levels of Calcium Carbonate on Measuring Exchangeable Calcium Concentration

The effects of increasing levels of CaCO₃ on the five different soil test techniques for measuring exchangeable calcium are presented in Figure 1 and Table 4. Much more Ca was dissolved when the silica sand was mixed with reagent grade CaCO₃ compared to mixing with calcareous sand. The extractable Ca concentrations from sands amended with reagent grade CaCO₃ were nearly double compared with Ca concentrations from sands amended with calcareous sand (Fig. 1). This is to be expected and is attributed to particle size and purity. The lab grade CaCO₃ was a finely ground pure powder, whereas the sand had a much larger particle size and the individual particles of sand-based CaCO₃ probably contained impurities, both of which could have reduced dissolution rate.

Fig. 1. Extractable Ca measured from different soil extraction methods of sand samples mixed with increasing volumes of CaCO3 from either reagent-grade CaCO3 (top) or calcareous sand (bottom). NH4OAc pH 7.0 and 8.1, Mehlich 3 and NH4Cl all measure exchangeable Ca, whereas the Water Extract method only measures the solution and soluble phase. Symbols represent actual measured values. Lines represent regression curves. Regression equations and correlation coefficients are presented in Table 4.

Table 4. Regression equations and correlation coefficients from the nonlinear (NLIN) procedure of Statistical Analysis Software (SAS Institute Inc., Cary, NC) of the sand samples mixed with increasing volumes of either reagent grade CaCO₃ or calcareous sand to quantify the effects of CaCO₃ on different soil testing methods.

The different extractants affected the solubility of CaCO₃ in different magnitudes (Fig. 1). Small increases in CaCO₃ content resulted in large changes in measured exchangeable Ca.

The average nutrient concentration recorded using the Water Extract procedure was considerably lower than the extractable cation concentrations (Fig. 1 and Table 4). Moreover, the nutrient concentrations from the Water Extract procedure did not directly correlate with the extracted nutrient concentrations.

Procedures for Measuring Cation Exchange Capacity

Six procedures were used to estimate cation exchange capacity (Table 5). Subsamples were taken from the 24 manufactured sands for this experiment. The exchangeable cations determined from the NH₄OAc pH 7.0 and 8.1, NH₄Cl, and Mehlich 3 extractions were summed together to estimate CEC values. It is common for soil testing labs to estimate the CEC by summing the basic cations, calcium, magnesium, and potassium that were measured in the soil testing procedure with an estimate of exchangeable hydrogen obtained from the buffer pH. Two more procedures that utilize double extractions were used for a more direct CEC measurement: CaCl₂/Mg(NO₃)₂ (19); and NaOAc-NaCl/Mg(NO₃)₂ (12).

Table 5. List of methods used to determine cation exchange capacity (CEC) and estimated cation exchange capacity (ECEC) of the 24 sand mixed with amendment samples. ECEC determined by summation of exchangeable basic cations.

The procedure described by Sumner and Miller was performed as follows: a 5-g sample of the silica sand/amendment was placed into a 50-ml polypropylene centrifuge tube to which 33 ml of the 0.2M CaCl₂-0.0125M CaSO₄ saturating solution was added. The tubes were shaken for 15 min, centrifuged for 5 min, and then the supernatant was discarded. This process was repeated two more times. The tubes were then filled with 33 ml of 0.2M Mg(NO₃)₂, shaken for 15 min, centrifuged at 2000 × g for 5 min, and the supernatant was filtered through a Whatman 42 paper into a 100-ml volumetric flask. This process was repeated two more times and the final extractant was brought to a 100-ml volume. Calcium was determined from the extractant by ICAP analysis following the techniques listed above. To correct for the amount of CaCl₂ solution remaining in the sand sample between the saturating and extracting steps, chlorine was determined by titration with silver nitrate (AgNO₃) using potassium chromate (K₂CrO₄) as an indicator.

The NaOAc-NaCl/Mg(NO₃)₂ (12) procedure consisted of placing 5 g of the silica sand/amendment sample into a 50-ml polypropylene centrifuge tube. Then 33 ml of the 0.4M NaOAc-0.1M NaCl, 60% ethanol, pH 8.2 saturating solution was added. The tubes were placed on the shaker for 15 min and centrifuged at 2000 × g for 5 min. The supernatant was discarded, and the process was repeated two more times. After which, 33 ml of 0.5M Mg(NO₃)₂ extracting solution was added. The tubes were shaken for 15 min, centrifuged for 5 min, and filtered through a Whatman 42 paper into a 100-ml volumetric flask, and repeated two more times. The resulting extractant was brought to volume and Na was determined by the same ICAP techniques describe above. To correct for the amount of entrained NaOAc-NaCl solution, chlorine concentration was determined by titration with silver nitrate (AgNO₃) using potassium chromate (K₂CrO₄) as an indicator.

Effects of Increasing Levels of Calcium Carbonate on Measuring Cation Exchange Capacity

The effects of increasing levels of CaCO₃ on the six different soil test techniques for measuring CEC are presented in Figure 2. When measuring the CEC by double extraction techniques (CaCl₂/Mg(NO₃)₂ and NaOAc-NaCl/Mg(NO₃)₂), the CEC ranged from 1.1 to 2.0 cmolc/100 g and 1.6 to 3.0 cmolc/100 g, respectively. The CEC values from these double extraction methods were not affected by the type of carbonate source added to the sand samples in this experiment; therefore, the double extraction methods would more accurately measure CEC than current soil testing procedures that estimate the CEC by summing the exchangeable cations.

Fig. 2. Plots of the average CEC (top) from CaCl₂/Mg(NO₃)₂ and NaOAc-NaCl/Mg(NO₃)₂ and CEC from Summation (ECEC) (bottom) from NH₄OAc ph 7.0, NH₄OAc pH 8.1, Mehlich 3, and NH₄Cl analyses of sand samples with increasing CaCO₃ levels from reagent-grade CaCO₃ (left) and from calcareous sand (right). Regression equations and correlation coefficients calculated from the nonlinear (NLIN) procedure of Statistical Analysis Software (SAS Institute Inc., Cary, NC).

The CEC values created by summing the exchangeable cations measured from Ammonium Acetate pH 7.0 and 8.1, NH4CL, and Mehlich 3 procedures that were performed in the above experiment increased with increase CaCO₃ and also differed greatly between the two types of carbonate sources (Fig. 2). As expected, the CEC values increased proportionately to the increase in exchangeable Ca due to the dissolution of the CaCO₃.

Recommendations

Since there is a great potential for CaCO₃ dissolution, Mehlich 3 should not be used to measure exchangeable cations or CEC of calcareous sand samples. The NH₄Cl method (18), which makes corrections for the amount of dissolution, produced exchangeable calcium concentrations that were lower compared to NH₄OAc pH 7.0 and Mehlich 3. But, due to the labor involved with several post-extraction procedures needed to make the alkalinity and sulfate corrections, it is doubtful that many routine soil-testing laboratories will use this procedure.

Raising the pH of the industry standard NH₄OAc pH 7.0 procedure to a pH of 8.1 to limit CaCO₃ dissolution is recommended for arid soils (18). Of the procedures tested in this paper, NH₄OAc at pH 8.1 is the procedure that offers the least dissolution of CaCO₃ and is most commercially available. It is not the best method, but one that can be offered my most laboratories without a difficult change in operations. There is still some dissolution with the pH 8.1 procedure and the turf manager should be aware of it when he/she interprets the results for exchangeable cations and CEC. More work needs to be done to develop soil test procedures that will more accurately measure exchangeable cations of calcareous sands.

The effect of CaCO₃ dissolution on CEC was nearly negligible when using a double extraction CEC technique like CaCl₂/MgNO3 or NaOAc-s/Mg(NO₃)₂ compared to creating a CEC by summation of extractable cations. However, these procedures are more complicated and expensive and will likely only be needed when studying samples for research or soil classification.

When it comes to CEC, it is important to know the CEC of the soil, whether it is a small CEC or large CEC. The CEC determines the amount of nutrients a soil can hold and can have large implications when dealing with fertility programs and leaching of nutrients into ground water. However, the manager should not get caught up in worrying as to whether the sand has a CEC of 2.1 or 2.5, but just know that his or her sand has a low CEC and manage it accordingly. Managers of low CEC sands should use appropriate fertilizing techniques like light fertilizer applications or spoon feeding, and/or using slow release fertilizers. Furthermore, the CEC is also used when making cation saturation ratios. Much research has been done that shows that most crops and turfgrass can thrive on a wide range of cation ratios and the Basic Cation Saturation Ratio (BCSR) theory doesn’t apply (6,13,14,17). When dealing with nutritional status and CEC values from calcareous sands, it is also important to test soils annually, at a minimum, at the same laboratory and monitor the trends from year to year. Sending samples to many different laboratories and at random time intervals will make following fertility trends more difficult. Don’t get caught up in the numbers game. Sand based soils have low cation exchange capacities. Treat them as such, with light frequent fertilizer applications and use slow release fertilizers to minimize the potential for leaching.

Since the water analysis method only analyzes the soluble and solution phase elements, nutrient concentrations from water extraction techniques are going to be very small compared to exchangeable nutrient concentrations derived from chemical extractions. Moreover, the solution and soluble portions of nutrients in the soil are going to change easily and rapidly throughout the season, due to fertilizer, irrigation and rainfall inputs. The water analysis does not measure the nutrients that are slowly soluble or the nutrients held on the cation exchange sites, both of which can be large reserves of nutrients for the plant. Furthermore, it has become common to make cation ratios from water analysis results. Making ratios of small numbers carries with it the risk of overestimation. Small changes in concentrations due to seasonal variability or differences in testing procedures will make large changes in the resulting ratios. It can be misleading when using water analysis as the sole means of estimating the nutritional status of sand-based rootzones, since water analysis techniques result in such small concentrations, they do not access nutrients held on the cation exchange sites or the nutrients that are only slowly soluble, and they are easily influenced by fertilizer, irrigation, and environmental conditions (1).

Pure silica sand samples can potentially be analyzed with any procedure studied in this research. But, if the silica sand sample has a possibility of containing any carbonates, then they should be treated as calcareous sands and analyzed and interpreted as such.

It is clear that the results from analysis of calcareous sands are affected by the concentration of CaCO₃. The higher the concentration of CaCO₃ in the sand, the more misleading the exchangeable Ca, CEC, any cation ratios, and cation saturation percentages will be. If the soils you manage are calcareous, use the results presented here to help guide you in choosing your testing laboratory and interpreting your results. Choose laboratories that are experienced in processing and interpreting soil tests from turfgrass areas based upon calcareous sands. Develop a relationship with your laboratory and find out which methods they use.

Good fertility programs are based upon good soil and tissue test results. More importantly, quality fertility programs are based upon a history of several seasons of test results.

Literature Cited

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