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¶ Introduction
This Crop File discusses use of the buffer pH method to make agricultural lime rate recommendations. It also discusses the rationale ServiTech Laboratories used for adopting the Sikora-1 and Sikora-2 methods.
¶ Buffer pH Background
ServiTech lime recommendations depend on the buffer pH method. Other lime requirement methods are available, like titration or a second “double-buffer” measurement, but they tend to be more expensive or time-consuming.
The “single buffer” pH measurement is used to estimate the amount of reserve acidity in a soil sample. The soil pH is measured first in a 1:1 mixture of soil and deionized water.
If the pH is below an acceptable level, a buffer solution with an alkaline pH (e.g., pH 7.5) is added to the slurry mixture. The mixture is allowed to equilibrate, and the pH measured a second time.
The alkaline buffer solution will neutralize a portion of the soil acidity during the equilibration step. The difference between the buffer solution pH and the second pH measurement of the soil and buffer solution slurry is then used to calculate the lime requirement.
The greater the difference between the two pH values, the greater the amount of acidity, and the greater the lime requirement.
Lime requirements are calibrated to buffer pH values using laboratory incubation studies. These studies compare the soil pH response using different lime rates to the measured buffer pH in a set of soil samples.
The recommended lime rate can then be calculated from the single buffer pH value as illustrated in Figure 1. The measured soil pH is not included in the lime rate calculation.
¶ SMP and Sikora-1 Methods
ServiTech originally used the SMP buffer pH method, the most widely used method in the U.S. at the time. The SMP solution contains potentially hazardous materials that had to be stored and disposed as hazardous waste, an expensive process.
The SMP method has been largely replaced by the Sikora-1 buffer pH method, introduced in 2006. The Sikora-1 buffer pH values are essentially the same as SMP buffer pH values, but the Sikora solution does not include hazardous materials.
The SMP method was originally designed for soils with significant reserve acidity requiring large amounts of lime (2 tons per acre or more). It was less effective when used in poorly buffered soils, soils with 10% or greater organic mater, and soils with significant levels of kaolinite, aluminum oxides, and iron oxides in their clay fractions.
¶ Figure 1. Lime Recommendations Based on Sikora-1 Buffer pH Method
The SMP method also tended to underestimate lime requirements when the buffer pH was 6.9 or greater. Thus, it was advised not to recommend lime or to extrapolate a lime recommendation with a buffer pH above 6.8, regardless of the measured soil pH.
This creates confusion, for example, when the soil pH is 5.5, the buffer pH is 6.9, and yet no lime is recommended. We found this to be a rather common problem because the SMP (or Sikora-1) method was unable to accurately quantify the comparatively small amount of exchangeable acidity in sandy soils.
¶ Figure 2. Average County Soil pH in Kentucky
¶ Sikora-2 Method: Soil pH
The Sikora-1 method measures the initial soil pH in a 1:1 slurry of soil and distilled water. The 1:1 soil pH using distilled water can be variable, depending on field conditions that affect the soluble salt content. Salts can displace exchangeable H+ and Al3+ from the exchange surface into the soil solution, increasing soil acidity.
Soluble salts oen increase as the soil dries and may decrease with rain or irrigation, so the soil pH may fluctuate during the year and between years.
The pH of a soil with dissolved salts can be lower than the pH of the same soil with litle to no salt. The soil pH can vary from year-to-year and even within the year (e.g., from fall to spring).
We have noted that soluble salt readings oen increase by 0.3 to 0.5 mmho/cm or more when drought conditions prevail. This depresses the 1:1 soil:water pH by 0.4 to 0.5 units or more.
Figure 2 shows the impact of a drought on soil pH levels due to soluble salt accumulation. The drought occurred in Kentucky during 2008. Figure 2 shows the average soil pH in counties affected by the drought compared the palmer Drought Index.
Salt levels may also be elevated following a recent fertilizer application with a resulting effect on the soil pH (see Figure 3). In these situations, the effect of the soluble salts on soil pH may result in recommendations for lime, even though lime is not needed.
This effect can be eliminated by measuring the soil pH in a solution with a standard amount of salt that is slightly greater than the expected saltiest level in most situations. Some soil pH methods use a calcium chloride solution (0.01 M CaCl2) which is about 0.6 units less than the soil:water pH.
The Sikora-2 method uses a potassium chloride solution (1 M KCl) which offsets the soil pH depression from soluble salt accumulations that may occur during drought conditions or due to fertilizer applications.
The initial pH measured in the KCl solution is about 0.9 units less than soil:water pH. We adjust this pH reading and report an equivalent soil-water pH, allowing producers and crop advisers to maintain soil test histories.
¶ Figure 3. Potential Effect of Fertilizer Application to Soil pH
¶ Sikora-2 Method: Lime Recommendation
The Sikora-1 method did not solve the SMP method problem of low pH with no lime recommended. We tried using other buffer pH methods for sandy and poorly buffered soils. The Adams-Evans and Woodruff buffer methods were developed for use in the South and Southeastern U.S. states which oen have poorly buffered (low CEC), low organic mater soils.
We found that these two methods seemed to be more effective on sandy soils, but they were also formulated with hazardous materials requiring special storage and disposal.
Another option we considered for sandy soils was to use a “double-buffer” method. This approach uses the measured difference of results between two different buffer solutions to beter identify the amount of acidity for an individual soil sample. However, existing double-buffer methods were time consuming.
The Sikora-2 method, introduced in 2014, uses two measured points – the soil pH and the buffer pH – to determine the amount of reserve acidity and calculate a lime recommendation for the specific soil sample. This method provides the same benefit obtained with double buffer methods.
This two-point lime recommendation method allows the user to identify any target soil pH. The lime recommendation is then further adjusted by the individual soil pH of the sample.
Figure 4 illustrates the various lime recommendations that would be calculated for various combinations of buffer pH and soil pH, all with a target soil pH of 6.5. This compares to the single lime rate that is based only on the buffer pH value, illustrated in Figure 1.
The Sikora-2 method has seemed to work effectively with our sandy, low CEC soils (4 to 7 mEq/100g) and with our medium-textured to fine-textured soils. The Sikora-2 soil pH and buffer pH reach stability quicker than pH measured in deionized water due to buffering capacity and the electrolytes in the buffer solution. This allows for beter analytical quality and more efficient laboratory operation.
¶ Figure 4. Lime Recommendations Based on Sikora-2 Buffer pH Method
¶ References
Harrold. 2011. Buffer pH methods. Personal communication.2 pg.
Kissel & Vendrell. 2012. Circular 875: Soil pH and Salt Concentration. Univ. of Georgia Coop. Ext., Athens GA. 2 pg.
Sikora, F.J. 2006. A buffer that mimics the SMP buffer for determining lime requirement of soil. Soil Sci. Soc. Am. J. 70:474-486.
Sikora & Howe. 2010. “A New Method for Testing Soil pH”. IPM Training School, 03 March 201,0 Univ. of Kentucky, Coop. Ext. Serv. https://slideplayer.com/slide/5793692/ accessed 20 Feb 2019
Sikora, F.J. 2014 “Sikora-2 Buffer for Lime Requirement” in Soil Test Methods From the Southeastern United States. Southern Coop. Series Bulletin No. 419. Southern Ext. & Research Activity Info. Exchange Group – 6. Clemson Univ., Clemson SC.
Peters, Nathan, & Labowski. 2015. "Chapter 4: pH and Lime Requirement (Revised August 2012)" in Recommended Chemical Soil Test Procedures for the North Central Region, Rev. August 2015. Pub. SB 1001. Missouri Agric. Expt. Sta., Univ. of Missouri, Columbia MO.