New UGA methods for measuring soil pH and the lime requirement

1. New UGA methods for measuring soil pH and the lime requirement David E. Kissel University of Georgia

2. Collaborators on the project • Min Liu (graduate student) • Bob Isaac • Wick Johnson • Rick Hitchcock • Paul Vendrell • Miguel Cabrera

3. We started using the new methods on November 1, 2004

4. How we measured pH and lime requirement before Nov. 1, 2004. • Soil pH was measured in a 1:1 ratio of soil to water with a combination pH electrode while stirring the sample. • For samples with pH less than 6, the Adams-Evans buffer was added and the pH (buffer pH) was read on those samples again. • Both water pH and buffer pH are used to calculate the lime requirement.

6. Soil pH • Soil pH is a measure of the “active” acidity in soil solution • What is measured with a pH meter • Soil pH alone does not tell us much about how much limestone to apply!!!

7. Lime Index • The lime index (buffer pH) is a measure of exchangeable acidity • This is the acidity that must be neutralized in order to increase soil pH • Limestone needs are determined by using both the soil pH and the lime index

10. Why change something that is working?

11. Reasons we changed our method for measuring buffer pH • The Adams Evans method is cumbersome for automation • The Adams Evans buffer contains para nitro phenol • a toxic chemical.

12. The new method employs two pH measurements • The first pH measurement is done in 0.01M Calcium chloride rather than in water. • The second pH measurement will be taken 30 to 45 minutes after adding a measured amount of calcium hydroxide, a very fast reacting form of lime.

13. Why measure pH in 0.01 M calcium chloride? • The measured Soil pH is more stable between years and during the year. • Unusually high pHs during wet Winters are avoided. • pH will not be affected by fertilizer application and by manure application.

14. When soil pH is measured in water, soil pH values are sometimes inaccurate (too high) in years with heavy winter rainfall, because of low salt content in the soil solution.

15. The soil testing period of Fall/Winter 00-01 was such a period, when some extremely high pH values were measured on some samples.We selected some of these soil samples for measurement of pH in 0.01 M calcium chloride.

16. Soil pH in water and 0.01 M CaCl2

17. Salt added from fertilizer application and from soil organic matter decomposition. • Typical nitrogen and potash fertilizer application for cotton or corn will add salt about equal to the 0.01 M calcium chloride. • Soil pH measured in water will be lower after fertilizer application than before. • Organic matter decomposition during summer will add about half that from fertilizer.

18. 120 lb K2O+ 150 lb N/acre (0.006 Molar)

19. What is different when interpreting soil pH values measured in calcium chloride?

20. Differences in pH values • Soil pH values in 0.01M calcium chloride are lower than soil pH values in water. • North Georgia samples were 0.55 pH units lower for a group of samples during Apr-July. • South Georgia samples were 0.68 pH units lower. • Soil pH values are more stable. • Unaffected by wet winters. • Unaffected by fertilizer application.

21. Effect of measuring pH in dilute calcium chloride vs in water

23. Fertilizer and manure application, average Georgia soil pHw wet year average pHw wet year pHw 5.9 dry year ΔpHaverage = 0.6 pHw dry year 5.3 pHCaCl2 January July December

24. Lower pH measured in 0.01 M calcium chloride means that we have to adjust our thinking about pH values.

25. How we operate differently regarding pH • We measure pH in 0.01 M calcium chloride; we report this value and the equivalent water pH. We add 0.6 to the pH in calcium chloride to obtain the equivalent water pH. • A lime recommendation is made for all samples with pH less than 5.4 in calcium chloride (6.0 equivalent water pH).

26. How to understand salt pH

27. Lime recommendations are based on the change in pH following the addition of calcium hydroxide. • Bigger pH changes are for sandier soils with less organic matter, they require less lime. • Smaller pH changes are for soils with more clay and higher in organic matter. They require more lime.

28. The lime requirement is then calculated using the following information. • Target pH • Soil pH • Soil pH after adding calcium hydroxide.

29. New UGA Lime Buffer Capacity Test • Measures the amount of lime needed to raise soil pH to the target pH. • The Lime Buffer Capacity (LBC) is the amount of pure calcium carbonate needed to raise soil pH one unit, ie, from 5 to 6. • The LBC varies for different soils. • Depends on soil organic matter. • Depends on soil clay content.

33. Automation

43. Ca(OH)2 was added to soils equal to the recommendations for target pH 6, pH read 3 days later. Early Jan. 04.

44. Ca(OH)2 was added to soils equal to the recommendations for target pH 6, pH read 3 days later. Late Jan. 04.

45. Take home message • New method will give more stable pH readings during the year and from year to year. • Lime recommendation will be similar for the new and the present procedure, except initially, more samples will receive a lime recommendation with the new method. • Measured pH values will average 0.5 to 0.7 pH units lower for the new method. We will provide an equivalent water pH on the report.

46. Time out for discussion and comments

47. The other part of handling lime management is to sample fields properly. This may mean taking separate samples from different field areas.

48. pH 4.4 pH 5.3 pH 6.0 pH 4.0

50. Consider separately sampling field areas that are distinctively different • Areas that are sandier and lower in organic matter than normal. • Areas higher in organic matter and clay content. • Areas that are lower in elevation.