Rapid Determination of the pH Buffering Capacity of Soils

1. Rapid Determination of the pH Buffering Capacity of Soils David E. Kissel University of Georgia Athens, GA

2. Why is pH buffering capacity important? • It can be used in making ag lime recommendations for acid soils. • It can potentially be used in making S recommendations for decreasing the pH for some soils (example, blueberry). • It is a valuable soil property for estimating pH changes from some nitrogen reactions in soils such as urea hydrolysis, nitrification, or soil retention of ammonia.

3. Soil pH Buffering Capacity can be expressed in units of meq H+ (kg soil)-1 (pH)-1multiplied by 50 mg CaCO3 (meq)-1 it can be expressed in units ofmg CaCO3 (kg soil)-1 (pH)-1

4. Example of Soil pH Buffering pH -2 3 2 1 0 -1 meq H+ (kg soil)-1 pH-1 pH meq H+ (kg soil)-1 pH-1

5. Acid/non-acid cations vs. soil pH pH - 3.5 H+ Soil Al3+ pH – 5.3 pH - 7.0 Ca2+ H+ Ca2+ Al3+ Mg2+ Mg2+ Ca2+ K+ Soil Ca2+ Soil Al3+ Mg2+ Ca2+ Ca2+ K+ Al3+ Al3+ Ca2+ Mg2+ K+ BS = 0% BS = 50% BS = 100%

6. Soil pH Buffering Capacity can be expressed in units of meq H+ (kg soil)-1 (pH)-1multiplied by 50 mg CaCO3 (meq)-1it can be expressed in units ofmg CaCO3 (kg soil)-1 (pH)-1

8. The UGA titration method employs two pH measurements • The first measurement is soil pH prior to adding calcium hydroxide (pH is measured in 0.01M calcium chloride, rather than in water). • The second pH measurement is taken after adding a measured amount of calcium hydroxide and 30 minutes equilibration.

9. How we do this operation? LBC = mg CaCO3/kg/dpH = (2.7 ml X 0.047 X 50) /0.020 kg/(6.21-5.27) = 338 mg CaCO3/kg/pH

10. Issues with Single-Addition Titration • The equilibrium pH was not reached after 30 minutes of equilibration. • By depending only on a stirrer to mix the soil with the added base, the stirrer motors and stirrers must be robust to keep all soil in the cup moving to interact with the added calcium hydroxide.

11. To address the lack of equilibrium, we adjusted the recommendation equation lb/a ag lime = LBC X [target pH-(measured pH-0.6)] X 2 X 1.5 X (soil depth/6)

12. Objectives (John Thompson) ● Determine the time of Ca(OH)2 reaction needed to reach an equilibrium LBC. ●Determine the relationship between the LBC from 30 minutes equilibration and the LBC at the time when a true equilibrium pH is reached.

13. Materials and Methods • Soils were identified and bulk samples collected from three geographic provinces in Georgia • Ridge and Valley (northwest GA) • Coastal Plain • Atlantic Coast Flatwoods • Soils with high organic matter content were obtained from North Carolina

14. Materials and Methods (cont’d) • Soils were dried overnight, then ground, sieved (2 mm), and stored in Ziploc freezer bags • Soils were analyzed in groups of five due to tray size • Each tray can hold 40 samples: 5 wide (columns) x 8 deep (rows)

16. Results

17. LBC = mg CaCO3/kg/dpH = (2.7 ml X 0.047 X 50) /0.020 kg/(pHtr-pHck)

18. LBC = mg CaCO3/kg/dpH = (2.7 ml X 0.047 X 50) /0.020 kg/(pHtr-pHck)

19. Summary • All soils were near equilibrium by 96 hr with respect to their LBC. • Ratio of LBC 96 hr/LBC 0.5 hr for Soil 15 and Soil 4 were 1.8 and 2.7 respectively. Most of the other soils were within this range. • Short term incubations are being carried out to test the validity of the new lime equation. • Based on our incubation results we will be able to improve our rapid method for soil pH buffering capacity used to make lime recommendations.

20. Summary • LBC equilibr. = LBC 30 min X approx. 2 • The LBC equilibr will be used to calculate the lime requirement without using the added factor of 0.6.

21. mg/kg= mg/kg/pH X (delta pH)where (delta pH) is (target pH-measured pH)

22. Present Equationlb/a ag lime = LBC30min. X [target pH-(measured pH-0.6)] X 2 X 1.5 X (soil depth/6) New Equation lb/a ag lime = LBCEquil. X [target pH-measured pH] X 2 X 1.5 X (soil depth/6)