Soil pH and Lime Rate
Jarrod O. Miller, Ph.D.University of Maryland Extension
Somerset County
• Neutralizes acidity
• Reduces toxicity of Al and Mn
• Adds Ca and Mg
• Improve soil biology
• Improve pesticide efficacy
What are the benefits of lime?
• Pure water has H and OH in equal concentrations
• Any addition of H+ makes the solution acidic
• Any addition of OH- makes the solution alkaline
• pH = Concentration of Hydrogen• Then why does a lower pH mean more acid (H)?
What is pH?
H2O ↔ H+ + OH-
What is pH?
Brady and Weil, 2000
pH = - log10 [H]
x10
SORRY IF YOU FIND THIS BORING, CUZ I DON’T.
What causes acidity in soils?
• H – Hydrogen• Al – aluminum hydrolysis (splits water)
• Fe – iron to a small extent• Ca, Mg, K, and Na are all considered bases
• They don’t split water
Al+3 + 3H2O Al(OH)3 + 3H+
Al and Fe can be related to soil mineralogy and weathering!H can come from manure and fertilizers!
Comparing soil pH
Brady and Weil, 2001
CO2 + H2O
↔
H+ + HCO3-
Most of our agricultural soils: pH 5 to 7
What causes soils to be acidic?
Due to weathering
• Minerals breakdown and release Al3+
• Basic cations leach out of the soil before Al3+
• Al > Ca > Mg > K > Na
Warm Wet
Climate
Cool Climate
The Eastern US gets more rainfallTherefore more leaching and weathering
Soil Types Precipitation
USDA Oregon State
Roadside Geology
Parent material affects1. Texture – sand, silt, clay2. Mineral weathering (i.e.
nutrients available)
Soil test reports have two pH’s!?!
• pH – acidity active in the soil water• Buffer pH – acidity held in reserve on the soil
Why do we need buffer pH?
• With a regular pH we only measure acidity dissolved in the water -“active”
• The pH the crop and soil biology sees• Indicates toxicity of Al and micronutrient
availability
• Buffer pH looks at acids held by the soil – “reserve”• Exchangeable (CEC)• Indicates lime additions needed
H
pH p
robe
H H
H
H
H
Al
HAl
H
H
CEC helps explain reserve acidity
• Soils have negative charge (clay and organics)• Many soil nutrients (Ca+2, Mg+2, K+1, Na+1) and acids (H, Al)
have positive charges (cations)• Therefore, acidity can be absorbed to the soil and dissolved
in the soil water
Ca
K
H
HH
Mg
Soil water
Exchange--
---
-
- Ca
Ca
Mg
KNa
H
Ca
H
-
Soil particle
Buffer pH vs Buffer Capacity
• Buffer capacity is related to CEC – the amount of buffering• Soils with higher CEC have greater “buffer capacity”
• Slows rapid pH change up or down – good for soil biology• Greater buffer capacity should correlate to lower buffer pH!
H
Mg
CaAl
K
HCa
Mg
K
Base Saturation
• Bases – Ca, Mg, K, Na• Acids – Al, H (reserve)
--
---
-
- H
Al
Mg
HCa
KNa
Al
-
--
---
-
- Ca
Al
Mg
KCa
KCa
H
-50% B.S. 75% B.S.
Lower pH
High CEC vs Low CEC
--
---
-
- H
Al
Mg
HCa
KNa
Al
-
--
---
-
-
Al
Mg
Ca
KCa
H
-50% B.S. 50% B.S.
--
----
--
Mg
Ca
KNa
H
Al
H
Al
Al
H
Same amount of lime has different effects
Soil with low CEC can quickly become
over limed
= lime added
Variation in CEC occurs with pH
Mostly due to organic matter, some due to oxides
Pratt and Bair, 1962 Helling et al., 1964
Whole soil CEC with pH CEC due to organic matter and clay
Buffer capacity
• Al/Fe oxides buffer low pH• Carbonates buffer alkaline soils• CEC buffers neutral pH
Brady and Weil, 2001
How is buffer pH measured?
• Soil is placed in a buffer solution with alkaline pH
• Soil acidity reacts with the buffer
• Lowers the solution pH
Solution pH = 8.0
Ca
Ca
Mg
K
Na
H
Ca
H
Therefore, lower relative buffer pH = more soil acidity
Soil variability = different buffer pH tests
Sims, 1996
Ask what kind of buffer pH the lab runs (Check each lab)
• Shoemaker-McLean-Pratt Single Buffer (SMP)• Meant for soils with high lime requirements but low organic
matter (OM) content of <10%. Based out of Ohio soils.• Underestimates in high OM soils
• Adams-Evans• For soils with low CEC and low OM• Good for extremely small differences • May over-estimate pH dependent acidity
• Mehlich Single Buffer• Good for reducing exchangeable acidity (Al) at pH 5.5• Best for soils with low CEC• What about crop needs?
Each method will have tables to determine the lime addition
All tests have their own calibrations and tables
Sims, 1996
Brady and Weil, 2001
Target pH is important!
Adams-Evans lime table for target pH 6.0
• You may have a recommendation related to your crop type
• Target pH become very important
You have your buffer pH, now what?
We don’t need a pH 7 for crops
• Al is toxic to plant roots below pH 5.5• Hydrogen isn’t an issue until below pH 4.0• Mn and Fe can also be toxic
• You may lose access to micronutrients when pH is too high
• Organic matter can bind Al, so you can have a lower pH in highly organic soils
• pH can go as low as 5.0 without Al problems!
Target pH can be related to crop response(not just Aluminum toxicity)
Penn State
Target pH can be related to P availability
Target pH can also be related to micronutrients
Micronutrient availability varies with pH
Predicting micronutrients is tough
• It’s easier to figure out how to limit Al toxicity
• Soil type and texture may predict micronutrients availability/toxicity
Soil Mn levels necessary at different pH
Camberato, 2000 – Clemson Extension
Low pH causes toxicity though!
Fe/Al oxide coatings on soil are only a problem if they dissolve
pH drops below 5.5
Dissolved Fe and Al
Mn can be toxic in soils at low pH, but we probably don’t have high enough concentrations
Sandy vs Clay
Recommend pH 6.0
• Low CEC and nutrient holding
• Low weathering and release of nutrients
• Low micronutrients
Recommend pH 6.5
• Higher CEC• May weather faster• Greater micronutrients
Soil tillage and mixing
pH 7
pH 6.5
pH 6.0
0-2 inches
2-4 inches
Soybean with Mn deficiency
Soil tillage and mixing
• Soil and leaf tissue were deficient in Mn
• Leaf was deficient in Cu, soil was not
Soil pH = 7.0
What does Cu deficiency look like?
http://www.atpnutrition.ca/plant-nutrition/copper/
Jarrod Miller
Do I have a lime issue?Scout your fields and take tissue samples
Jarrod Miller
Jarrod Miller
Mn or Fe?
Nebraska
• Interveinal chlorosis on upper leaves
• Dark green veins
• Brown necrosis when severe
• Occurs on high pH soils
• Interveinal chlorosis on upper leaves
• Dark green veins
• Brown necrosis when severe
• Occurs on high pH soils
Nebraska
Mn or Fe?
Purdue
Michigan State
Virginia Tech
Iowa State
Pioneer
Mississippi State
Evaluating the Lime Requirement
• Lab tests are not equal for all soils
• Understand soils have varying amounts of acidity
• Buffer capacity will vary with texture and organic matter
• Scout your fields when limed
• Give lime plenty of time to react before planting
Questions?