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?