Thomas ObrezaSoil and Water Science Dept.

Univ. of Florida

Soil Fertility and pH

Soil fertility: How soils hold nutrientsOrganic matterCation exchange capacitySorption (Sorb = To take up and hold).

Soil pH, liming, and acidification

Soil fertility

Ridge

Flatwoods

1. The ability of a soil to store nutrients in a form easily available to plants.

2. The degree to which nutrient leaching is prevented by a soil.

1. Contained in organic matter (humus).Nitrogen, phosphorus, sulfur, etc.

2. Nutrient Soil electrostatic attraction, a.k.a. “Cation Exchange Capacity” (CEC).

Cations like H+, Ca2+, Mg2+, and K+.

3. Sorption by certain soil components.Phosphorus.

Movement of a non-retained nutrient like nitrate-N (NO3

-)is driven by water.

Property Influence or Effect

Texture CEC; Sorption

Organic matter Nutrient storehouse; CEC

pH CEC; Sorption

Sand grain coatings Sorption

Water-holding cap. Movement of soluble nutrients

Organic Matter

Nutrient retention mechanism #1

Element Percentage

Carbon 50

Nitrogen 5

Phosphorus 0.5

Sulfur 0.5

Other nutrients Trace amounts

For every 1% organic matter in the top 6 inches of soil, N mineralization =About 20 lbs N per acre per year.About ½ lb N per 1000 square ft per

year.

Cation Exchange Capacity

Nutrient retention mechanism #2

Clay or humus particle

Ca

Ca

Ca

KK K Mg

HH

H

H

_

_

_

__

_ __

__

__

_

Apply ammonium nitrate fertilizer: NH4+, NO3

-

CationExchangeCapacity

_

Clay or humus particle

__

_

__

_ __

_

__

__

+ 2H+, K+, NO3-

HH

NH4

KK

NH4

NH4 Ca

Ca

CaMg

Cation exchange capacity:clay vs. organic matter

MaterialApproximate CEC

(meq/100 g)

Quartz 1 – 2

Al and Fe hydrous oxides ≈ 4

Kaolinite clay 1 – 10

Illite clay 10 – 30

Montmorillionite clay 80 – 150

Vermiculite clay 100 – 200+

Organic matter 150 – 500

Soil seriesCEC

meq/100 g% due to

clay% due to

OM

Candler(Central FL Ridge)

3.5 14% 86%

Immokalee(Gulf flatwoods)

4.8 6% 94%

Riviera(Indian River flatwoods)

4.8 19% 81%

Sorption

Nutrient retention mechanism #3

Nearlyinsoluble

Fe-P, Al-P

Nearlyinsoluble

Ca-P

Solublephosphate

H2PO4-

At low pH

Fe, Al

At high pH

Ca

pH 6.5Best P availability

Sorptionexample

Is there a limit to the amount of P asoil can hold?

Yes.

What determines P-holding capacity?

Fe, Al, Ca

Why is sorption important?

Keeps P from leaching.

Can a sandy Florida soil hold P?

Depends on what’s in it.

Coated sand Non-coated sand

Tavares sand Immokalee sand

NutrientIonic form in soil

solutionPrecipitation or sorption?

Mobile in sandy soil?

N NH4+, NO3

- No Yes

P PO43- Yes Yes/No*

K K+ No Yes

Ca, Mg Ca2+, Mg2+ Yes No

S SO4- Yes Yes/No*

Cu, Mn, Fe, Zn

Cu2+, Mn2+, Fe3+, Zn2+ Yes No

B, Mo, Cl H3BO3, MoO42-, Cl-

Mo – Yes

B, Cl – No

B, Cl – Yes

Mo – No

*Depends on soil properties

Soil pH, liming, and acidification

Measure of soil acidity or alkalinity.Importance:

Nutrient availability.Soil microbial activity.

0 7 14

ACIDIC BASICNeutral

1 2 3 4 5 6 9 10 11 12 138

A decrease of 1 pH unit equals a10X increase in acidity.

0

100

4.0 5.0 6.0 7.0 8.0

Plantbenefit

Plantinjury

Toxicity DeficiencyRel

ativ

e pl

ant

yiel

d (%

)

Soil pHandnutrientavailability

Portable pH meter

Causes:Over-liming.Calcareous fill material.Alkaline irrigation water.

Symptoms:Micronutrient deficiencies.

Over-liming

Calcareous soil

High pH construction residue

Calcareous fill material

Alkaline irrigation water

High pH effects

Magnesiumdeficiency

Iron deficiency

Iron deficiency

Measuring soil pH

1:2 soil/water mixture.

Wait 30 minutes.Check pH meter

calibration.Measure pH of

liquid above soil.

Supplies calciumNeutralizes acidityRaises soil pHUsually means calcium carbonate

Acidification

Can’t do much with soils filled with lime rock or shell.If you have to landscape these soils,

choose plants that grow well at high pH.

Marginally-alkaline soils can be acidified with elemental sulfur.

Sandy soils = Low fertility.Improve fertility with organic matter.Soil pH affects nutrient availability.Best pH range is 6.0 to 6.5.Apply lime to acidic soils.Apply sulfur to alkaline soils.It is not practical to acidify soils

containing lime rock or shell.

CreditsAuthor: Thomas ObrezaDepartment: Soil and Water SciencePhotographs: IFAS Communication ServicesCopyright 2008 University of Florida