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
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