Thomas Obreza Soil and Water Science Dept. Univ. of Florida Soil Fertility and pH

Soil fertility: How soils hold nutrients  Organic matter  Cation exchange capacity  Sorption (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 +, Ca 2+, Mg 2+, and K +. 3.Sorption by certain soil components.  Phosphorus.  Movement of a non-retained nutrient like nitrate-N (NO 3 - )is driven by water.

PropertyInfluence or Effect TextureCEC; Sorption Organic matterNutrient storehouse; CEC pHCEC; Sorption Sand grain coatingsSorption Water-holding cap.Movement of soluble nutrients

Organic Matter Nutrient retention mechanism #1

ElementPercentage Carbon50 Nitrogen5 Phosphorus0.5 Sulfur0.5 Other nutrientsTrace 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 K K K Mg H H H H _ _ _ _ _ _ _ _ _ _ _ _ _ Apply ammonium nitrate fertilizer: NH 4 +, NO 3 - Cation Exchange Capacity _ Clay or humus particle _ _ _ _ _ _ _ _ _ _ _ __ + 2H +, K +, NO 3 - H H NH 4 K K Ca Mg

Cation exchange capacity: clay vs. organic matter

Material Approximate CEC (meq/100 g) Quartz  1 – 2 Al and Fe hydrous oxides  ≈ 4 Kaolinite clay  1 – 10 Illite clay10 – 30 Montmorillionite clay 80 – 150 Vermiculite clay 100 – 200+ Organic matter  150 – 500

Soil series CEC meq/100 g % due to clay % due to OM Candler (Central FL Ridge) 3.514%86% Immokalee (Gulf flatwoods) 4.8 6%94% Riviera (Indian River flatwoods) 4.819%81%

Sorption Nutrient retention mechanism #3

Nearly insoluble Fe-P, Al-P Nearly insoluble Ca-P Soluble phosphate H 2 PO 4 - At low pH Fe, Al At high pH Ca pH 6.5 Best P availability Sorption example

Is there a limit to the amount of P a soil 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 sandNon-coated sand

Tavares sandImmokalee sand

Nutrient Ionic form in soil solution Precipitation or sorption? Mobile in sandy soil? NNH 4 +, NO 3 - NoYes PPO 4 3- YesYes/No* KK+K+ NoYes Ca, MgCa 2+, Mg 2+ YesNo SSO 4 - YesYes/No* Cu, Mn, Fe, Zn Cu 2+, Mn 2+, Fe 3+, Zn 2+ YesNo B, Mo, ClH 3 BO 3, MoO 4 2-, 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.

ACIDICBASIC Neutral A decrease of 1 pH unit equals a 10X increase in acidity.

Plant benefit Plant injury Toxicity Deficiency Relative plant yield (%)

Soil pH and nutrient availability

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

Magnesium 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 calcium Neutralizes acidity Raises soil pH Usually 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.

Credits Author: Thomas Obreza Department: Soil and Water Science Photographs: IFAS Communication Services Copyright 2008 University of Florida