1

Soils of Oahu

Jonathan DeenikAssistant Specialist, Soil Fertility

Department of Tropical Plant and Soil Sciences

Concepts in Soil Fertility

Outline• Concepts in Soil Fertility

- Clays - pH- CEC - N & P

- Organic matter - Al & Mn toxicity

• Soils of Oahu

2

Air and water: C, H, O

Macronutrients: N, P, K, Ca, Mg, S

Micronutrients: B, Cu, Fe, Mn, Zn, Mo, Ni, Co, Cl

Essential Plant Nutrients

Soil Factors Affecting Plant Nutrients

Type and amount of claypHOrganic matterWater

3

Idealized Soil Composition

25% H2O

25% Air

45% Mineral

5% Organic Matter

50% Solids 50% Pores

Clay is Where the Action is!

Clay Properties:Microscopic size (<0.002 mm)

Extremely high surface area- water retention- chemical reactions- biological activity

Clay surfaces carry charge (-/+)

4

In Hawaii Type of Clay is Critical2:1 layer silicates

1:1 layer silicates

oxides

Clay MineralogyMontmorillonite

Properties:

• Expanding• Constant charge (-)• High CEC (80-120 cmolc kg-1)• High surface area• Sticky

5

Clay MineralogyClays

Properties:

• Non-expanding• pH dependent charge• low CEC (1-10 cmolc kg-1)• Relatively low surface area• Non-sticky

Kaolinite

Clay Mineralogy

Al & Fe Oxides

Properties:• Non- expanding• pH dependent charge• Very little CEC • High anion retention• Non- sticky, form strong aggregates

6

Cation Exchange CapacityCEC

Negatively charged sites that adsorb cations: Ca2+, Mg2+, K+, NH4+

Surface ChargepH- Dependent Charge• Net Negative or net positive• Charge determined by H+/OH-

• Oxide- rich soils have + charge at low pH

Protonation De-protonation

7

Organic Matter and CECOrganic matter has a high CEC- OM high in surface horizon- CEC always higher in surface soil

Weathering Intensity and Clay Type

Smectite Kaolinite Oxides

8

Soil AcidityNatural Sources of Acidity:

Precipitation and cationleaching Carbonic acid and organic acidsOrganic matter

Human Induced Acidity:

Acid rainUreaAmmonium fertilizersMono and diammonium phosphateElemental S

Soil pH and Nutrient Availability

Soil pH controls nutrient solubilityIdeal range 6.0-6.5CEC decreases at low pHP fixation increases at low pH

9

P-Fixation

P-FixationChemical bonding of phosphate with Aluminum/iron on clay surfacesHigh in weathered soils rich in oxides (Koolau soils and Central Oahu)P-fixation increases as soils become more acidHigh P-fixing soils require high P inputsOrganic matter inputs reduce fixation capacity

10

Aluminum Toxicity

At pH below 5.0 Al solubility increasesUltisols most likely to have Al toxicity under acid conditionsSoils acidified by pineapple production may be prone to Al toxicityCa deficiency may be the more serious limitationLiming (CaCO3/CaSO4) and/or organic matter inputs alleviate Al toxicity

ManganeseA mineral in basalt Mn2+ is an essential plant nutrient, but at high concentrations it becomes toxicMn2+ concentration depends on pH, O2availability and organic matter• As Soil pH decreases Mn toxicity increases• As Oxygen is depleted (saturated soil) Mn

toxicity increases• Adding organic matter increases Mn toxicity

increases

11

Soils with Potential Mn ToxicityOxisols exisiting at low to moderate elevation (200-750 ft) with moderate rainfall (20-60 in/yr)Molokai, Lahaina,

Wahiawa (1.5% MnO2) series

Soil SeriesWahiawaLahainaMolokaiEwaWaialua

Organic Matter

Humus

12

Importance of Organic Matter in Soils

Physical Properties- Improves aggregation (glue)- Improves water holding capacity (surface area)

Importance of Organic Matter in Soils

Chemical- Increases nutrient availability (N cycling, P and

micronutrient solubility)- Increases CEC (200 cmolc kg-1)- Buffers the soil against pH changes

N mineralizationConversion of organic N to inorganic N

13

Importance of Organic Matter in SoilsBiological- N fixation- Increases microbial diversity- Assists in pathogen suppression

Microbial Functional GroupsBacteria- decomposers, primary players in N, P and S cycling- Actinomycetes act on more complex compounds to

form humus - fundamental to composting

Fungi- Decomposers, attack lignin- Nutrient acquisition (mycorrhiza)

Protozoa and Nematodes- Consume bacteria and fungi releasing plant nutrients

(N)- Activity increases decomposition rates

14

Nitrogen Dynamics

MineralizationMineralization: microbial conversion of organic N into plant available inorganic forms (NH4

+, NO3-)

Wahiawa

0 10 20 30 40

N M

iner

aliz

ed (m

g kg

-1)

0

20

40

60

80

100

120

140 Molokai

Weeks

0 10 20 30 40

Waialua

0 10 20 30 40

Nitrogen Dynamics

ImmobilizationImmobilization: Conversion of inorganic N to organic N by microbes

soil

soil + coconut leaves

15

DenitrificationDenitrification: Conversion of NO3- into NxO

gasesN gases

Denitrification occurs when:

Soils are saturated and lack oxygen

Denitrification rate increases with temperature and organic matter content

Soil Fertility Depends on:

Amount of clayType of Clay Organic Matter

Fertile SoilsSlightly acid to neutral pHHigh CECHigh organic matter

Infertile SoilsAcid to strongly acid pHLow CECHigh P fixation

16

Summary

Clays interact with soil nutrientsSoil pH affects nutrient availabilityOrganic matter makes a differenceSoils vary on the landscapeIf we know our soils we can manage them well

17