elemental toxicities in hawaiian soils: myths and realities

Elemental Toxicities in Hawaiian Soils: Myths and

Realities

Jonathan DeenikAssistant Specialist, Soil Fertility

Department of Tropical Plant and Soil Sciences

Soil Aluminum and Manganese

Outline• Soil Aluminum

• Mineralogy and chemistry of Al• Al and soil weathering• Al toxicity in Hawaiian soils

• Soil Manganese• Chemistry of Mn• Mn toxicity in Hawaiian soils

• Management Strategies

Source: http://www.ga.gov.au/education/minerals/images/edexp3.gif

http://www.solarviews.com/browse/earth/earthint.jpg

Soil Aluminum3 Hawaiian BasaltsEarth’s Crust

Young Soil

Moderately Weathered

Ultisol

Highly Weathered

Oxisol

(Source: Bohn et al., 2001)

Si:Al+Fe: 2.6 4.5 4.4 0.4

Al and Clay MineralsSmectite

Unstable under leaching conditions and as pH< 6.0

Brady & Weil, 2004

Al and Clay MineralsKaolinite

Unstable at low pH, < 5.0Kaolinite has low zero point of charge (pH = 2.0-4.6)Soils rich in kaolinite under leaching environment can be very acidic and release Al3+ into solution

Brady & Weil, 2004

Al Oxides (Gibbsite)

Source:http://www.icmab.es/multimetox/docs_lectures/lectures_html/Gale_J/img033.JPG

Stable at low pHZero point of charge (pH = 5.9-6.7)Al only dissolves under very acidic conditions

Al and Clay Minerals

Fe Oxides (Geothite)

Source: www.cnr.berkeley.edu/.../soilmineralogy.html

Stable at low pHHigh zero point of charge (pH = 5.9-6.7)Does not readily dissolve

Al and Clay Minerals

Silicate clays are unstable under acidic conditions

- Relatively high Si:Al+Fe ratio- Kaolinite releases Al as pH < 5.5

Al & Fe oxides are more stable under acidic conditions

- They have relatively higher zero point of charge so they tend to keep soil pH well above 5.0 as they increase in dominance

• As Si:Al+Fe ratio decreases (more weathered), Al less soluble

Aluminum Solubility and pH

Al stable as Al(OH)3 down to pH 4.5

Source: Bohn et al., 2001

KCl extractable Al and pH in a Kauai Oxisol

Source: Ayers et al., 1965

Weathering Intensity and Al Toxicity

Smectite Kaolinite Oxides

Fox et al., 1991

Weathering Intensity and Al SolubilityVertisol (Lualualei)

Si:Al+Fe = 0.69

Soil Conservation Service, 1976

Weathering Intensity and Al SolubilityVertisol (Waihuna) under Pineapple

20% Al saturation

Si:Al+Fe = 0.62


Weathering Intensity and Al SolubilityMollisol (Pamoa)

15% Al saturation

Si:Al+Fe = 0.63


Weathering Intensity and Al SolubilityUltisol (Haiku)

Si:Al+Fe = 0.310.35

52% Al saturationSoil Conservation Service, 1976

Weathering Intensity and Al SolubilityUltisol (Paaloa)

Si:Al+Fe =0.38


Weathering Intensity and Al Solubility

Oxisol (Lahaina)

- Relatively dry environment

http://ssldata.nrcs.usda.gov/querypage.asp

Weathering Intensity and Al SolubilityOxisol (Makapili)

Si:Al+Fe = 0.200.12


Kauai Ultisols with potential for Al toxicity

Oahu Ultisols with potential for Al toxicity

Maui Ultisols with potential for Al toxicity

Molokai Ultisols with potential for Al toxicity

Hoolehua series and Al toxicity

Si:Al+Fe = 0.54

Al saturation = 37%


Al and Big Island Soils

Hamakua

Puna

Kona

Kohala

Waimea

Al and Big Island Soils Waimea

Medial AndisolsWaimea Series - Ustand


Hydrus AndisolsHonokaa Series - Udand


Al and Big Island Soils

UdifolistsPapai

UstifolistsKekake

Al and Histosols

UstifolistsKekake

UdifolistsPapai

http://ssldata.nrcs.usda.gov/querypage.asp

Aluminum Summary

At pH below 5.0 Al solubility increasesUltisols most likely to have Al toxicity under acid conditionsSoils acidified by pineapple production may be problematic especially if Si content is high (>20%)Ca deficiency may be the more serious limitationLiming (CaCO3/CaSO4) and/or organic matter inputs alleviate Al toxicity

Manganese

A mineral in basalt - MnO2, Mn2O3, Mn3O4In solution as Mn2+, Mn3+, Mn4+

Mn2+ is an essential plant nutrient, but at high concentrations it becomes toxicMn2+ concentration depends on pH, O2availability and organic matterMn toxicity depends on:• Soil pH• Oxygen status• Organic matter status

Critical level

Mn solubility and soil pH

Source: Hue et al., 1998

Mn solubility Depends on O2 Status

- O2

+ O2

Source: Porter et al., 2004

Oxisol Wahiawa, high Mn

Mollisol, moderate Mn

Mn solubility Increases with Organic Inputs

Source: Hue et al., 2001

Soils with Potential Mn Toxicity

Average MnO2 content of soils = 0.1%Oxisols exisiting at low to moderate elevation (200-750 ft) with moderate rainfall (20-60 in/yr)Molokai, Lahaina, Wahiawa (1.5% MnO2) series

Kaolinitic Mollisols and Inceptisols in dry environmentsKeahua (0.4%), Ewa, Paia (1.7%), Hoolehua (1.5%),

kahana series

Makaweli Soil with Potential for Mn Toxicity

0.98% MnO2 in surface horizon

Oahu Soils with Potential MnToxicity

Hue et al., 1998

Soil SeriesWahiawaLahainaMolokaiEwaWaialua

Molokai Soils with Potential for Mn Toxicity

% MnO20.31.49

0.81

Maui Soils with Potential for MnToxicity

% MnO20.33

0.4

1.38

Manganese SummaryMn toxicity occurs in soils with relatively dry climate. Mn toxicity increases as pH drops below 5.5.Mn toxicity can occur in wet soils where organic inputs are highManage Mn toxicity with lime, gypsum, water management, and careful attention to organic inputs

Summary

Tropical soils are not always Al toxicAl toxicity is most common in acid soils with appreciable amounts of Si. As weathering intensity increases likelihood of Al toxicity decreasesIn most of Hawaii’s highly weathered soils Ca and P deficiency are the limiting factorsAl toxicity is managed with lime and organic matter

Summary

Managanese toxicity may be a serious problem in HawaiiMn toxicity occurs in dry lowland soils with high MnO2 content that have been acidified, don’t drain well, and/or received organic inputsMn toxicity is managed with lime and careful water management

elemental toxicities in hawaiian soils: myths and realities

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