Elemental Toxicities in Hawaiian Soils: Myths and Realities Soil Aluminum and Manganese Jonathan Deenik Assistant Specialist, Soil Fertility Department of Tropical Plant and Soil Sciences
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 Aluminum Earth’s Crust
3 Hawaiian Basalts
(Source: Bohn et al., 2001)
Young Soil
Si:Al+Fe:
2.6
4.5
Moderately Weathered Ultisol
4.4
Highly Weathered Oxisol
0.4
Al and Clay Minerals z
Smectite
Brady & Weil, 2004
z
Unstable under leaching conditions and as pH< 6.0
Al and Clay Minerals z
Kaolinite
Brady & Weil, 2004
z z z
Unstable at low pH, < 5.0 Kaolinite 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
Al and Clay Minerals z
Al Oxides (Gibbsite)
Source:http://www.icmab.es/multimetox/docs_lectures/lectures_html/Gale_J/img033.JPG
z z z
Stable at low pH Zero point of charge (pH = 5.9-6.7) Al only dissolves under very acidic conditions
Al and Clay Minerals z
Fe Oxides (Geothite)
Source: www.cnr.berkeley.edu/.../soilmineralogy.html
z z z
Stable at low pH High zero point of charge (pH = 5.9-6.7) Does not readily dissolve
z
Silicate clays are unstable under acidic conditions - Relatively high Si:Al+Fe ratio - Kaolinite releases Al as pH < 5.5
z
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
KCl extractable Al and pH in a Kauai Oxisol
Source: Ayers et al., 1965
Source: Bohn et al., 2001
Weathering Intensity and Al Toxicity
Smectite
Kaolinite
Oxides
Fox et al., 1991
Weathering Intensity and Al Solubility Vertisol (Lualualei)
Si:Al+Fe = 0.69
Soil Conservation Service, 1976
Weathering Intensity and Al Solubility Vertisol (Waihuna) under Pineapple
Si:Al+Fe = 0.62
20% Al saturation Soil Conservation Service, 1976
Weathering Intensity and Al Solubility Mollisol (Pamoa)
Si:Al+Fe = 0.63
15% Al saturation Soil Conservation Service, 1976
Weathering Intensity and Al Solubility Ultisol (Haiku)
Si:Al+Fe = 0.31 0.35
52% Al saturation Soil Conservation Service, 1976
Weathering Intensity and Al Solubility Ultisol (Paaloa) Si:Al+Fe =0.38
86% Al saturation Soil Conservation Service, 1976
Weathering Intensity and Al Solubility Oxisol (Lahaina) - Relatively dry environment
http://ssldata.nrcs.usda.gov/querypage.asp
Weathering Intensity and Al Solubility Oxisol (Makapili) Si:Al+Fe = 0.20 0.12
4% Al saturation Soil Conservation Service, 1976
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% Soil Conservation Service, 1976
Al and Big Island Soils Kohala Waimea
Hamakua
Kona
Puna
Al and Big Island Soils
Waimea
Medial Andisols Waimea Series - Ustand Soil Conservation Service, 1976
Hydrus Andisols Honokaa Series - Udand
Soil Conservation Service, 1976
Al and Big Island Soils
Udifolists Papai
Ustifolists Kekake
Al and Histosols Ustifolists Kekake
Udifolists Papai
http://ssldata.nrcs.usda.gov/querypage.asp
Aluminum Summary z z
z
z
z
At pH below 5.0 Al solubility increases Ultisols most likely to have Al toxicity under acid conditions Soils acidified by pineapple production may be problematic especially if Si content is high (>20%) Ca deficiency may be the more serious limitation Liming (CaCO3/CaSO4) and/or organic matter inputs alleviate Al toxicity
Manganese z
A mineral in basalt - MnO2, Mn2O3, Mn3O4 In solution as Mn2+, Mn3+, Mn4+
z
z
z
Mn2+ is an essential plant nutrient, but at high concentrations it becomes toxic Mn2+ concentration depends on pH, O2 availability and organic matter Mn toxicity depends on:
• • •
Soil pH Oxygen status Organic matter status
Mn solubility and soil pH Source: Hue et al., 1998
Critical level
Mn solubility Depends on O2 Status Mollisol, moderate Mn
Oxisol Wahiawa, high Mn
- O2
+ O2
Source: Porter et al., 2004
Mn solubility Increases with Organic Inputs
Source: Hue et al., 2001
Soils with Potential Mn Toxicity z z
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
z
Kaolinitic Mollisols and Inceptisols in dry environments Keahua (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 Mn Toxicity
Soil Series Wahiawa Lahaina Molokai Ewa Waialua
Hue et al., 1998
Molokai Soils with Potential for Mn Toxicity
% MnO2 0.3 1.49
0.81
Maui Soils with Potential for Mn Toxicity
% MnO2 0.33 0.4
1.38
Manganese Summary z
z
z
z
Mn 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 high Manage Mn toxicity with lime, gypsum, water management, and careful attention to organic inputs
Summary z z
z
z
Tropical soils are not always Al toxic Al toxicity is most common in acid soils with appreciable amounts of Si. As weathering inten sity increases likelihood of Al toxicity decreases In most of Hawaii’s highly weathered soils Ca and P deficiency are the limiting factors Al toxicity is managed with lime and organic matter
Summary z z
z
Managanese toxicity may be a serious problem in Hawaii Mn toxicity occurs in dry lowland soils with high MnO2 content that have been acidified, don’t drain well, and/or received organic inputs Mn toxicity is managed with lime and careful water management
