Acid soils are prevalent in tropical areas and forests.
Topsoils tend to be more acidic than subsoils.
In Western Australia, many soils are below the target pH of 5.5 (in calcium chloride) and critical pH.
Liming is expensive, which prevents some farmers from applying it even when needed.
The critical pH in subsoil is set at 4.5 due to the sensitivity of crops like canola; wheat can tolerate lower pH.
Soil acidification is influenced heavily by soil processes.
High rainfall leads to leaching of basic cations (calcium and magnesium), resulting in acidic soils.
Weathering of soil minerals also contributes to the leaching of calcium and magnesium.
Western Australia's soils are acidic due to weathering and their sandy composition with low organic matter, leading to low pH buffering capacity.
Disturbances in the carbon and nitrogen cycles also affect soil acidity.
Acid rain is less of a problem now due to sulfur scrubbers in chimney stacks reducing sulfur emissions.
Major Problems in Acidic Soils
Hydrogen toxicity can be a problem at very low pH (less than 4), primarily in native areas.
Aluminum toxicity is a major concern in food producing areas with pH between 4 and 5.
Manganese toxicity occurs at pH 5 to 5.5, particularly in areas with manganese-rich parent material.
Leaching of magnesium and calcium is common across the range of acidic soils.
Deficiency of anions such as monohydrogen and dihydrogen phosphate (phosphorus) and molybdate is prevalent.
Root growth and water uptake are inhibited in acidic soils.
Nutrient Availability and pH
Nutrient availability is significantly influenced by pH.
Maximum nutrient availability generally occurs around pH 6.
Micronutrients (metals) are highly soluble and available in acidic soils, potentially causing toxicity, except for molybdenum, which is an anion and has lower availability in acidic conditions.
The term "phosphate" is often incorrectly used to refer to PO4^{3-}. Plants actually take up dihydrogen (H2PO4^-$) and monohydrogen phosphate (HPO4^{2-}$).
The forms of phosphate present depend on pH, with dihydrogen phosphate dominating under acidic conditions and monohydrogen phosphate under slightly alkaline conditions.
H3PO4 (phosphoric acid) is present at extremely acidic conditions.
Practical Effects of pH on Nutrient Availability
Increasing pH levels lead to a decline in manganese concentration.
Availability of manganese increases with acidification, showing a significant difference over two pH units.
Different nutrients behave differently with pH changes; manganese, iron, and potassium uptake vary depending on pH levels.
Oats exhibit relatively strong tolerance to acidity, with only a slight decline in dry weight production under acidic conditions.
Molybdenum availability is problematic in the acidic range; increasing pH improves molybdenum uptake by plants like soybeans, a legume with high molybdenum requirements.
Adding molybdenum can temporarily alleviate the effects of low pH on legumes, but liming is eventually necessary as pH continues to decline.
Liming and Soil Reactions
Calcium containing materials are considered soil amendments rather than fertilizers.
Common liming materials include calcium carbonate, calcium magnesium carbonate, magnesium calcite, and dolomite.
Dolomite can be more effective due to its surface area, despite being chemically less soluble.
Lime application involves replacing aluminum ions on cation exchange complexes with calcium ions.
Aluminum precipitates as gibbsite (Al(OH)_3) as pH increases.
Liming leads to the emission of CO2, a greenhouse gas.
Lime usage in Western Australia has been increasing, but the amount applied is still less than what is needed based on target pH levels.
Lime moves slowly in the soil profile, primarily affecting topsoil pH.
Plant Strategies for Coping with Acidic Soils
Plants can either avoid or tolerate problems associated with acidic soils.
Avoiding stress involves excluding aluminum or manganese uptake.
Efficient nutrient uptake from acidic soils helps plants avoid stress.
Tolerance involves enduring high concentrations of aluminum in plant tissues; tea plants are an example of aluminum-tolerant species, accumulating aluminum in their leaves.
Aluminum intake has been linked to neurodegenerative diseases, such as Alzheimer's, but is more likely a consequence rather than the cause.
Plants may have low internal nutrient demand or be efficient in nutrient retranslocation to survive under stress conditions.
Alkaline Soils
Calcareous, saline, and sodic soils are types of soils with high pH.
Calcareous soils have relatively lower pH compared to saline and sodic soils.
Problems include deficiency of micronutrients and phosphorus, excess sodium and boron, bicarbonate, and issues with water and mechanical impedance.
Sodic soils have poor aeration.
Lupines prefer acidic soils, while fababeans prefer slightly higher pH.
Iron uptake decreases in high pH soils, especially with the presence of bicarbonate.
Managing Alkaline Soils
Washing salts down with good quality fresh water can help, but is often impractical in arid areas where these soils develop.
Adding calcium in the form of gypsum (calcium sulfate) helps replace sodium on the cation exchange complex and acidifies the soil due to the sulfate component.
Adding acids is theoretically possible but not practical due to cost and the need for continuous application.
Adding elemental sulfur allows microorganisms to produce sulfuric acid, which is a cheaper way to decrease pH.
Sandy soils with low clay content require smaller amounts of lime compared to soils with high clay content due to the lower cation exchange capacity.