Why is soil acidification a problem?
The increased concentration of hydrogen ions in soil resulting from soil acidification:
- increases the solubility (dissolution) of any aluminium and manganese in soil; this can induce manganese toxicity in plants and aluminium toxicity can affect growth and function of plant roots
- increases sorption (binding) of molybdenum by soil, inducing deficiency in plants
- is toxic to the Rhizobia bacteria required for symbiotic nitrogen fixation in legumes.
Aluminium toxicity
Aluminium is a component of clays and oxides in soil, and aluminium compounds are present on the surfaces of many soil constituents. At soil pHCa greater than about 5.0, most aluminium is present in sparingly soluble form so there is negligible soluble aluminium in soil solution. However, as soil pHCa drops below 5.0, there is increased solubility of aluminium.
Aluminium has no known role in plants but, when its concentration in soil solution increases, it becomes toxic to plant roots. Aluminium toxicity greatly reduces root growth and function, reducing the ability of the roots to grow in soil and take up water and nutrients. Aluminium toxicity is the major soil acidity problem for agriculture in WA.
Manganese toxicity
In most agricultural areas, manganese toxicity is usually the first problem resulting from soil acidification. Below about pHCa 5.5, solubility of manganese increases as the concentration of hydrogen ions increases. Plant roots take up soluble manganese from the soil solution. As the concentration of manganese increases, plants eventually take up toxic amounts, reducing yield. Fortunately, the amount of manganese present in most agricultural soils in WA is low. Though solubility of manganese continues to increase as soils acidify, the amount of manganese dissolved is insufficient to cause toxicity for pastures.
Testing for exchangeable soil aluminium
Exchangeable soil aluminium is the best measure we have for estimating the potential for aluminium toxicity. The cation exchange capacity of a soil is a measure of the amount of negative charge sites on the soil surface and is estimated by measuring the amount of cations balancing the negative sites.
Cation exchange sites are typically balanced by the major soil cations calcium, magnesium, potassium, sodium and, for acidified soils, hydrogen and aluminium.
The amount of aluminium balancing negative charge, called exchangeable aluminium, is used as an indicator of when aluminium toxicity is likely to reduce plant yield.
The proportion of the major cations that balance the total cation exchange capacity of most productive soils is usually 65 to 80% calcium, 10 to 20% magnesium, 3 to 8% potassium, less than 4% sodium and less than 5% aluminium. If the proportion of aluminium on the soil exchange sites rises to over 30%, aluminium toxicity is highly likely to affect production of even the most tolerant crop and pasture species.
Tolerance of subclover and ryegrass to aluminium toxicity
Subclover and annual and Italian ryegrasses, the major pasture species in WA high rainfall pastures, are all relatively tolerant of aluminium toxicity. Aluminium toxicity becomes a major problem for these species when the soil pHCa falls below 4.3. For more sensitive species, such as lucerne and annual medics, aluminium toxicity becomes a major problem when the soil pHCa falls below 5.0.
Molybdenum deficiency and molybdenosis
Plants require very small amounts of molybdenum and soils with pHCa values greater than 5.0 usually have enough molybdenum for pasture production. Very little molybdenum is sorbed (bound) by soil when the pHCa is greater than 5.0. However, as the pHCa of a soil drops below 5.0, its capacity to sorb molybdenum rises, inducing deficiency in pastures.
When land was newly cleared, molybdenum deficiency only occurred on naturally acidic soils and molybdenum fertiliser was applied. However, if too much molybdenum fertiliser is applied to pastures grazed by ruminants, the animals can develop molybdenosis.
Molybdenosis results from molybdenum reacting with sulphur and copper in the rumen to form a poorly soluble compound. This prevents the animals from taking up sufficient copper from their gut and they become copper deficient. When this happens, animals need to be injected with copper supplements. Also, the animals should be grazed on pastures that are lower in molybdenum.
Molybdenum deficiency is best ameliorated by applying sufficient lime to raise soil pHCa to greater than 5.0. It is nearly always better to lime a soil than apply molybdenum fertiliser.
Effect of soil pH on nitrogen fixation
The major pasture species for high rainfall pastures in WA are the annual species annual ryegrass (Lolium rigidum), Italian ryegrass (Lolium multiflorum) and subterranean clover (Trifolium subterraneum). Subterranean clover is the only one of these three pasture species capable of biological nitrogen fixation.
Biological nitrogen fixation is brought about by Rhizobia bacteria in the soil which ‘infect’ the roots of emerging subclover plants to form nodules. In these nodules, the bacteria convert atmospheric nitrogen to nitrogen compounds required by the clover — mainly amino acids and proteins. The dead remains of subclover plants — roots, stems, leaves, burrs and seed — are processed by soil organisms to release nitrogen to the soil, initially as ammonium ions. These are then converted by soil bacteria to nitrate and hydrogen ions, as previously discussed, to provide nitrogen for ryegrass.
The Rhizobia bacterium which infects subclover (Rhizobia trifolii) is more sensitive to low soil pH than the host plant; when the soil pHCa falls below about 4.5, the bacterial population declines. Eventually, nitrogen fixation cannot occur and clover has to obtain nitrogen from the soil. If there is insufficient soil nitrogen, nitrogen deficiency reduces clover and ryegrass production and persistence. Though fertiliser nitrogen can be applied to overcome nitrogen deficiency, aluminium toxicity greatly reduces the ability of roots to take up water and nutrients from the soil.