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Effect of boron, soil acidity and aluminium toxicity on barley
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Barley is very susceptible to the high levels of boron that are often found in duplex soils with an alkaline and sodic subsoil.
Boron toxicity symptoms tend to develop during the latter stages of plant growth. This 'spring flush' of boron may be due to roots coming into contact with higher concentrations of boron in the subsoil, and absorbing relatively higher levels of moisture from this depth (passively absorbing more of boron) during spring.
However, other environmental conditions may influence the pattern of development of boron toxicity in barley crops. Under controlled conditions, moisture stress has been shown to increase the level of boron in the tops of Stirling barley.
Boron toxicity appears as small dark spots, often located more towards the tip and along the margins of the leaf. In some cases these spots have a yellow margin similar to spot-type net blotch but they can be differentiated by their size and location on the leaf. Boron toxicity symptoms should not be confused with physiological leaf spotting or disease symptoms from the net blotch diseases. Table 1 outlines the likely differences in symptoms caused by physiological leaf spotting, boron toxicity and net blotch (net and spot type). It may be difficult to distinguish between the symptoms on varieties such as Hamelin which is prone to physiological leaf spotting, intolerant to high levels boron and susceptible to both net and spot type net blotch.
Table 1. Guide to differentiating between brown spotting on barley leaves.
|Physiological leaf spotting||Boron toxicity||Net blotch|
|Location on leaf||over entire leaf||leaf margins and tips||irregular|
|Yellow margins around leaf||no||sometimes||yes|
Photographs of physiological leaf spotting and the net blotch symptoms can be found in these two Farmnotes:
- Farmnote 62/2005: Hamelin: a new barley variety with superior malting quality for medium to low rainfall zones [PDF 213KB]
- Farmnote 65/2001: Leaf diseases of barley [PDF 201KB]
The occurrence of boron toxicity symptoms vary from year to year, largely depending on seasonal conditions. Boron toxicity occurs mainly in the dryer years in medium to low rainfall zones. In Western Australia, barley crops with boron toxicity symptoms grow almost exclusively on the duplex soils with alkaline and fine-textured subsoils, which were generally non saline but probably sodic.
It is likely that the only available solution to the problem will be to avoid growing the more susceptible varieties where boron toxicity symptoms have been observed.
The degree of leaf damage of varieties in response to high levels of boron in the soil is shown in the following table:
Tolerance of Barley Varieties to Boron
Table 2 lists the likely expression of boron toxic symptoms on barley leaves during spring. It should be noted that the presence of leaf symptoms does not necessarily mean that the variety will not perform on soils with high levels of boron. For example, trials on alkaline soils (31 observations, 2001 - 2004) with high sub-soil pH show that Vlamingh (which is rated as intolerant to born toxicity) out yielded Schooner (which is rated as moderately tolerant to high levels of boron) by 9% or 0.23 t/ha.
Table 2. Likely expression of boron toxicity symptoms on leaves of a range of barley varieties during spring.
|Moderately Tolerant||Moderately Intolerant||Intolerant|
|Displays low levels of leaf damage||Moderate levels of leaf damage||High levels of leaf damage|
A boron level of 50 ppm in whole shoot tissue, sampled immediately before awn emergence, can indicate an expected loss in yield of up to 10 per cent.
Boron levels in the grain can be a useful indicator of toxicity. Levels of 3 ppm or more indicate that the crop was grown on a soil with the potential to develop boron toxicity problems.
Soil acidity and aluminium toxicity
Soils with low pH often have high levels of aluminium in the soil solution. This aluminium is toxic to barley, even at low levels, as barley is more sensitive than other cereals. In general, barley should not be grown on soils with a pH in calcium chloride of less than 4.5 (sampled at 0 to 10 cm depth). Yield losses of up to 30 per cent have been associated with a soil pH in calcium chloride of 4.3 or less.
Many light textured soils in Western Australia's cropping areas were slightly acid before they were cleared. The Wodjil soils of the eastern wheatbelt and peaty sands in the southwest are naturally highly acid. However, with the introduction of agriculture, all soils are becoming more acidic due to:
- Leaching of nitrogen from the root zone
- Removal of produce (grain pasture, meat and wool)
- Use of legumes in rotations
Agricultural production increases the concentration of hydrogen ions in soil. The increasing concentration of hydrogen ions in the soil increases dissolution of aluminium from soil constituents (clays, oxides, organic matter) increasing the concentration of aluminium ions in soil solution. Eventually the aluminium in soil solution becomes toxic to plant roots, reducing root growth and the ability of the roots to explore soil to take up water and nutrient elements from soil reducing grain yields. The acidity problem is greatest in the 10 - 40 cm zone of the soil and is called subsurface acidity.
Removing produce from the paddock can be thought of as being equivalent to removing lime, leaving the soil more acid. Grains such as barley, wheat and canola contain approximately 2.5 kg of lime per tonne of grain.
Soil acidity affects the availability of soil nutrients, producing toxicities and deficiencies. In Western Australia, aluminium toxicity is the main cause of yield loss from soil acidity. Plant roots will not grow into a soil layer that is high in aluminium which limits the ability of the plant to scavenge for nutrients and restricts the rooting depth. Restricted root systems can lead to nutrient deficiencies or drought symptoms of the crop even if moisture is available further down the soil profile. Roots affected by soil acidity often look short and stubby.
At low pH, nutrients such as nitrogen and phosphorus are less available to the plant and may even become deficient at normal rates of fertiliser. While fertiliser rates can be increased to overcome this, the loss of efficiency can be costly.
Applications of lime to the soil can ameliorate surface pH problems. Soil samples need to be collected from the top 10 cm of soil, and also from the 10 - 20 cm depth. If the pH in the 10 - 20 cm sample is less than 5.0, subsurface acidity is a problem. Sufficient lime needs to be applied to the topsoil to raise the pH of the top 10 cm of soil to 5.5 or greater. Only then will alkali produced by the lime in the topsoil start to move down into the subsoil to ameliorate subsurface acidity. If pH values in the topsoil are below 4.0 then about 4 t/ha lime needs to be applied which can be achieved by several applications in successive years.
Applications of lime can induce zinc and manganese deficiency, so applications of fertiliser zinc and manganese are often required when applying lime to ensure deficiency does not reduce grain yields. Before liming, collect tissue samples from crops to test for zinc and manganese. If the tests indicate these elements are marginal or deficient then apply fertiliser zinc and manganese to the next crop.
To reduce yield losses of barley due to soil acidity:
- Measure the pH in CaCl2 of the topsoil (0 to 10 cm), before growing barley
- If the pH is below 4.5 and previous barley crops have performed below expectation, apply lime before growing barley or grow a more tolerant species and sow barley on paddocks with a higher pH
- If the pH is above 4.5, soil acidity will not affect production of barley. If the pH is 4.5 to 4.7, remeasure in two to three years time
- Choose a more tolerant variety of barley.
Of the barley varieties tested for their tolerance to soil acidity, Schooner is rated as highly sensitive and Stirling, Hamelin, Baudin, Skiff and Doolup are rated as sensitive. New South Wales Agriculture have released the feed varieties, Brindabella with moderate tolerance and Yamba with high tolerance to acidity. While these varieties perform well on acidic soils in Western Australia, they are lower yielding than WA varieties on other soil types.
A number of barley crossbreads are being evaluated for their tolerance to low soil pH and/or aluminium toxicity in Western Australia. Research trials from the last three seasons suggest that a number of crossbreads from the NSW Department of Agriculture show improved tolerance relative to Stirling and are better adapted than Yambla and Tulla to Western Australia. The Western Australian barley breeding program has made crosses to these lines to develop Hamelin and Baudin back cross barley lines with improved tolerance. It is hoped that these lines will be available for commercial production by 2010, subject to commercial malting and brewing trials being completed. The potential for the release of lines with improved tolerance however does not decrease the necessity to use lime to increase the productivity of these soils.
Page updated: June 2006