- Soil pH is a measure of the concentration of hydrogen ions in the soil solution and describes the soil�s acidity or alkalinity
- Soil acidification (low pH) is a natural process, but can be accelerated under agriculture
- Soil pH can be measured either in water (pHw) or in calcium chloride (pHCa)
- As a general rule, pH measured in calcium chloride is 0.6 to 0.8 of a pH unit lower than pH measured in water
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- Soil pH effects generally limit plant growth and biological activity below pHCa 5.5, though this can vary between different plant species
- Soil testing will tell you the current acidity status of your paddock and is relatively easy and inexpensive
- Aim for a target pHCa of greater than 5.0
- Surface soil acidification is found throughout the Corangamite region and is most commonly corrected with lime
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Understanding the question
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| Why is it important to me as a farmer?
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- Soil pH problems represent a significant constraint to production in the Corangamite region
- By not having pH at the optimum levels in your soils there can be a negative impact on
your farm business by:
- Limiting plant choices
- Reducing plant growth
- Although soil pH is a measure of the hydrogen ions in soil solution, it is the associated chemical changes in the soil that impact on the availability of essential plant nutrients and increase the availability of toxic elements that are important
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- Specifically, acid soils, which are as a result of lower than optimum soil pH:
- decrease the availability of essential nutrients
- increase the impact of toxic elements
- decrease plant production and water use
- affect essential soil biological functions like nitrogen fixation
- make soil more vulnerable to soil structure decline and erosion
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- The pH of soil indicates the strength of acidity or alkalinity in the soil solution which
bathes soil constituents, plant roots and soil micro-organisms
- pH is based on a numerical scale of 0 to 14 where pH below 7 (neutral) is acid and pH
above 7 is alkaline. The pH scale is logarithmic, meaning that a soil of pH 8 is 10 times
more alkaline than a soil of pH 7
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- In the laboratory, soil pH can be measured either in water (pHw) or in calcium chloride (pHCa) and the pH will vary depending on the method used
- As a general rule, pH measured in calcium chloride is 0.6 to 0.8 of a pH unit lower than pH measured in water
- pHCa is the preferred measurement when monitoring pH changes over time
- In the field, soil pHw can be measured using relatively cheap and easy to use field test kits
Figure 2 - Relationship between pH measured in Calcium Chloride and Water - Source: DEPI, Victoria
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| How do changes in pH occur?
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- Soil acidification is a natural process but is accelerated by agriculture
- The major processes that increase the rate of soil acidification are:
- The leaching of nitrate nitrogen from the root zone and exchangeable bases (magnesium, potassium and particularly calcium) from the soil in periods of high rainfall. Soil acidification is a natural process but is accelerated by agricul
- The application of acidifying fertilisers, such as those that contain elemental sulphur or that contain nitrogen as ammonium or urea
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- The addition and accumulation of organic matter, which creates humic acid, a weak acid
- The removal from the paddock of plant and animal products that typically contain high levels of calcium, magnesium and potassium. These three elements are all bases (as opposed to acids), and thus their removal increases acidity
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- Acid soils pose a problem in the south west of Victoria, whereas alkaline soils are prevalent in the north west of Victoria
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- Soil acidification is a process that is more common in sandier soils, where
nitrogen/nitrate leaching occurs more readily
- Soil acidification is found throughout the Corangamite region, but appears to be most prevalent in areas of higher agricultural
production (below). These areas have high levels of agriculture-related inputs and
outputs, which also increase the likelihood of soil acidification
Figure 4 - Areas of moderate, high and very high soil acidification susceptibility in the
Corangamite region. - Source: CCMA
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- Plants have preferred pH ranges
- Majority of soils in south west Victoria are pHW 5.5 � 6.0 in 0-10cm depth
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Table 1 � Soil pH � Plant preferences. Source: *Target 10 Soils and Fertiliser Program � State Chemistry Laboratory (1995)
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Species |
pHW range |
pHCa range |
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Pasture |
Medic
Phalaris
White clover
Lucerne
Sub Clover
Perennial rye
Cocksfoot
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6.0 � 8.5
6.0 � 8.0
5.8 � 6.5
5.8 � 8.0
5.5 � 7.0
5.0 � 6.5
5.0 � 7.5
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5.3 � 8.0
5.2 � 7.3
5.0 � 6.0
5.2 � 7.5
4.8 � 6.5
4.3 � 6.0
4.3 � 6.8
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Highly Sensitive
Highly Sensitive
Highly Sensitive
Highly Sensitive
Sensitive
Moderately Tolerant
Moderately Tolerant
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Crop |
Canola
Barley
Wheat
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6.0 � 7.5
6.0 � 8.0
5.5 � 8.0
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4.9 � 6.5
4.9 � 6.9
4.4 � 6.9
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Highly Sensitive
Highly Sensitive
Sensitive
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- To minimise the impact on agricultural production, it is recommended that farmers should aim for a pHCa of greater than 5.0
- For most plant species, the optimum pHCa is 6.5. As soil pH drops, the following occurs:
- pHCa 5.5 � balance of major nutrients and elements available
- pHCa 5.0 � aluminium may become soluble in the soil depending on soil type; phosphorus combines with aluminium and may be less available to plants
- pHCa 4.5 � manganese becomes soluble and toxic to plants in some soils; molybdenum is less available; soil biological activity slows down; aluminium becomes soluble in toxic quantities
- pHCa 4.0 � soil structural damage begins to oc
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| How can you reach optimum pH?
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1. Conducting soil tests for pH
- Soil pH is one of the most routinely measured soil parameters. It is used as a benchmark
to interpret soil chemical processes and governs the availability of many essential or
toxic elements for plant growth
- Soil pH is a common measure of the soils acidity or alkalinity because:
- testing is relatively easy
- field equipment to measure pH is relatively inexpensive
- Note that within the Corangamite region, only about 10% of landholders regularly
conduct soil tests
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2. Applying Lime
- Soil acidity is corrected by applying agricultural lime or dolomite. Lime (calcium
carbonate) is the most common product applied to dairy pastures to increase the pH and
neutralise the effects of soil acidity
- In the Corangamite region, more than half the cropping and dairy land is subjected to
regular applications of agricultural lime (Bluett and Ward pers. comms. 2003). On
broadacre grazing, less than 10% of land has lime applied regularly, because it is
generally not cost-effective
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Other related questions in the Brown Book
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Brown Book content has been based on published information listed in the Resources and References sections below
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- Soil Acidity - Heytesbury District Landcare Network
- Soil pH - Soil Health Knowledge Bank � Department of Agriculture, Fisheries and
Forestry
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- Victorian Department of Primary Industries (2011), Soil Types and Structures Module.
- Clarkson T, Department of Primary Industries on behalf of the Corangamite Catchment
Management Authority (2007). Corangamite Soil Health Strategy 2007. - Corangamite
Catchment Management Authority, Colac, Victoria.
- Soil pH -. Soil Health Knowledge Bank � Department of Agriculture, Fisheries and
Forestry.
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- Surface Soil pH. Victorian Resources Online - Department of Primary Industries,
Victoria.
- Managing Soil Factors That Can Limit Plant Growth. Department of Primary Industries, Victoria.
- Acid Soils. Victorian Resources Online - Department of Primary Industries, Victoria.
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