Surface Soil pH
see also:
- Impact of Acid Soils in Victoria publication. This report was prepared by DPI in consultation with the Acid Soil Steering Committee. It aims to improve the awareness of the impact of soil acidification and identifies issues that should be considered in developing a strategy to manage acid soils.
- Report from the Inquiry on the Impact and Trends in Soil Acidity (external link) in Victoria (March 2004) - available for download on the Environment & Natural Resources Commitee website (external link).
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What is soil pH?
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. Soil is neutral when pH is 7, it is acid when pH is less than 7 and alkaline when it is greater than 7. The pH scale is logarithmic, so a difference of a unit is a tenfold difference in acidity or alkalinity (eg. pH 5 is ten times more acid than pH 6).
Measuring soil pH
Traditionally, soil pH has been measured in the laboratory by inserting a pH electrode into a suspension of one part soil and five parts water (1:5).
The calcium chloride method has been used in recent times: here a weak calcium chloride solution is used in place of water. It is a more robust method of measuring pH as it is less affected by soil salts. This newer method gives a lower reading by up to 1.3 pH units. It is important therefore to state which method has been used when assessing pH. All pH values presented in this Website have been measured in water.
 Surface Soil pH | Surface soil pH map Soil pH was mapped across Victoria using a statewide soil chemistry data set based on samples submitted from farms, vineyards and orchards between 1973 and 1994. Each sample was a composite of 20 to 30 cores representing the 0-10, 0-15 or 0-30 cm depth of soil taken from the main soil type in each paddock. Samples from national parks, urban land and sport and recreational turf were excluded from the data. Collated data included nearest location and pH (1:5 soil:water). |
The map was generated by applying geo-statistical techniques (‘kriging’) to the mean pH of the locations. It indicates the geographic trends in the acidity and alkalinity of surface soils across Victoria's agricultural land. This map cannot indicate soil pH at the paddock scale and it should only be used as an indicator of likely pH at a regional scale. Considerable variations in soil pH will occur within a region. This map does not substitute for a soil test. It is important to note that the map only considers surface soil pH. In many Victorian soils, pH will increase with depth. | |
Geographic trends in soil pH across Victoria
A number of Victoria’s soils are naturally acid while others are naturally alkaline. This is due to the effect of natural processes, (such as leaching, erosion, deposition) occuring over hundreds to thousands of years on the soil parent material. Consequently, areas of high rainfall such as those on the coast and mountains, have more acidic soils compared to more arid areas, due to the more intense leaching in wetter climates.
Surface soil pH is generally acidic in the Eastern and western Uplands, the Strzelecki and Otway Ranges and north-eastern Victoria. The acid soil conditions in these areas are known to limit plant production in agricultural industries. For example, crops and pastures such as barley, phalaris and lucerne have been limited by aluminium toxicity in north-eastern Victoria and the Central Highlands. Molybdenum deficiencies have limited pasture production in Gippsland.
Soil pH is high and alkaline in north-western Victoria, ie. Mallee and Wimmera. Field crops grown on these soils have experienced zinc deficiencies. Herbicide persistence has also occurred in the strongly alkaline soils.
Agricultural practices have also acidified soils. For example, soils under subclover based pastures, leguminous crops such as lupins, and where ammonium fertilisers are used. Management practices can be used to reduce the impact of acidification. These include use of perennial and deep rooted species (eg. phalaris), and avoiding acidifying fertilisers. Ultimately, application of lime will be needed to combat acidification.
In contrast, alkalinization has also occurred in some soils. Specifically, irrigation with alkaline bore water and fertilising with nitrate fertilisers will increase soil pH. Movement of an alkaline water table will also alkalinize soil. Of course, applying lime will alkalinize soil.
Significance of soil pH to plant growth
Soil pH affects the availability of soil constituents to plants and soil micro-organisms. For most plants, the ideal soil pH (water) test result is pH 6 - 7.5, although many will tolerate pH 5.5 - 8.5. However, the tolerance to extremes in pH varies between plant species and within species. Some plant species have quite different preferred pH ranges (eg. lucerne 6.0 - 8.5, celery 6.0 - 7.0, potatoes 5.0 - 6.0). Therefore, consideration of the need for soil amelioration will depend on individual circumstances.
Soil pH problems
The pH of the surface soils in Victoria ranges from pH 4 to pH 10. These extremes in alkalinity and acidity present problems for the production of many agriculturally important plant species and their symbiotic rhizobia. Due to the complexity of soil chemistry, it has often been difficult to confidently identify the cause of poor plant growth or nodulation. However, aluminium and manganese toxicities and molybdenum and phosphorus deficiencies are probable causes of poor production in many strongly acid soils. On the other hand, lime induced zinc and iron deficiencies often occur in alkaline soils.
© The State of Victoria, 1996 - 2012.
This document was last reviewed on 14/11/2011