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Soil pH


Soil pH describes the soil's acidity or alkalinity. Extremely high or low pH (on a scale from 0 to 14, and in which 7 is neutral) can have a negative effect on the health of plants and soil biota. In soils with pH below 5.5, acid-sensitive agricultural plants are adversely affected and the risk of subsoil acidification increases. Soil pH is an important indicator of chemical processes that occur in soil, affecting soil processes governing nutrient availability. Although soil acidification is a natural process, it can be accelerated under agriculture. Soil pH represents a signficant constraint to production in many areas of Australia, with the economic loss from soil acidification across Australia, estimated to be up to six times higher than from dryland salinity.
Key Messages

  • Soil pH effects generally limit plant growth below pH 5.5 (measured in CaCl2), though this can vary between different plant and crop species

  • Soil acidification is a natural process caused primarily by the removal of cations and organic matter, and by nitrate leaching. This process can be accelerated in agricultural production systems, for example by the application of acidifying fertilisers and greater removal of cations in hay

  • Liming is usually required to neutralise acidity associated with agricultural production, although it can also be managed through changed farming practices

  • Lime can be used to treat surface acidity, usually with rapid results, but subsoil acidity is much harder and more expensive to correct

What is soil pH?

The chemical environment of the soil can be acid, neutral or alkaline. The movement of strongly acid balancing ions such as sulphate or nitrate through the soil generates excess acidity and the accumulation of strongly basic ions (i.e. calcium and sodium) generates alkalinity.

Soil pH indicates the net acidity or alkalinity of a soil. Specifically pH measures the concentration of hydrogen ions in soil solution on a logarithmic scale that goes from 1 (extremely acid) to 14 (extremely alkaline). In a neutral soil the concentration of hydrogen ions (H+) equals the concentration of hydroxyl (OH-) ions. Acidity develops as the concentration of hydrogen ions increases in the soil at the expense of hydroxyl ions and as a result soil pH declines. In drier areas or in saline soils alkalinity develops as concentration of OH-exceeds the concentration of H+ ions. Soil pH can vary significantly between different soil horizons. For example sandy topsoil with a pH of 5 may have clay subsoil with a pH of 8 or sandy subsoil where pH continues to decline. This suggests it is important to measure soil pH in different horizons of the soil to ensure appropriate management.

Soil pH can be measured in soil extract obtained with deionised water or a dilute calcium chloride (CaCl2) solution. Soil pH values measured in CaCl2 solution are less variable than pH measured in water. For most soils pH values in water are between 0.5 and 0.8 pH unit higher than values in CaCl2.

A soil pH (in CaCl2) between 6.5 and 7.5 is considered close to neutral, less than pH 6.5 slightly acid, less than pH 5.5 moderately to strongly acidic and extreme acidity associated with soils of pH less than 4.5. Acid sulphate soils (ASS) can have pH values much less than 4. Above pH 7.5, soils are considered alkaline with soil between pH 7.5 and 8 often associated with free calcium carbonate (lime equivalent) and above pH 8.5, sodium salts.


pH is a logarithmic scale, so soil with a pH of 6 has ten times more hydrogen ions and is much more acid than soil with a pH of 7, and 100 times more than a soil with pH 8. Australian soils range from pH 3 (peat bogs) to pH 10 (arid desert soils), though most agricultural soils are in the pH range 4.5 to 9. A pH range of 5 to 7 is ideal for most plants, with some showing a preference for more acid soils, and others preferring alkaline soils (Figure 1).

Areas of high rainfall such as those on the coast and mountains often have more acidic soils compared to more arid areas, due to the more intense leaching in wetter climates. Other soils contain large amounts of carbonates (limestone derived from marine sediments) or alkaline salts in their groundwater - these soils can be very alkaline, even in high rainfall areas.

The National Land and Water Resources Audit (2001) estimates that approximately 50% of surface soils and 25% of agricultural subsoils in Australia (about 50 and 23 million hectares respectively), have surface pH values less than 5.5. In addition, between 10 and 22 million hectares is extremely to highly acidic with pH values less than 4.8.
How to recognise the effects of soil acidity and alkalinity

Possible effects of an acid soil (pH < 5.5 in Cacl2)


  • Nutrient deficiencies (phosphorus, magnesium, calcium) (see diagram below)

  • Reduced boron, molybdenum and availability

  • Aluminium or manganese toxicity

  • Failure of legumes to nodulate

  • Poor root growth (stubby roots and few fine roots)

  • Reduced microbial activity
  • Presence of acid sulphate soils indicated by low pH readings (pH < 4)Source: Chris Gazey (DAFWA)Figure 1 Indicator dye and powder can be used to show acidification in soil in the field (acid soils turn orange/yellow). Photo courtesy of Chris Gazey (DAFWA)


Effects of an alkaline soil (pH >8.3 in Cacl2)

  • High pH can indicate the presence of calcium carbonate, high sodicity or the presence of toxic compounds like sodium carbonate (see diagram below)

  • Surface sealing and crusting problems due to excess sodium

  • Zinc deficiency

  • Boron toxicity

  • Reduced microbial activity

  • Increased salt concentrations

Figure 1 Impact of pH on nutrient availability

(Source: http://www.odellengineering.com/newsletters/2013-09/2013-09.html)

What factors influence soil pH?

Soil acidification is a natural process, but it can be accelerated under agriculture and horticulture. Soil processes that lead to a decrease in hydrogen (H) ions will increase pH, whilst an increase in free H ions will lower soil pH.



Increasing acidification may be attributable to some of the following factors:
  • Nitrification - Nitrification is the conversion of ammonium to nitrate. During this process, hydrogen ions are produced and contribute to the release of aluminium (Al) ions and displace calcium (Ca), magnesium (Mg) and potassium (K) ions - contributing one unit of acid to the soil for each unit of N mineralised. Decaying organic matter can be a significant source of NO3-especially in legume based pastures. Negatively charged nitrate ions are generally not retained by soil and are subject to leaching if they are not used by plants. However when plants take up nitrate (NO3-) ions they release hydroxyl (OH-) ions which help to counter act the acidity generated by excess hydrogen ions. Perennial pastures need to be composed of at least 60% grasses to maximise the uptake of nitrate and reduce the risk of acidification.


  • Fertiliser application

  • Nitrogen: Nitrogen fertilizers provide significant amounts of nitrogen in two forms ammonium and nitrate. Ammonium ions are positively charged and so may be retained in soil by negative surface charge sites present on organic matter and clay. Nitrate however is far more mobile and if applied in excess quantities or at times when plants are growing slowly Nitrate N can move through the root zone before being taken up by crops or pastures leaving residual hydrogen ions and thus acidifying top soils. Ammonium sulphate and mono-ammonium phosphate (MAP) are the most acidifying N fertilisers, followed by di-ammonium phosphate (DAP). Ammonium nitrate and urea acidify at a slower rate, whilst calcium nitrate does not result in net acidification.



  • Sulphur When pure sulphur is added to soil it combines with water and oxygen to form sulphuric acid which of course will acidify soils. Sulphur applied in the sulphate form in gypsum or superphosphate will have no impact on pH.

  • Phosphorous Phosphate fertilisers are not directly acidifying, but indirectly add to soil acidity by improving plant growth and stimulating N fixation. The increase in N cycling, particularly under legume dominant pasture phases and grazing systems can lead to accelerated soil acidification.

  • Buffering capacity The amount of soil organic matter and clay in soil influences its buffering capacity by providing charged exchange sites where excess ions can be held or exchanged, and this influences its ability to resist pH changes. Soil pH drops more rapidly in sandy soils which have a lower buffering capacity than heavy soils, and are less able to retain nutrients against leaching. Soils with a higher buffering capacity require higher lime rates to change pH.

  • Intensity of legumes in the rotation If the supply of nitrate is less than plant uptake, then the potential acidification risk is low. If nitrate supply is greater than plant demand then nitrate can be leached, resulting in more rapid soil acidification. Grain crops typically acidify the soil faster than pastures, due to their inefficient use of nitrate. In contrast, perennial pastures are able to establish earlier and have a longer growth phase, increasing nitrate uptake and reducing the rate of acidification. The rate of acidification is typically higher where grain legumes are used in the rotation and is dependent on the frequency of legume use, as well as soil factors such as organic matter and soil type.


  • Removal of plant and animal material Calcium and magnesium provide a buffer against acidifying processes, but are continually removed in plant and animal produce and waste products from the paddock (Table 1). Once the soil store of these available cations nears exhaustion, further removals will lead to rapid acidification.

Table 1 Lime application (kg) needed to counteract soil acidification caused by the removal of alkaline organic products (per tonne of product removed)

Product Lime required to counteract acidity from product removal (kg lime/ tonne product removed)

Hay (lucerne) 55-70

Hay (legume dominant) 45-50

Hay (mixed legume/grass) 35

Hay (grass dominant) 15-35

Hay (cereal) 22-25

Meat (10 DSE/ha) 30

Wool (10 DSE/ha) 25

Grain (canola) 25

Grain (Lupin) 20

Grain (triticale) 7

Grain (wheat) 3-10

Grain (barley) 5-10

Milk (1000 L) 4



  • Less commonly the loss of soil organic matter can also contribute to increasing pH and a loss of buffering capacity as seen on Vertosols in northern Australia (P. Wylie, pers. comm.).

  • Oxidation of sulphide minerals resulting from mining or land development Acid sulfate soils are formed by the introduction of air into either i) soil rich in non-oxidized sulfidic materials which form in waterlogged saline sediments with a supply of easily decomposed organic matter or ii) sediments containing the mineral pyrite. The increasing amount of acid production exceeds the soil’s ability to neutralize it, resulting in the production of sulfuric acid and a soil pH below 4.

  • Acid deposition Acid deposition from industrial atmospheric pollutants such as sulphur dioxide and land contamination by acidic pollutants can contribute to increasing soil acidification.

  • Parent material Acid soil formation occurs on naturally acidic parent material such as mountain peat, whilst alkaline soils tend to be associated with naturally alkaline parent material such as limestone. Read more on Soil Groups.

  • Liming and input of carbonate salts Lime is commonly used as a soil ameliorant on acid soils to increase soil pH. The input of carbonate salts via alkaline dust in arid regions can increase soil alkalinity.

  • Water quality Irrigation with alkaline bore water which contain carbonate salts will increase soil pH, as will upward movement of an alkaline water table. Read more on Soil Drainage.

How does soil pH affect soil health?

Plants and soil microorganisms generally prefer a soil pH range between 5 and 7 (measured in CaCl2). In soils with pH below 5.5 in CaCl2, acid-sensitive agricultural plants are adversely affected and the risk of subsoil acidification increases. Acidity can build up gradually and may not be noticed if agronomic management is optimal - yet yield penalties of 20-30% may be occurring. Subsurface soil acidity can have as much effect on plant growth as surface acidity, but is more difficult and costly to treat.


  • Ion toxicity

In strongly acid soils (less than pH 4.0 to 5.0), aluminium and manganese may become available in sufficient quantities to become toxic, inhibiting root growth and reducing crop yields (Figure 3). An increase in the uptake of toxic heavy metals such as cadmium can also occur as the soil becomes more acidic.



Source: Steve Carr 

Figure 2 Negative effect of aluminium toxicity on wheat roots (increasing Al concentration from left to right). Photo courtesy of Steve Carr

  • Nutrient deficiency

Nutrient deficiencies that affect plant growth occur at both low and high pH. In very acid soils all major plant nutrients (nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg)) and also trace element molybdenum (Mo), may be unavailable to plants, or only available in limited quantities (Figure 4).



Figure 3 The influence of soil pH on nutrient availability. A narrowing of the band signifies lower nutrient availability.

  • Subsurface acidification

If left unmanaged, increasing soil acidity on the surface can contribute to more rapid acidification of subsoil (10-30 cm depth). Susceptible soil types include the deep sands, sandy earths, gravels and duplex soils with low clay and organic carbon content and low pH buffering capacity. Subsurface acidity is more difficult to treat due to the slow movement of neutralising materials such as lime and base rich organic matter down the soil profile. It is much cheaper to avoid the risk of subsurface acidity by keeping the surface soil at pH 5.5 or more through regular liming or other methods.


  • Water use efficiency

The ability of plants to use subsoil moisture may be limited due to poor root exploration in a highly acid or alkaline subsoil. This can result in higher leaching rates, loss of nutrients and increased deep drainage. Read more on Plant Available Water and calculate your water use efficiency.

  • Biological activity and function

Biological activity is depressed in acid soils, slowing SOM decomposition. Soil acidity affects the survival and persistence of bacterial groups such as rhizobia which are responsible for nitrogen fixation, slows soil processes such as nitrification , and results in fungi becoming more dominant. Earthworm and termite populations decline in acid soils. Read more on Beneficial Soil Organisms.

  • Other

Extreme acidification can result in poorly structured or hard-setting topsoils, as well as increasing the risk of soil erosion caused by poor ground cover (read more on Soil Stability). Soils may also acidify to the point where acid, nutrients, sediment and heavy metals are exported and impact nearby inland waters (see 'Acidification of inland waters').
How do I manage my soil pH?

Soil tests should be used to monitor paddocks showing poor growth for surface and subsoil pH, testing the same area each time to check any changes in soil that occur over time. Unless it is severely acidified, most land won't show a rapid response to amelioration and records should be maintained for at least five years to see differences in your soil and yields. The following checklist should be used in the management of soil acidity or alkalinity.


Low pH (acid) soils

Soil acidification can be slowed by liming, changing land use, crop mix and management of the crop.


  • Replace annuals with deep-rooted perennials

In Australia, naturally acid soils have become more acid since the land was cleared of native vegetation and sown to annual crops and legume-based pastures. Crop and pasture legumes cause acidification if grasses are unable to use the nitrate produced and prevent nitrate leaching - acidification risk can be reduced by dropping legume content.

  • Fertilise appropriately

Higher fertiliser inputs and the increased frequency of legumes in rotation support increased production but also increase losses of N associated with nitrate leaching, contributing to increasing soil acidification. Matching N fertilizer to crop demand and timing applications to match the period of most rapid crop growth will reduce the losses of N from soil.

Urea, ammonium nitrate and other nitrate based forms of fertiliser are less acidifying than ammonium based N fertilisers such as ammonium sulphate, mono-ammonium phosphate (MAP) and diammoniurn phosphate (DAP). Composted materials generally have no acidifying effect.



  • Minimise nitrate leaching

High leaching environments (i.e. high rainfall areas, irrigated industries) increase the rate and intensity of alkalinity leaching, and have the potential to increase nitrate leaching.

Efficient irrigation management, including not over-irrigating crops and not fertilising prior to irrigation/rainfall events minimises the risk of nitrate leaching - especially when using fertigation.



Early sowing after fallows and cover crops minimise nitrate leaching, whilst growing deep-rooted perennials or extended pasture phases will maximise nitrate uptake from the subsoil.

  • Use a nitrification inhibitor

Although nitrification inhibitors (coatings applied to nitrogen fertilisers eg Entec) effectively slow the conversion of ammonium based fertilisers to nitrate and reduce the rate of acidification, they are expensive and only likely to be economically viable in industries with high commodity values.


  • Choose acid-tolerant crops

If practical, chose acid-tolerant crops (e.g. oats, triticale, serradella) or acid-tolerant varieties of crops such as wheat, barley and canola.

  • Apply lime or other neutralising agents

Lime can neutralise surface soil acidity and its regular application can prevent future subsurface acidity. Subsurface acidity is harder to correct, requiring repeated surface applications over long periods of time, or placement of lime at depth during ripping or slotting operations. Application rates vary according to soil type, but as a guide an increase of 0.2–0.3 pH units can be expected from one tonne of lime per hectare (rate is strongly dependent on soil texture, buffering capacity, soil moisture, solubility of the neutralising product, particle size and other soil constituents such as organic matter). If lime is to be used, take the following steps: 

    • Determine the depth of the acid soil layer to be ameliorated (To test for the presence of subsurface acidity, soil samples need to be collected at 0-10 cm and 10-20 cm).

    • If the pH for the 10 to 20 cm depth is less than 5, then subsurface acidity is potentially present and sample depths below 20 cm should subsequently be tested. If the topsoil pH is less than 5.5, apply lime to raise topsoil pH. Lime in the topsoil will gradually move into the subsoil and either prevent or start reversing subsurface acidification and aluminium toxicity.
    • If the subsurface is already below pH 5.0 in CaCl2, it may be worthwhile to treat it directly with injected lime, or it may be preferable to regularly lime the topsoil to maintain it at pH 5.5 or above and to grow acid-tolerant crops or pastures while alkalinity moves to the subsoil.


    • Determine amount of lime required to reach the target soil pH for your particular land use. This can be done using the following technique.

You can determine your soil’s lime requirement using the following technique.

  • Subtract your present pH(Ca) from your target pH.

  • Divide this number by:

  • 0.3 if your soil is clay

  • 0.4 clay loam

  • 0.5 sandy clay loam

  • 0.6 sandy loam

For example

  • You have a pH(Ca) of 4.0 and a target pH(Ca) of 5.2

5.2 -4.0 = 1.2

For a clay soil 1.2/0.3 = 4 tonnes lime/ha



For a sandy loam 1.2/0.6 = 2 tonnes lime/ha

  • If your soil has an organic carbon level above 2% then apply an additional 0.2-0.4 tonnes per hectare.

    • Calculate maintenance rates required to deal with annual acidification caused by product removal and leaching (See fact sheet on ‘liming materials http://www.dpi.nsw.gov.au/agriculture/resources/soils/acidity/publications/best-lime ).

    • Lime works best when it is finely ground (check effective neutralising value) and incorporated into soil.

    • It can take some years before the full effects of lime application are seen in yield and profit, but the benefits are then usually maintained for several seasons. Regular maintenance applications of lime should be included when setting the farm or enterprise budget.

Selecting a Liming Material 
  • Lime is usually taken to mean calcium carbonate (CaCO3) , but may also include hydrated lime, burnt lime and various other materials such as marl, slag and stack dust, some of which are of importance in Australian agriculture. In some areas, for example in the Adelaide, Geelong, Wollongong and Marulan areas, precipitator dusts and slag account for quite large proportions of the liming materials used.


  • Important considerations when selecting a liming material are its:

  • Calcium and Magnesium content

  • Neutralizing value;

  • Particle size (Fineness);

  • . Cost.

Calcium and Magnesium Content:

  • Most materials are important for their calcium content but some may also contain significant amounts of magnesium. Agricultural lime is the commonly used commercial form of lime and is a finely ground calcium carbonate.

  • Dolomite or Dolomitic limestone and blends of dolomite and lime are variable mixtures of calcium and magnesium carbonates which rarely contain more than about 10% Mg. It is of greatest value in light soils in high rainfall areas where deficiencies of both calcium and magnesium are common.

  • Other forms of 'lime' are hydrated lime or calcium hydroxide [Ca(OH)2] produced by hydrating burnt lime, and burnt lime (COO) itself. Neither of these forms is widely used in Australia on their own but can have a place in certain situations provided care is taken with their use. They commonly occur as components of industrial wastes which are used as liming materials. Hydrated lime and burnt lime react more quickly than agricultural lime.

Neutralizing Value (NV):

  • The relative ability of different liming materials to counteract the effects of soil acidity is expressed as an index. It is determined by comparing them to the neutralizing value of pure calcium carbonate. By setting the NV of CaC03 at 100, a value for the other materials can be calculated, This value is called the 'relative neutralizing value'* ,

  • 1 calcium carbonate equivalent' The NV for several liming materials are shown in the table below.
  • Relative neutralizing value of liming materials.


  • Liming Material

  • Relative Neutralising Value

  • Calcium carbonate

  • Dolomitic limestone

  • Agricultural lime

  • Burnt lime

  • Hydrated lime

  • Gypsum

  • Basic slag

  • 100

  • 95-108

  • 85-100

  • 150-175

  • 120-135

  • nil

  • 50-70

Particle size:

  • When a given quantity of lime is incorporated into the soil, its reaction rate and degree of reactivity are affected by particle size. Coarse particles react more slowly and less fully. Fine particles react more rapidly and much more completely. In general, lime that is coarser than about 250 microns (0.25 mm has very little value in raising soil pH, at least in the short term. Most agricultural lime contains 60% or more by weight finer than 150 microns (0. 15 mm), as illustrated in Figure 2.8.




  • Figure 2.9 tells a dramatic story about lime particle size and degree of reactivity. Large particles, of 2.4 to 4.5 mm (4 to 8 mesh) were only 10% efficient in terms of reacting with the soil. Smaller particles of 150 175 microns (80 to 100 mesh) reacted completely.


Price:

  • The cost of lime increases with the fineness of grind. As a result, agricultural liming materials contain both coarse and fine materials. This guarantees the lime will be of sufficient quality to neutralize acidity. The importance of particle size is shown in Figure 2.9.

  • Although the rate at which lime reacts depends on particle size, initial pH, and degree of incorporation in the soil, the chemical nature of the liming material itself is an important consideration. For example, calcium oxide and calcium hydroxide react more quickly than calcium carbonate. In fact, hydrated lime can react so quickly it can partially sterilize the soil. If applied too near planting time, it can induce a temporary potassium deficiency because of the high calcium availability. Retarded plant growth and some plant death can result, in extreme cases.

  • When selecting a liming material, the cheapest material per tonne is not always the most cost effective. For example, which is better: Product A with a NV of 96% costing $50 per tonne spread or Product B with a NV of 85% costing $45 per tonne spread. On this basis, Product A is a better buy because it costs $50/0.96 or $52.08 per effective tonne while Product B costs $4510.85 or $52.94 per effective tonne. The fineness of the material and its chemical make up should also be taken into account (refer Chapter 13   Lime dolomite and liming materials for methods of calculating effective neutralising value of lime which take particle size into account).

Common Liming Materials

  • Although the common liming materials have been referred to in previous sections, a short description of some of the important materials follows:

  • Calcitic limestone (CaCO3 ) and dolomitic limestone [Ca Mg(CO3 )
  • Widespread deposits of high grade calcitic and dolomitic limestone are common in many parts of Australia and are often associated with production of cement and various materials used in industrial processes. Their quality is affected by the impurities they contain, particularly sand and clay. Their NV is usually in the range of 70 to 100 or slightly more, with the lower NV limes being generally somewhat less expensive. Most good quality lime is generally considered to have a NV of around 94 or more;


  • Calcium oxide (CaO):

  • Also known as burnt lime or quicklime, CaO is a caustic white powder, which must be handled with great caution to avoid contact with eyes, skin and soft tissues. It is manufactured by roasting calcitic limestone in an oven or furnace. Its purity depends on the purity of the raw material. When added to the soil, it reacts almost immediately, so when rapid results are required. it (or calcium hydroxide) is ideal. It should be mixed completely with the soil, because it rapidly cakes and can become less effective;

  • Calcium hydroxide [Ca(OH)2] :

  • Frequently referred to as slaked lime, hydrated lime or builders' lime,

  • Ca(0H)2 is a caustic, white, powdery substance, which must be handled with great caution to avoid contact with eyes, skin and soft tissues. As with CaO, it reacts very quickly when applied to the soil. It is produced by adding water to CaO;

  • Basic slag:

  • Slag is a by product of the steel industry and is extensively used overseas, mainly as a source of phosphorus. Locally produced slag normally contains a much lower level of phosphorus than many of those produced overseas. However, the local material does have some useful neutralizing value, but for a number of reasons, it is used only in areas adjacent to the steel works where it is produced;

  • Other materials: Other materials sometimes used overseas as liming materials include marl, stack dust and water softening sludge. These materials are of no importance in Australian agriculture.

  • Build up organic matter

Return organic matter to the soil including animal manures, stubble and cover crops. Whilst decaying organic matter releases organic acids and may increase soil acidity, it adds nutrients to soil, stops nutrients already in soil from leaching away, and enhances the soils cation exchange capacity. This means the soil will be more heavily buffered against future changes in soil pH.. Addition of organic materials to soil may also assist in ameliorating soil acidity by binding active aluminium. Read more on Soil organic matter.


  • Replace what you remove

Higher intensity production systems and greater alkalinity removal in farm produce (Table 2) increases the rate of acidification. Soils become acid if crops and produce are taken off-farm and not returned to the soil. This also applies to strip-grazed pastures where animal manures do not fall where the pasture was grazed. Replacing these nutrients will keep your soil chemistry balanced and so prevent your soil acidifying.

Table 2 The amount of lime product required to balance acidification caused by removal of alkaline products (kg lime/ha) (Source: Farming systems and soil acidity, Primary Industries and Resources SA, Fact Sheet 510)



High pH (alkaline) soils

  • Choose alkaline-tolerant crops

Choose a crop that is naturally tolerant of alkaline conditions.

  • Amelioration of soil by adding acidifying materials

Where soil alkalinity inhibits uptake of micronutrients such as Fe, Zn and Cu (pH >8.5), acidifying materials such as elemental sulphur can be applied to decrease pH.

Although it is possible to acidify a strongly alkaline soil by applying acid generating N fertilizers (e.g. ammonium sulphate, MAP), this is usually a slow process, especially if the soil has a high buffering capacity.



  • On strongly alkaline soils (pH > 8.3 to 9.1)

    • Only alkaline-tolerant plants will survive, and applications of micronutrients may be required.
    • If soil electrical conductivity (ECSE) is greater than 4 dS/m and the exchangeable sodium percentage (ESP) is >6 then the soil is saline-sodic (see ‘Soil Salinity’ module).


    • If ECse is less than 4 dS/m and the ESP is >6 then the soil is sodic

    • Lime and gypsum may be effective in reducing exchangeable sodium.

Profiting from managing soil pH

Across Australia, the economic loss from soil acidity is estimated to be five to six times higher than from dryland salinity (Commonwealth of Australia, 2001b). A liming program is an essential part of good farming where farm soils are acid or acidifying. If lime is not regularly applied to acidifying soils, productivity can decline over time - often substantially and unnoticed. Rehabilitation of acid soils takes time and frequent applications of lime. Maintaining soil pH by regular application of maintenance levels of lime will avoid the need for expensive and slow remediation management, with data from Primary Industries and Resources South Australia (PIRSA) showing an investment in lime on cropping land likely to return $3 for each $1 invested.

Some case studies are given below of how land managers have tackled soil acidity.



Case study 1

Economic returns from monitoring and remediation of acid soils with lime application

Source: Stephen Carr1, David York1, Joel Andrew1 *& Chris Gazey2 (1Aglime of Australia, Precision SoilTech; 2Department of Agriculture and Food Western Australia). Avon Catchment Council SI002 Case Study 2006

A long term soil acidity monitoring study was conducted in the Gabby Quoi Quoi catchment located in the central agricultural region of Western Australia, measuring change in soil pH at GIS located sample points in 1999 and 2006. Of the 300 sites sampled, 75% of the topsoil and 85% of the midsoil sampled in 1999 had pH values < 5.0, with 15% of these soils having a pH value of < 4.0. Liming recommendations were provided to growers, and after resampling seven years later an overall increase in soil pH was measured, with approximately 15% less sites recording a pH < 5.0 of of soils and no samples found to be < pH 4.0. Grain yield in limed paddocks was found to be on average 6-7% higher than in unlimed paddocks, though recorded significantly higher yield in drier seasons.


PDF Link 3.7MB

Case study 2

Long term lessons from liming soil


Source: MASTER Trial, Wagga Wagga; Tarlee long term rotation trial; Rutherglen SR1

Long term trials have shown continuous cropping under a cereal legume rotation with district practice application of fertiliser N and retention of stubble caused a decrease in soil pH of up to 1.6 units over 14 years. Reduced N inputs can lessen this impact (as can stubble removal) but also decreases yield.

At Wagga, the application of lime in combination with phosphorous (P) increased grain yields by almost 100%, whilst stocking rates increased by 25% after six years. Soil pH initially increased from pH 4.0 to 5.5 after application of 3.7 t/ha lime, but subsequently decreased back to 5.0 over the next six years.


  • High initial lime rates will improve soil pH more rapidly and allow excess lime to move deeper into the profile addressing subsoil acidification

  • Maintenance lime is required to maintain changes in soil pH

  • Liming may increase N leaching

  • P application is more efficient

  • Excessive N application and leaching of nitrate is likely to accelerate the decline in soil pH

Further information:

http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0007/167209/soil-acidity-liming.pdf

Best practice principles for the management and monitoring of soil pH


An important indicator for rainfall conversion to grain is water use efficiency and this can be used as a means of benchmarking cropping system performance. In southern farming systems, each mm of rainfall should result in 15-20 kg grain. A practical way of monitoring progress is the ‘best practice’ approach together with observations on soil condition, as follows.


  • Have a program of on-going monitoring for topsoil and subsoil pH

  • Map the extent of soil with acidity and/or alkalinity problems

  • Where economical to overcome acidity problems through liming, modify as much of the root zone as possible

  • Where it is not practical or economic to alter soil pH, select acid tolerant crop species/varieties

  • Be aware of the annual acidifying effect of the available cropping and pasture options

  • Apply lime regularly – maintenance rates will stop further soil acidification, but higher rates may be required to provide optimal soil conditions for crop production

  • Deal with associated limitations to plant growth such as compaction and nutrition - otherwise, the response to applied lime may be limited

  • With high value crops (trees/vines), high rates of lime (> 20 t/ha) ripped in prior to planting and trellising often can be justified economically

  • Sow perennial pasture - deep rooted, more summer active, reduce nitrate leaching.

  • Use fertilisers wisely - match to plant requirements, monitor plant and soil levels, and use the least acidifying N fertilisers

  • Feed hay onto paddock in which it was cut where possible - recycles nutrients and alkalinity

  • Rotate grazing paddocks

  • Irrigation with bore water can supply carbonate which helps to neutralise acidity, but check water pH before application

  • Avoid irrigating when soil nitrate levels are high

  • Buy in hay/grain where practical

Sources. Most of the information included in this topic is taken from the Soil Health Knowledge Bank (http://www.soilhealthknowledge.com.au/ ) Sadly this site is no longer available.




Soil pH 2015 v2



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