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 1Impact 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
o 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.
o 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.
o 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 2Negative 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).
Figure3 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 Waterand calculate your water use efficiency.