Guideline Index

Chapter 7: Managing Limiting Soil Factors

7.6 Soil pH

Soil pH is a measure of the concentration of the positively charged hydrogen ions (H+) in the soil solution. When a soil solution contains more H+ ions, it is acidic. When there are fewer H+ ions, the soil solution is alkaline.

The pH scale ranges from 0 to 14; a value of 7 is neutral – See Figure 7.11. Values less than 7 are acidic, and greater than 7 are alkaline. As the soil pH value decreases, the level of acidity increases. In other words, the soil solution becomes more acidic. As the pH value increases, alkalinity increases or in other words, the soil solution becomes less acidic ).

Figure 7.11  The pH scale
Figure 7.11 The pH scale

 

The pH scale is ‘logarithmic’. A one-unit decrease in the pH value signifies a tenfold increase in acidity. So a soil with a:

  • pH of 6 is 10 times more acidic than a soil at pH 7.
  • pH of 5 is 100 times more acidic than a soil at pH 7.
  • pH of 4 is 1000 times more acidic than a soil at pH 7.

7.6.1 Measuring pH

7.6.2 Causes of soil acidification

Soil pH is influenced by many factors, including soil type, organic matter, rainfall, fertiliser use and farming practices.

Soil acidification is a natural process in which the soil pH decreases over time.

Many of our farming practices increase the rate of acidification.

 

Soils with a light texture (in other words, a high sand content) and low organic matter content are most susceptible to acidification, particularly if high levels of nitrogen fertilisers are applied.

The major processes that increase the rate of soil acidification are:

  • The addition and accumulation of organic matter, which creates organic acids; a weak acid.
  • The removal from the paddock of plant and animal products that contain high levels of calcium, magnesium and potassium. These three elements are all bases and thus their removal increases acidity. The degree of acidification will depend on how alkaline the product is and how many kilograms of product are removed.
  • The leaching of the exchangeable bases (magnesium, potassium and particularly calcium) from the soil caused by high rainfall.
  • The leaching of nitrate nitrogen from the root zone.
  • The application of acidifying fertilisers, such as those that contain elemental sulphur or that contain nitrogen as ammonium or urea – See Chapter 12.6.2.

7.6.3 Potential problems of acid soils

Problems that can occur in acid soils include:

  • Aluminium and manganese toxicity to plants.
  • Decreased availability of nitrogen, phosphorus, potassium, sulphur, molybdenum, magnesium, boron and calcium to plants.
  • Decreased biological activity of soil microbes and thus reduced recycling of nutrients. For more information refer to Soil Biology Chapter 5.
  • Suppression of rhizobia bacteria, which affects legume nodulation.
  • Suppression of root growth and the plant’s ability to take up water and nutrients.

For more information on soil acidity please refer to link: http://soilquality.org.au/factsheets/soil-acidity

7.6.4 Symptoms of soil acidity

The symptoms of soil acidity include:

  • Uneven pasture growth.
  • Poor nodulation of legumes.
  • Stunted root growth and high incidence of root diseases.
  • Invasion of acid-tolerant weeds (for example, fog grass, sorrel, geranium).
  • Difficulty establishing lucerne, phalaris, and medics.
  • Formation of organic mats on the ground surface due to reduced biological activity.

7.6.5 Optimum pH range for pasture plants

Most pasture plants grow best in medium to slightly acid soil ranging from pH (CaCl2) 4.8 to 5.8 (see Table 7.4), although they can tolerate levels below this.

Table 7.4   The optimum pH range of pastures and crops  Source: Adapted from ¹New South Wales Acid Soil Action Program, (2000), *Havlin et al, (1999).
Table 7.4 The optimum pH range of pastures and crops
Source: Adapted from ¹New South Wales Acid Soil Action Program, (2000), *Havlin et al, (1999).

7.6.6 Effect of pH on soil nutrients

One of the most significant impacts that acid soils have is the effect that the acidic environment has on the availability of important soil nutrients. As the pH of a soil changes, soil nutrients can become either more or less available for uptake by plants. This change in availability of nutrients can result in pastures showing either a nutrient toxicity or deficiency.

The effect that soil pH has on the availability of soil nutrients is shown for the two soil pH test methods : pH (1:5 water) – Figure 7.12; and pH (1:5 CaCl2) – Figure 7.13. (Note: There are slight differences between mineral soils and organic soils in the availabilities of the various nutrients ).

Figure 7.12  Effect of pH (1:5 water) on nutrient availability. Source: Incitec Pivot LTD (2008) Agronomy Advantage Manual.Figure 7.12 Effect of pH (1:5 water) on nutrient availability. Source: Incitec Pivot LTD (2008) Agronomy Advantage Manual.

Figure 7.13  Effect of pH (1:5 CaCl2) on nutrient availability.  Source: http://www.sibelco.com.au/LinkClick.aspx?fileticket=VhVwW7xQEsc%3D&tabid=162

Figure 7.13 Effect of pH (1:5 CaCl2) on nutrient availability. Source: http://www.sibelco.com.au/LinkClick.aspx?fileticket=VhVwW7xQEsc%3D&tabid=162

The width of the bar represents the relative availability of that particular nutrient at that pH level.

In strongly acidic soils (pH (CaCl2) less than 4.0), all the major plant nutrients (nitrogen, phosphorus, potassium, sulphur, calcium and magnesium) and the trace element molybdenum may become less available to plants (see Figure 7.13). If the pH (CaCl2) is greater than 6, some trace elements, such as zinc, copper and boron, become less readily available, which may lead to deficiencies in plants.

Soils that are deficient in molybdenum may show a pasture response when lime is applied because the chemical reactions increase the availability of molybdenum for plant growth, especially legume growth. In many cases, what is seen as a lime response is actually a molybdenum response. For soils low in molybdenum, applying a fertiliser mix that includes molybdenum will cost less than applying lime where the pH is known to be adequate for plant growth. Caution should be exercised with the application of molybdenum as an over application of molybdenum can have an antagonistic effect on copper uptake from pastures. Molybdenum status is best assessed using plant tissue analysis of white clover (or other legume) or by applying a test strip – See Chapter 8.7 for more information on fertiliser test strips.

7.6.6.1 Aluminium and manganese

As soils become more acidic, it is common to see a rise in the plant availability of both aluminium (Al) and manganese (Mn), which can both be toxic to pasture plants and crops (see Figure 7.12). Aluminium toxicity is particularly common in acid soils and restricts root growth and function in sensitive plant species. For more information on critical aluminium levels for various pasture species see Chapter 9.2.9.7.

7.6.7 Effect of pH on biological activity

Living organisms are an important component of the soil. Good organic matter levels, good drainage and appropriate pH levels encourage their presence.

Earthworms are less active in very acidic soils, fungal organisms prefer a wide range of pH, and bacteria prefer slightly acid to neutral soils. Some of the important beneficial organisms (for example, nitrifying bacteria) are inhibited in both very acid and very alkaline soils. On strongly acidic soils and, in particular, those with a pH (CaCl2) less than 4.5, the activity of the bacteria responsible for the conversion of organic material into plant-available nitrogen is significantly reduced. For more information refer to Soil Biology Chapter 5.

7.6.8 Soil pH across the dairying regions of Australia

For information on soil pH across the dairying regions of Australia see the links below:

Figure 7.14   A screen shot of a pH map from Northern Tasmania downloaded from the ASRIS website. Source: http://www.asris.csiro.au/index.html
Figure 7.14 A screen shot of a pH map from Northern Tasmania downloaded from the ASRIS website.
Source: http://www.asris.csiro.au/index.html

7.6.9 Correcting soil acidity

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. Dolomite may be used where magnesium is required.

7.6.9.1 How does lime work?

Liming materials consist of calcium and magnesium carbonates. When applied, the carbonates slowly dissolve in the acid soil solutions and consume hydrogen ions and soil pH rises. Consumed exchangeable hydrogen ions are replaced by the calcium and magnesium ions. Figure 7.15 shows a simplified version of these chemical reactions.

Figure 7.15   How lime works
Figure 7.15 How lime works

The amount of lime required to lift a soil’s pH to a desired level is determined by how acidic the soil is and by the soil’s pH buffering capacity. Some soils have a higher pH buffering capacity than others. The pH buffering capacity is the soil’s ability to resist a change in its pH level and is largely determined by the soil texture. Soils containing high proportions of clay and organic matter, such as clays and clay loams, have a higher pH buffering capacity than sandy soils. Soils with a high pH buffering capacity acidify at a slower rate than soils with a low pH buffering capacity. As a result, these soils can tolerate acidifying processes, such as product removal and nitrogen fertiliser use, for a greater period before acidity begins to affect plant growth. However, once they do become too acidic, they will require larger quantities of lime to raise the pH level compared to soils with a low pH buffering capacity.

7.6.9.2 How and when to apply lime

Most lime is spread by contractors because of the need for specialised equipment due to the nature of the product and the large quantities applied. Lime is a salt and usually applied prior to sowing a pasture. It is preferred to incorporate the lime into at least the top 10cm to allow greater interaction with soil volume.

7.6.9.3 Lime application at sowing

Lime is relatively insoluble so does not dissolve easily (see Table 7.1 ). Thus, it is slow to react. For maximum benefit, it should be worked into the soil when resowing a pasture or sowing a fodder crop. Table 7.5 shows the recommended application rates of lime if applied to an area to be sown.

 

Table 7.5   Limestone required (fine and neutralising value (NV) > 95 ) to lift pH (CaCl2) of the top 10 cm of soil to 5.5.  Source: AgFacts NSW DPI, Soil acidity and liming.
Table 7.5 Limestone required (fine and neutralising value (NV) > 95 ) to lift pH (CaCl2) of the top 10 cm of soil to 5.5.
Source: AgFacts NSW DPI, Soil acidity and liming.

*It is recognised that low rates of lime are impractical to apply, but over-liming can cause nutrient imbalances, particularly in these light soils

 Table 7.5b

** Do not apply greater than 4 t/ha in a single application, so as to minimise any problems that could arise from over liming.

In order to use Table 7.5 the amount of lime applied is dependent on both the soil pH and effective cation exchange capacity (ECEC). Soil pH is represented in a logarithmic way; meaning that soils having a pH (CaCl2) 5.0 are 10 times more acidic than a soil of pH (CaCl2) 6.0 and a soil with a pH (CaCl2) 4.0 is 100 times more acidic than a soil of pH (CaCl2) 6.0. Therefore, proportionately it takes greater amounts of lime to correct a lower soil pH than it does to correct a higher soil pH as reflected in table 7.5.

Soil texture plays an important role in the effectiveness of the lime application, due to its effects on the ability of limestone to move through the profile and the soils buffering capacity. The effective cation exchange capacity recognises that as the soil pH drops below pH (CaCl2) 5.0, aluminium is becoming more soluble and plant available; increasing to possible toxic levels when less than pH (CaCl2) 4.5. The higher ECEC values would indicate that the soil to be limed has a higher clay or organic matter content and a higher buffering capacity and as a consequence will require more lime to adjust the soil pH.

Two examples using Table 7.5:

1. Your soil has a pH (CaCl2) of 4.7 in the surface 10 cm and an ECEC of 6 cmol(+)/kg. Your aim is to increase the soil pH (CaCl2) from 4.7 to 5.2 by incorporating the limestone into the top 10 cm prior to sowing the pasture. Follow the pH column down and the ECEC row across, and where they intersect is the limestone application rate of 1.2 t/ha. This amount in practical terms would be applied at 1.5 t/ha.

2. Your soil has a pH (CaCl2) of 4.0 in the surface 10 cm and an ECEC of 6 cmol(+)/kg. Your aim is to increase the soil pH (CaCl2) from 4.0 to 5.2 by incorporating the limestone into the top 10 cm prior to sowing the pasture. Follow the pH column down and the ECEC row across, and where they intersect is the limestone application rate of 5.5 t/ha. This amount in practical terms would be best applied in a split application of 3.0 t/ha in the first year and repeated in a couple of years or at the earliest convenience.

Liming is an expensive input into soil and pasture management, therefore it is crucial that an accurate soil sample is taken from the field which considers preferably both the surface and subsurface acidity.

The pH of the soil and aluminium levels will be the guide to the likely need for lime when resowing pastures. Lime is unlikely to be of benefit for dairying pastures on moderately acid (those above pH (CaCl2) 5.1), neutral or alkaline soils.

For soils below pH (CaCl2) 5.1, an application of lime incorporated into the soil top 10 cm by cultivation is recommended, depending on soil pH and sowing method.

Note: Table 7.5 is a rough guide only. For a more accurate estimate of lime application rates, ask your laboratory to do a pH buffering test – see Chapter 9.2.4.
7.6.9.4 Lime application as a topdressing

Lime can be applied as a topdressing (in other words, spread over uncultivated soil or existing pasture) if a paddock is to remain in the pasture phase for several years.

In the past, surface-applied lime was not recommended because earlier research indicated that 18 to 24 months might be required before a rise in soil pH was measured. This period of time was often needed to allow movement of the lime into the soil (0.5 to 1 cm each year) and to allow for the chemical reactions to occur.

However, much uncertainty still surrounds the likely responses to surface-applied lime.

 

Without doubt, most pastures on very acidic soils (less than pH (CaCl2) 4.3) will respond to surface-applied lime over a period of time. Pastures on soils with very high levels of aluminium or manganese will also respond. Recent experiments illustrate the lime quandary.

A Tasmanian experiment where lime was surface-applied over a range of soil types found that the pH (water) level rose by 0.1 unit for each 1 tonne/ha of lime applied. The more acidic the soil, the greater the pasture response. Even on soils with a pH (water) of 5.8, responses still occurred. Pasture responses varied between 1% to 15% extra annual pasture growth on most plots. If milk solids returned $2.85/kg and lime cost $42, then a pasture response of 0.7% per tonne of lime applied is the breakeven point. This trial measured responses of in excess of 1% to 2% per tonne of lime applied.

However, two separate research experiments conducted by DPI at Ellinbank and Hamilton also produced some interesting results:

  • The research conducted at Ellinbank demonstrated that the rate of downward movement of lime may be much quicker than previously thought in high-rainfall areas and on lighter soil types, than it is in drier areas and on heavier soil types. The pH levels rose significantly between 0 to 5 cm depths and 5 to 10 cm depths (at the higher rates of 10 to 20 t/ha) within 12 months of application of the lime. Pasture responses to the lime treatments were variable and seasonal and came largely from the higher (above 10 t/ha) treatments. On many of the treatments, there was no immediate response; however, responses may or may not improve over time.

The work done at Hamilton also produced promising results for the effectiveness of surface-applied lime. After 3 successive years of a 5 t/ha application of lime, an increase in pH of up to 2 units was seen in the top 5 cm of the soil profile. In the 5-10 cm range of the soil profile, an increase of 0.5 to 1.0 unit was seen. Some of this response was seen after the first year of treatment, and the soils steadily improved over subsequent years. Another important observation from the Hamilton research was the effect of surface-applied lime on the available aluminium in the soil profile. Exchangeable aluminium levels on the limed treatments were up to 50% to 80% lower after 3 years than on those treatments that received no lime.

On the tropical grass pastures of the Atherton Tablelands region, Far North Queensland, the Krasnozem soils are naturally acidic. This, and the use of nitrogen fertilisers, means that lime is applied at 2.5t/ha every 5 years to maintain a soil pH (1:5 water) of at least 5.0, and preferably 5.2 to 5.4. Where these paddocks are planted to irrigated temperate pastures over winter, lime is applied every 3 years to maintain a soil pH of at least 5.5, and preferably 5.7 to 5.9. Where soil tests indicate magnesium is required, a lime dolomite blend (typically 3% Mg) is applied.

The current advice among many, although not all, advisers is that:

  • If the soil is strongly acidic (in other words, below pH (CaCl2) 4.3), then surface-applied lime (2.5 t/ha) is likely to improve pasture productivity (see Table 7.6).
  • In the pH (CaCl2) range of 4.3 to 4.6, either apply 2.5 t/ha of lime or lay down lime test strips and observe for responses over several years.
  • Above pH (CaCl2) 4.7, lime is unlikely to result in a pasture response in the short term, but a pasture response may occur over the longer term.
Table 7.6   Lime recommendations on existing pasture
Table 7.6 Lime recommendations on existing pasture
Note: Table 7.6 is a rough guide only. For a more accurate estimate of lime application rates, ask your laboratory to do a pH buffering test – see Chapter 9.2.4 .

When surface-applying lime, a maximum rate of 5 t/ha is recommended for a single application to avoid smothering the plants and to avoid possible animal health problems.

If lime is to be applied to pastures as a topdressing, there is an advantage in using superfine or microfine lime to increase the rate of movement from the surface of the soil to depth.

Check your understanding of lime application rates by working through Excercise 2.

7.6.10 The effective neutralising value of lime and dolomite products

There are many sources of lime and they vary in their ability to change the soil pH and the speed at which this happens. The effectiveness of lime is determined in two ways:

  • Neutralising Value (NV) – The amount of calcium or magnesium as oxides or carbonates. Neutralising value is expressed as a percentage relative to pure calcium carbonate, which is given a value of 100 per cent (Gazey, 2011).
  • Effective Neutralising Value (ENV) – Considers the purity (Neutralising Value), as well as particle size or fineness. The finer the product, the greater the surface area for the neutralising chemical reactions to occur.

The key indicators of agricultural lime quality are neutralising value and particle size, regardless of the lime source.

Figure 7.16   Approximately 1.7 t/ha of 60 per cent NV lime is required compared to 1.1 t/ha of 90 per cent NV lime to achieve the same pH change.

Figure 7.16 Approximately 1.7 t/ha of 60 per cent NV lime is required compared to 1.1 t/ha of 90 per cent NV lime to achieve the same pH change.

Figure 7.17   Finer particles of agricultural lime were more efficient in changing soil pH at an application rate of 2.5 t/ha (from Cregan et al., 1989).

Figure 7.17 Finer particles of agricultural lime were more efficient in changing soil pH at an application rate of 2.5 t/ha (from Cregan et al., 1989).

Source: Gazey, 2011

Figure 7.16 shows the rate of lime (t/ha) required to achieve the same pH change using lime products with neutralising values ranging from 60 to 100 per cent. The example shows that only 1.1 t/ha of 90% NV lime is required to achieve the same result as 1.7 t/ha of a 60% NV lime.

Figure 7.17 compares the relative efficiency (%) of agricultural limes with different particles sizes applied at 2.5 t/ha. It shows that lime with a particle size of 0.25 mm is five times more efficient in changing soil pH than lime with a particle size of 1 mm.

Different codes of practice for labelling of agricultural limes apply in each Australian state. For example, Figure 7.18 shows a product information sheet for a Lime WA Inc. accredited supplier. It shows particle sizes, neutralising value of each fraction, and the overall neutralising value of the bulk product. It also shows the levels of Calcium, Magnesium and Sodium (quoted in pure, not carbonate form).

Figure 7.18   Example product information sheet. Source: Gazey 2011 http://www.agric.wa.gov.au/objtwr/imported_assets/content/lwe/land/acid/liming/bn_2011_lime_quality_audit.pdf  
Figure 7.18 Example product information sheet. Source: Gazey 2011 http://www.agric.wa.gov.au/objtwr/imported_assets/content/lwe/land/acid/liming/bn_2011_lime_quality_audit.pdf

7.6.11 How to calculate the cost of lime or dolomite

When you compare lime products, make sure that you select the most economical product available in your region.

The value of limes of various types and from various sources can be compared by making the following calculations:

  1. Gather quotes from suppliers for the total cost per tonne to have various limes applied to the paddock (including the purchase price and the transport and spreading costs).
  2. Obtain the Effective Neutralising Value for the limes. Most limes on the market have been tested to determine their ENV, and this information should be available from the supplier. This will provide a ‘per unit’ basis for comparison.
  3. Divide the total cost by the effective neutralising value of each product:

Unit cost = Total cost per tonne spread ÷ Effective neutralising value

Example.

Say that there are two lime products available in your area.

Lime A has an ENV of 95 and costs $60/t spread.

Lime B has an ENV of 70 and costs $50/t spread.

Which is more economical?

Lime A: $60 divided by 95 = $0.63 per unit of ENV (as received basis).

Lime B: $50 divided by 70 = $0.71 per unit of ENV (as received basis).

Lime A is the lower cost lime to use based on its effective neutralising value and the total price.

Knowing these characteristics about lime (including dolomite) allows you to compare the cost-effectiveness of a variety of lime products and purchase the product that will be most cost-effective for your farm. However, you must also take into account other considerations, including the handling requirements of some products. The lime comparison calculator on the soilquality.org.au website can be used to calculate and compare the cost-effectiveness of agricultural limes. It considers the cost of: lime, transport, spreading; particle size distribution of the lime; and the neutralising value of each particle size range in the lime.

7.6.12 Lime products

By-product and natural limes contain calcium carbonate (CaCO3), calcium hydroxide (Ca(OH)2), or calcium oxide (CaO). Dolomitic limes contain magnesium carbonate (MgCO3) in addition to the CaCO3. Pure lime is 100% calcium carbonate (CaCO3).

Agricultural limestones usually occur in limestone rock deposits with calcium carbonate (CaCO3) contents ranging from 48% to 97%. Agricultural lime is the most commonly used product for increasing soil pH in pastures and is usually the most cost-effective.

Burnt lime (also called quick lime) is calcium oxide (CaO). It is a faster-acting lime and has the highest neutralising value. This lime is mostly used in horticultural enterprises and is not usually applied to pastures. However, it needs to be used soon after its production because in time it reverts back to lime.

Slaked lime (also called hydrated lime or builder’s lime) is calcium hydroxide (Ca(OH)2) and has a higher neutralising value than agricultural lime but is more expensive and not usually applied to pastures.

Lime kiln dust is the very fine dust (particle size of less than 0.1 mm) produced by kilns used to burn lime. It contains both limestone and burnt lime and is difficult to handle due to its fineness, so a contractor experienced in spreading the product should be used. Cement kiln dust has similar properties, plus it can contain significant amounts of potassium (commonly 3% to 5%).

Wet lime is also known as liquid lime. The effectiveness of liquid lime is determined by its NV, not its ENV. There are extra handling costs with wet lime. Wet lime is not usually applied to pastures.

Dolomite is a mixture of calcium carbonate and magnesium carbonate (CaCO3 and MgCO3). As the magnesium carbonate content of limestone increases, it is firstly called dolomitic limestone and finally dolomite (pure magnesium carbonate). The Limestone Association of Australia defines dolomite (as a product) as having a minimum magnesium carbonate analysis of 28% and a minimum calcium carbonate analysis of 35%. Dolomite is frequently used in horticulture as a source of magnesium (for example, in orchards) and is sometimes used on pastures.

Dolomite is used as a source of magnesium for magnesium-deficient soils. It can also be used as a source of magnesium for livestock. However, very high rates are required for this purpose (5 t/ha or greater). A Department of Agriculture study at Camperdown showed that 12.5 t/ha needed to be applied to obtain an effect. Experience is that dolomite is generally not effective in reducing grass tetany , and livestock should be treated directly.