Managing Soil Factors That Can Limit Plant Growth
Specific soil factors that can limit plant growth include:
- Slaking and dispersion.
- Compaction.
- Water logging.
- Salinity.
- Soil pH.
- To get the best from fertilisers, you must first eliminate or reduce soil-related factors that limit pasture growth.
What will you find out in this section?
At the completion of this section, you should:
- Recognise soil factors that limit plant growth.
- Understand the management options for minimising the limiting factors.
4.1 Slaking and dispersion
Slaking is related to soil structure and particularly to structural stability, which is the soil’s ability to retain aggregates and pore spaces under various environmental conditions. Slaking is the result of lack of organic matter.
Dispersion is usually a problem of soil chemistry (namely, high levels of exchangeable sodium), although it can occur in non-sodic soils due to excessive mechanical disturbance of the soil. Slaking and dispersion can occur together. When this happens, both problems will have to be managed.
4.1.1 Slaking
Slaking is the breakdown of aggregates into smaller aggregates or single particles. It occurs when a dry clay soil becomes wet. The clay swells and the air within the pore spaces in the aggregates is compressed. This builds up pressure, resulting in the ‘explosion’ of the aggregate (Figure 4.1).
Figure 4.1 Diagrammatic illustration of slaking and dispersion
Slaking is severe in some soils with low organic matter and can occur within minutes of the soil becoming wet. When a slaked soil dries, crusting (hardsetting) of the soil can occur. This limits water infiltration and seedling emergence. The hardsetting can be limited to the top few millimetres of soil or can extend through the entire soil profile.
4.1.2 Dispersion
Dispersion is the separation of the clay particles from the aggregates when the soil is wet (see Figure 4.1). Clay particles carry a negative electrical charge and tend to repel each other. Calcium, magnesium, sodium and potassium all carry positive charges and are attracted to the clay particles, forming a ‘bridge’, or bond, between the negatively charged clay particles.
Calcium (Ca++) ions, followed by magnesium (Mg++) ions, are the strongest ‘bridge formers’ because they have two positive charges. Potassium (K+) and sodium (Na+) ions only have one positive charge, and their bonding of the clay particles is much weaker. If calcium is forming the bridge, the clay particles will hold together when they are wet. However, if sodium is forming the bridge, the bonding is much weaker and the clay particles tend to separate and repel one another when they are wet (in other words, they tend to disperse).
Cloudy or muddy water in puddles is an indication that a soil may be dispersive. A continual stream of cloudy water running out of a mole drain outlet is also indicative of a dispersive clay-type soil. Mole drainage and open drains in dispersive soils may lead to severe soil erosion.
When dispersion occurs, the dispersed clay particles fill up the pores between soil particles and aggregates; and when the soil dries out, the dispersed clay blocks up soil pores. This restricts seedling emergence, water and air movement, and root penetration. Dispersed soils are generally hardsetting and may form a surface crust. Dispersion with no slaking results in a ‘concrete-like’ lump being formed.
Note: Soils prone to dispersion are very prone to tunnel and gully erosion if incorrectly managed.
Dispersion is an indicator of sodic soils (soils containing such a high level of sodium cations that it affects soil structure). However, ploughing or other mechanical treatment of some non-sodic soils can also result in dispersion.
4.1.3 Simple tests for determining slaking and dispersion
Place several large aggregates (5 to 10 mm in diameter) into a shallow dish containing distilled or rain water.
If the soil is a slaking soil, the aggregates will fall apart within a few minutes to a few hours. Cloudy water will not appear around the aggregates.
If the soil is dispersive, a cloud of clay-sized particles, or muddy water, will form around the aggregates. Dispersion can take several hours to occur, and a visual assessment should be made after about 2 hours and again after about 24 hours.
Another test can be done to determine whether a soil is dispersive and will respond to gypsum. Place a small handful of soil into each of two clear glass jars half filled with distilled or rain water. Add a small handful of gypsum to one of the containers only (label it with a marker). Shake the two jars and leave them for 24 hours. If the soil is dispersive and responsive to gypsum, the soil will settle out in the jar with gypsum and will remain cloudy in the jar without gypsum.
4.1.4 Management of slaking soils
Slaking, which is related to soil structure and particularly to soil stability, can be managed by increasing the level of organic matter in the soil. Organic matter reduces slaking by reducing the rate of aggregate wetting and by more strongly binding the soil particles together.
The best ways to increase the organic matter level in the soil are to:
- Grow highly productive pastures, especially perennial ryegrass and white clover and, where possible, deep-rooted legumes, such as lucerne.
- Use minimum tillage techniques for crop and pasture establishment.
Organic matter levels and stable soil aggregates can be easily destroyed by excessive cultivation, by cultivation when the soil is too dry or too wet, or by stock trampling, particularly when the soils are wet. Cultivation increases the rate at which organic matter oxidises from the soil. The cultivation machinery compacts the soil, as does stock trampling. In fine-textured soils, cultivating when the soil is too wet breaks down aggregates, and cultivating when the soil is too dry creates large clods that are not easily penetrated by roots or seedlings.
4.1.5 Management of dispersive soils
In the short term, gypsum will reduce dispersion on sodic soils. Lime can be used to reduce dispersion (to a lesser extent) on acidic sodic soils, but it is much less soluble than gypsum. In both cases, the sodium cations attached to clay particles are replaced with the stronger-bonding calcium cations. (Some magnesium cations will also be replaced by calcium cations.) The clay particles then bond together, or aggregate, by flocculation.
However, aggregates formed solely by flocculation generally are not very stable. In the longer term, dispersion management involves increasing the organic matter level in the soil, which will help to form stable aggregates (aggregates that hold together). See Section 4.1.4 for information on increasing organic matter levels.
The sodium cations that are exchanged for calcium cations on the clay particles don’t disappear. They enter the soil solution, where they can reattach to clay particles when the opportunity arises. Adequate drainage will give longer-term responses to gypsum applications.
What is gypsum?
Gypsum is the common name for hydrated calcium sulphate
(CaSO4
- 2H2O).
There are two basic sources of gypsum: mined gypsum (natural deposits) or by-product gypsum.
Natural deposits of gypsum occur in many parts of inland Australia and vary widely in purity. The effectiveness of these sources of gypsum depends largely on the purity of the deposit and how finely the gypsum is ground.
By-product gypsum, or phospho-gypsum, is produced at fertiliser plants during the manufacture of phosphoric acid. This is a crystalline form of gypsum that also contains a small amount (around 1% to 2%) of phosphorus that is available to plants.
Classification of gypsum products
Gypsum products were originally classified under the Fertiliser Act 1974 (Vic) as a material that must contain not less then 65% calcium sulphate dehydrate (gypsum), not less than 36% insoluble sulphate, not less than 15% calcium, and not more than 0.8% sodium chloride.
The new Fertiliser Regulations 1995 have classified natural gypsum into three grades, based on their sulphur and calcium content, and have set standards for the sulphur and calcium content of phospho-gypsum:
- Grade 1 gypsum must contain a minimum of 15% sulphur and 19% calcium.
- Grade 2 gypsum must contain a minimum of 12.5% sulphur and 15.5% calcium.
- Grade 3 gypsum must contain a minimum of 10% sulphur and 12.5% calcium.
- Phospho-gypsum must contain 17% sulphur and 21% calcium.
The regulations also require that the label on any gypsum product must specify the percentage of gypsum capable of passing a 2-mm sieve.
Uses of gypsum
The main use of gypsum is as a source of calcium to improve soil structure in dispersive soils.
Some farmers apply gypsum as a source of sulphur.
In the high-rainfall dairying areas of southern Victoria, some farmers apply gypsum on clay soils in the belief that it will help reduce waterlogging by improving soil structure. However, many clay loam soils, despite having waterlogging problems, will not improve in soil structure (and consequently drainage), after an application of gypsum. This is because these soils already contain adequate amounts of calcium ions, clay dispersion is not a problem, and adding further calcium in the form of gypsum is usually a complete waste of time and money.
In the Northern Irrigation areas on the red-brown earths (see Section 5.4.1), the soils generally have low calcium levels and in some cases are also sodic. In northern Victoria, particular attention should be given to lasered paddocks with exposed subsoils that will have low organic matter. In laser-leveled paddocks, gypsum should be incorporated into the plant root zone prior to sowing. It is usually applied prior to the final grading.
However, in northern Victoria, good pasture cover is the priority on recently lasered permanent pastures. Gypsum topdressing is considered if pasture cover is poor (usually related to poor structure or sodic soils) or on paddocks that have been badly pugged, exposing the subsurface soils. The aim of the topdressing is only to stabilise the surface soil. This will result in the soil retaining its crumb structure and improving water penetration in the summer by not sealing over.
Determining the need for gypsum
Requirements for gypsum can be predicted from soil tests (see ‘Exchangeable sodium’ in Section 8.1.8). However, predictions based on cation measurements alone may be inaccurate. Therefore, a clay dispersion test (see Section 4.1.3 above) should also be done.
Rates and application notes
Gypsum can be incorporated into the soil before sowing or before the final laser grading. Alternatively, it can be applied as a topdressing.
Gypsum does not dissolve readily, although it dissolves more readily than lime, so it should be applied well before sowing to allow time for a reaction in the soil. Application rates of about 2.5 to 5 tonnes per hectare are needed on many soils, and the effect lasts from 1 to 20 years depending on the soil type and rate used.
When gypsum is applied as a topdressing, the application rates are reduced. Rates of 1.2 to 2.5 t/ha are commonly used, and repeat applications may be required the following year.
Note that in a topdressing situation on irrigated dairy pastures:
- Wait until a long rotation in the autumn.
- Graze the paddocks out completely.
- Top dress the gypsum prior to natural rainfall.
Cattle may be reluctant to graze the pastures until the gypsum fines have been washed off the leaves.
Gypsum is a type of salt, so application to an already saline soil may temporarily increase the total soluble salt levels.
Unless drainage is adequate, the sodium (displaced by the calcium) may not be flushed or leached beyond the plant root zone. Improved drainage may be necessary so that the excess sodium can be leached out of the soil. If drainage is not adequate, it is possible for the soils to quickly revert to their waterlogged and dispersive nature.
When buying gypsum by the tonne, it is important to know the water content of the product when comparing products, as high freight costs may be incurred.
Pasture growth responses to topdressed gypsum on existing pasture are uncommon unless a sulphur response is obtained.
4.2 Compaction
A compacted soil is one in which the soil aggregates have been compressed so that the pore spaces between the aggregates have been substantially decreased. Compaction reduces water and air infiltration, restricts pasture root growth and slows pasture growth rates. The compacted layer may be at the soil surface as a result of wheel compaction; slightly below the surface from pugging; or at some depth in the soil profile as a hard pan, either naturally occurring or from the use of cultivation implements.
4.2.1 Naturally occurring compaction
Natural compaction occurs through leaching as the more mobile soil constituents move downward and form hard pans. For example, in some soils, iron is leached out of the surface layers and accumulates at depth to form a hard pan that is very difficult to penetrate. These hard pans are locally called ‘coffee rock’ and are found commonly in the Heytesbury district. Similar ‘buckshot’ layers are found in the older basalt plains soils. Clay particles can also be leached downwards to form a clay-pan layer that is virtually impermeable.
Compaction also occurs naturally within the soil profile. The lower horizons are usually more compacted than the upper ones due to the weight of the upper layers and the lower organic matter content of the lower layers.
4.2.2 Induced compaction
Induced compaction is soil compaction caused by farming practices. It can make rapid and severe changes to soil porosity (the amount of pore spaces in a soil), and some of these changes can have long-lasting effects.
Dry soils are relatively strong and usually do not compact. However, when soils are wet, they have less strength and are more easily compressed. Cultivation, grazing, or fertiliser spreading at this stage will reduce the soil’s ability to drain excess water away quickly, which can make waterloggingmore severe. Heavier-textured soils, such as clays and loams, are more affected because they naturally drain more slowly. Management should be adjusted to take these factors into account.
4.2.3 Degree of compaction
The degree of compaction of a soil affects the bulk density of the soil. Bulk density of a soil is the weight of a unit volume of dry soil and its pore spaces. It is usually measured in grams per cubic centimetre (g/cm3).
A friable, well-aerated soil will have a low bulk density (more pore spaces per cubic centimetre; therefore, a lower weight), while a compacted soil will have a high bulk density as the large soil pores are lost.
Bulk density is related to both texture and structure. Sandy soils have a higher bulk density, because their particles tend to be closer together and sandy soils also are usually lower in organic matter. Clays and loams usually have a lower bulk density because they are made up of smaller particles that are usually well-granulated (have formed aggregates). This is assisted by their higher organic matter content. The structure of a soil is affected by the balance of macropores (large soil pores) and micropores (small soil pores). Thus, as a soil becomes less well-structured (loses macropores), it increases in bulk density. Management as well as soil type affects this characteristic of any soil.
Compacted layers can be easily assessed by pushing a pointed rod into the soil. Often a hard pan is found around 10 to 15 cm from the surface.
4.2.4 Managing compaction
On compaction-prone soils, management practices can reduce the likelihood of compaction.
Compaction may be avoided or reduced by increasing the level of organic matter in the soil. This will improve soil structure, reduce the bulk density of the soil and promote freer drainage when soils are wet.
Use of large-area, low-pressure tyres and keeping animals and machinery off wet areas can help to reduce compaction. Cultivation of wet soils should be avoided.
If grazing is restricted while the soils are wet, there may be a need for investment in hard stand-off areas to protect the soil resource.
Overcoming waterlogging through surface and subsurface drainage (see Section 4.3) can substantially help to conserve the soil structure. It is imperative, where subsurface drains are installed, to allow 24 to 48 hours for excess water to drain away. Ideally, the watertable should be at least 300 mm below the soil surface before the pasture is grazed.
Frequent or fast cultivation pulverises soil aggregates, breaking down soil structure and macropores, therefore hastening compaction. Modern techniques of minimum tillage are less damaging to soil structure.
Tyning or ripping can loosen the compacted layer. This is a ‘temporary fix’ if the underlying cause of the compaction is not corrected. Also, unless water can get away, the tyned or ripped soil will wet up more easily (to the tyned or ripped depth) and be even more prone to compaction damage. When mechanically aerating, ensure soil conditions are not overly dry or wet.
4.3 Waterlogging
Waterlogging occurs when most or all of the macropores become filled with water rather than air. It occurs more easily in soils that have a greater proportion of micropores than macropores, because the macropores promote free drainage while the micropores tend to hold on to water. Both compaction, which presses the soil particles and aggregates closer together, and dispersion, which fills the pore spaces with clay particles, tend to promote waterlogging.
4.3.1 Causes of waterlogging
Waterlogging is a common problem on many soil types, particularly in the winter and spring. Waterlogging may be due to periods of heavy rainfall, poor irrigation management, rising watertables, or a combination of poor surface drainage (across the paddock) and poor subsurface drainage (down the soil profile). Figure 4.2 shows a well-aerated soil and a waterlogged soil.
Figure 4.2a Well-aerated soil
Figure 4.2b Waterlogged soil
4.3.2 Effects of waterlogging
Soil strength decreases as the soil gets progressively wetter, resulting in
a greatly increased potential for damage to the soil structure. A waterlogged soil reduces the amount of oxygen in the root zone, thus seriously inhibiting plant growth and encouraging anaerobic soil micro-organisms at the expense of aerobic micro-organisms. A build up of salt is also more likely under poorly drained irrigation pastures.
Waterlogging prevents air and many nutrients from reaching the roots. Also unfavourable gases and compounds are produced in the root zone due to the anaerobic (no oxygen) conditions.
Where soil drainage is poor, numerous effects are seen:
- Plants are stunted and yellow as nitrogen is lost due to denitrification (caused by a lack of aeration).
- Improved pasture plants are replaced by weeds (docks, smartweed, rushes, sedges, couch, etc.).
- Pastures become fouled with mud and utilisation is reduced.
- Pasture growth rates decline.
- Soils become pugged and water ponds on the surface.
- Responses to applied fertiliser are poor.
- Nutrient balance in the pasture is upset with lower nitrogen, potassium, magnesium and chlorine in the pasture.
4.3.3 Managing waterlogged soils
Prevention of damage to the pasture and soil structure is the first priority when waterlogging has occurred. This can only be done by keeping vehicles and animals off waterlogged areas. A grazing management technique called ‘on-off grazing’ can significantly reduce pugging damage and increase pasture utilisation. With the ‘on-off grazing’ technique, stock are only allowed to graze the paddock for a short period (2 to 4 hours) and are then held in a stand-off area, such as a feedpad, a laneway, an old sand quarry, or the dairy shed yard.
Long-term management involves removing the cause of the waterlogging. Waterlogging may be caused by a soil chemical or physical problem, such as dispersion or compaction, or by a high watertable or poor irrigation management.
In districts where waterlogging is caused by a high watertable, management strategies aimed at controlling the level of the watertable will be required.
4.3.4 Reducing waterlogging through irrigation management
Waterlogging is one of the most limiting factors affecting flood-irrigated pasture production. Unfortunately, white clover (the best milking feed) is the most susceptible pasture species to waterlogging.
In a flood irrigation situation, waterlogging can occur because it takes too long to get water:
- On to a bay.
- Off of a bay.
Water On
A compromise needs to be found between irrigating quickly (to reduce waterlogging) and allowing enough time for adequate soakage of water into the bay. For common soil types, an irrigation length of 4 to 6 hours is a good compromise. On lighter, more permeable soils, a shorter irrigation time is appropriate to reduce water losses beyond the pasture root zone.
Four factors commonly cause slow watering and can lead to a waterlogging problem. These are:
- Inadequately sized (small) farm channels and channel structures.
- Small bay outlets.
- Weeds in channels restricting the water flow.
- Irrigated ground that is high.
Any of these factors will restrict the flow of water, cause the wheel to slow down and slow down irrigation.
Leaking bay outlets may also be the cause of a waterlogging problem at the top of an irrigation bay. (Refer to the Target 10 Irrigation and Drainage Reference Manual to see how to address these problems.)
Water Off
Water should not be left lying on the bay. It should be drained away as quickly as practicable to prevent ponding and waterlogging. Ideally, there should be no water lying on the bay 24 hours after an irrigation has commenced.
Drainage off the bay will be affected by:
- Bay slope.
- Soil type.
- Condition of the drain at the bottom of the bay.
- Length of bay.
- Evenness of slope.
- Presence of spinner cuts.
- Pasture height, type and density.
Water will stay longer on longer bays, on bays that are flat or are on heavy soil types, or where the drains at the bottom of the bay are in poor condition.
Longer bays (longer than 400 m) do not drain well, particularly in winter when evaporation rates are low. The water has to move a longer distance to reach the drain, and the bay stays wetter longer.
Spinner cuts running down the bay will improve drainage. The spacing of the spinner cuts across the bay will depend on the severity of the drainage problem. A spacing of 15 to 20 m between spinner cuts is often used.
It is important that there is a well-defined drain at the bottom of the bay. This drain should connect with the rest of the farm drainage system so that it carries runoff to a reuse system, to a regional or community drain, or to a natural watercourse.
There are a large number of factors to consider when addressing an irrigation waterlogging problem. Developing a whole-farm plan provides a means to ensure that an irrigation layout is well planned and well designed. A good irrigation layout is essential to minimise waterlogging problems and optimise pasture production.
4.3.5 Reducing waterlogging through drainage
Overcoming waterlogging through drainage may help preserve soil structure. In addition, in high-rainfall dairy pastures, increases in pasture utilisation of 40% to 60% have been measured on drained (subsurface drainage) compared to undrained paddocks. Improved pasture yields and pasture composition have also been measured on drained paddocks. Soil salinity levels are also often lowered by subsurface drains because the drained water removes some of the salt.
Drainage systems need to be planned, constructed and maintained effectively to have a long-term, positive effect on both the on-farm and off-farm environments.
Because the drainage water often has to flow onto neighbouring properties, drainage works are best done in cooperation with the neighbouring landholders and in conjunction with the relevant water authorities.
Drainage of the soil can reduce waterlogging effectively but at a cost. Other strategies, such as ‘on-off grazing’ and agistment, are used on some farms.
Surface drainage
Improvement in surface drainage should be investigated first, as it is the simplest and cheapest option. Surface drainage involves maintaining existing drains and installing additional drains that are adequately sized and positioned (usually placed along fence lines or through depressions). If possible, emphasis should be placed on preventing water from the upper paddocks flowing over onto the lower paddocks.
However, many farmers mistakenly believe their waterlogging problems are due to surface water alone, when they are actually often due to subsurface water or to a combination of surface and subsurface water. In these cases, a combination of surface and subsurface drainage may be required.
Subsurface drainage
Poor subsurface water movement is caused by an impediment to the water moving down the soil profile. Possible impediments include a heavy soil texture, compacted layers, and natural or induced hard pans in the profile (which can create a raised watertable). Poor subsurface water movement can also be caused by subsurface water moving downhill from upper slopes or by springs.
In irrigation areas, groundwater pumping is a common form of subsurface drainage.
In high-rainfall areas, the two main forms of subsurface drainage are tile drains and mole drains. The type of drain installed depends on soil characteristics, rate of drainage required, topography, etc.
Note: In irrigation areas, your irrigation supply authority (for example, Goulburn-Murray Water) should be contacted before installing subsurface drainage. In some regions, permission is required from the local irrigation supply authority to discharge water from subsurface drainage systems off the farm.
Tile drains
Free-draining topsoils with an impermeable layer at a depth of more than 0.7 m or deep, free-draining soils subject to rising watertables require tile drains. Tile drains are constructed by placing a slotted PVC or corrugated plastic pipe in a trench and then surrounding the pipe with a permeable backfill, such as stone or gravel (Figure 4.3).
Figure 4.3 Tile drain
Tile drains are expensive to install but are very effective and economically viable in the correct situations. They can last for many years, provided they are correctly installed and consistently maintained.
Mole drains
Clay and clay loam soils with poor natural drainage and with clay less than 40 cm from the surface are generally suitable for mole drainage (Figure 4.4). Mole drains cost less than tile drains but require more maintenance.
Figure 4.4 Mole drain
The clay through which the mole is pulled must be plastic when wet (retains shape of mole) and stable (not prone to cracking, dispersion or slaking). If installed correctly in appropriate soil, mole drains may function adequately for 3 to 7 years.
4.4 Salinity
4.4.1 What is salinity?
When we refer to salinity in agriculture, we are referring to the level of salt in the soil and the soil solution.
The most frequently found salt in saline conditions is common salt, more correctly known as sodium chloride (NaCl). Other salts found in varying amounts include sulphates of sodium, calcium, magnesium and potassium and chlorides of calcium, magnesium and potassium. All of these are found in Victorian ground water at varying concentrations.
Saline soils are those with sufficiently high levels of salt in the root zone to adversely affect plant growth. High salt levels affect plant growth in two ways:
- They reduce the plant’s ability to take up water (an osmotic effect).
- They cause toxicities (usually chloride and sodium) and nutrient imbalances by changing the plant’s ability to take up a wide range of nutrients.
Natural saltland (primary salinity) existed in Victoria before European settlement; however, the extent and severity of salinity has increased markedly due to changes in the management of land and water (secondary salinity).
4.4.2 The processes of salinity
The processes of salinity are shown in Figures 4.5 (dryland) and 4.6 (irrigation).
Rainfall or irrigation water can leave the soil surface in several ways. It can be used by plants or removed by runoff and evaporation. The remaining water seeps through the soil until it reaches a dense rock layer (often referred to as bedrock) or a clay layer or other hardpan layer that it cannot penetrate. The soil then begins to fill up with water, starting from this layer. The top of the saturated zone in the soil is known as the watertable. Recharge, or groundwater recharge, is the name for water added to the watertable in this way.
When the watertable rises to within 1 or 2 metres of the soil surface, the water can move into the plant root zone by capillary rise. Capillary rise occurs when drier soil on top of the watertable sucks up the ground water, similar to the action of a sponge. The capillary zone is the area above the watertable that is affected by capillary rise.
Figure 4.5 Processes and causes of dryland salinity
The height of capillary rise depends on the soil type. It is greatest in loam soils, which have a variety of particle and pore sizes. In sandy soils, there are bigger spaces and the force needed to lift water through them is greater. In clay soils, small pore size slows water movement.
As the watertable rises, the salts in the soil are dissolved. If capillary rise brings salty water into the plant root zone, the plants are affected by the salts. In extreme cases, capillary rise will bring the salty water to the soil surface. This is known as discharge.
The salt makes it hard for the plants to take water from the soil, and the plants may show signs of water stress. Eventually, they may die of dehydration if the salinity levels are too high.
Figure 4.6 Processes and causes of irrigation salinity
Pastures suffering from the effects of salinity can become invaded by salt-tolerant weeds; and as salinity increases, the proportion of salt-tolerant grasses increases.
The early symptoms of salinity are often incorrectly thought to be caused by something else, such as waterlogging or a lack of fertiliser; and a yield loss of up to 30% can occur before definite signs of salinity become visible.
4.4.3 The causes of secondary salinity
The main cause of many of the secondary dryland salinity problems is the clearing of deep-rooted native trees from recharge areas. Trees and other large vegetation use and transpire much more water than pasture or cropping does. When these large plants are cleared from the recharge area, more water seeps into the watertable.
The main cause of irrigation salinity is poor irrigation methods, such as slow watering, that allow water to pond for long periods and seep into the soil. In addition, seepage from irrigation channels and drains and from dams results in recharge.
4.4.4 Soil classification and salinity measurement
Soils have been divided into five classifications to help identify the degree of the salinity problem. The classes range from very low to extreme levels of salinity and are named A+ (very low), A, B, C and D (extreme). Most of the Northern Irrigation area is type A+ or A soil, but there are significant areas of classes B, C and D, particularly to the west.
The growth of most pasture species is unaffected by salinity levels of class A or B soils. Only salt-tolerant plants like barley grass grow in class D soils.
Salinity is measured by determining the electrical conductivity (EC) of a water or soil sample. Electrical conductivity is a measure of the capacity of soil or water to carry an electric current. The main unit of measurement for soil salinity is decisiemens per metre (dS/M). ECe (dS/m) is a salinity measure used by researchers that allows for the effect of soil texture on salinity.
The estimated returns from applying irrigation water to dairy pastures growing on soils with different degrees of salinity are shown in Table 4.1. This shows the huge affect soil salinity can have on economic returns. Similar economic effects would occur with the application of fertiliser to the various classes of saline soil.
Table 4.1 Estimated returns from applying 1 megalitre of irrigation water to different classes of saline land (1992 data)
| Soil Salinity Class | ECe (dS/m) |
Approx. Amount of Salt (ppm) | Species That Will Grow | Gross Margin |
|---|---|---|---|---|
| A+ Very low | Less than 1.8 | n.a. | All pastures and clovers | $170 |
| A Low | 1.8 to 3.8 | Less than 1800 | Most pastures, crops, legumes | $120 |
| B Moderate | 3.8 to 6.5 | 1800 to 3000 | Grass, some legumes | $55 |
| C High | 6.5 to 8.6 | 3000 to 4000 | Grass, not clover | $5 |
| D Extreme | More than 8.6 | More than 4000 | Salt tolerant plants, some barley grass | -$15 |
Source: Adapted from Norman et al. (1995).
Soil salinity levels determine what grows in the soil and are a major reason for lower productivity. Lower salinity soils give better return for your money. Therefore, in salt-affected areas, it is important to know the classifications of your soil so areas of C and D class can be managed differently to the more productive A and B class soils.
4.4.5 What does salinity look like?
Salinity will affect plants differently depending on their stage of growth. They are usually most at risk during seedling emergence and early seedling growth. If salinity weakens them at an early stage in their growth, they are more prone to stress caused by other problems, such as poor soil structure, disease, insects, and nutrient deficiencies or waterlogging. Plants may die from the combined effect of several of these rather than just one.
The leaves of salt-affected plants can initially appear smaller and darker than normal. Shoot growth is also reduced. As salinity levels increase, the effects become more pronounced. Low germination rates and seedling deaths reduce establishment, and surviving plants grow more slowly. The tips of leaves can appear burnt, and this can spread until the whole leaf is yellow.
The photos in Figures 4.7a and 4.7b show examples of A, B, C, and D soil salinity classes and the effect of their salt levels on productivity.
Figure 4.7a Soil salinity classes A and B and their effect on productivity in the Kerang Irrigation Region
Figure 4.7b Soil salinity classes C and D and their effect on productivity in the Kerang Irrigation Region
General pasture symptoms
- Plant growth is poor and uneven.
- Grasses dominate because they generally are more tolerant to salinity than clovers and other legumes.
- Animals may lick and graze salty areas.
General soil symptoms
- White salt crystals may appear on bare soil surfaces in extreme cases.
- The surface soil may remain moist and greasy.
- Clay soils may appear loose and crumbly and when cultivated may have a soft and spongy texture.
4.4.6 Plant tolerance to salty conditions
The tolerance of the various pasture species to salinity does vary. In some cases, specific varieties may be recommended to assist more severe salinity problems because of their better tolerance to the particular conditions. Table 4.2 indicates the relative salt tolerance of some pastures and crops.
Table 4.2 Tolerance of some pasture and crop plants to various levels of salinity
| Salt Tolerance | Plant Species | Salt Tolerance | Plant Species |
|---|---|---|---|
| Low | White clover | Medium (cont’d) | Lucerne* |
| Sub clover | Berseem clover | ||
| Persian clover | Strawberry clover | ||
| Crimson clover | Millet | ||
| Balansa clover | High | Barley | |
| Medium | Perennial ryegrass | Wheat | |
| Annual ryegrass | Oats | ||
| Phalaris | Barley grass | ||
| Tall fescue | Tall wheat grass |
*Sensitive to salinity until established.
4.4.7 How can we best manage salinity?
The solutions to the salinity problem are not a matter of just treating the symptoms but of managing all the factors that influence the well-being of the water catchment.
As a whole, the community can help in lowering the watertable. Management of the whole catchment assists everyone’s problems and so benefits everyone. Some of the practices available are mentioned below.
Community surface drainage reduces accessions to the watertable as well as providing a wide range of benefits on farms. Surface drainage programs in irrigation areas provide financial and technical support to survey, design and construct subregional community drainage schemes.
Subsurface drainage removes excess water from below the ground surface. Common methods are groundwater pumping from suitable shallow aquifers (in irrigation areas) and tile or mole drainage.
Whole-farm plans can be used to protect and enhance environmental features while increasing farm profitability. A whole-farm plan is a drawing or photograph of the farm showing existing natural and built features and details of the improvements to be made on the property, such as fencing by soil salinity class, drainage plans, or improved irrigation management.
Trees strategically planted on recharge areas, along laneways and fence lines and on irrigation reuse systems and channel banks provide several benefits. A significant one is the consumption of ground water by the trees, which assists in lowering the watertable; others include providing shelter for livestock, improving property values and providing wildlife habitat.
In irrigation areas, incentives are available to producers and community groups for each of these four practices. For more information, contact your local DPI office.
As long as untreed recharge areas and irrigation practices continue to upset the natural water balance, the salinity problem will remain. Only when improved land and water management practices slow the rate of recharge to the groundwater system will it be possible to control salinity.
Specific steps for managing dryland salinity
Salinity is not a widespread problem in dryland areas at present. However, steps that dryland farmers can take to manage dryland salinity include:
- Using more water in recharge areas by:
- Retaining any remaining native vegetation.
- Planting perennial pasture species, as they are deeper-rooted and use more water than annual species.
- Replanting catchment areas with trees, which can be used for livestock fodder, firewood, posts and poles, sawlogs, honey, windbreaks, wildlife habitat, erosion control, etc.
- Controlling grazing in recharge areas so that as much pasture growth as possible remains on the soil.
- Fencing off saline areas and planting them in salt-tolerant trees or pasture species. These plants will help to keep the soil profile drier by using the saline water and will also help to prevent soil erosion, which often occurs on saline areas when the existing vegetation cover is killed by the salt.
In many cases, salinity in dryland areas is caused by problems on recharge areas that are not on the property on which the salinity occurs. In such cases, a catchment-wide approach to salinity management will be necessary, involving cooperation with neighbouring landowners and your local catchment management authority.
Specific steps for managing irrigation salinity
All irrigation farmers can take steps to improve irrigation management. These include:
- Using a whole-farm plan for a structural approach to farm development. Approximately 30% of the irrigated area of the Northern Irrigation Region has been covered by whole-farm plans since their introduction in 1986.
- Matching water application to the needs of pastures and crops.
- Improving pasture and crop growth to use more soil water.
- Watering paddocks in less than 6 hours.
- Draining paddocks as quickly as possible after irrigation.
- Reusing all drainage water in future irrigations.
4.5 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 (Figure 4.8). 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 (in other words, the soil solution becomes less acidic).
Figure 4.8 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.
4.5.1 Measuring pH
Two laboratory methods are currently used to measure pH: the water method and the calcium chloride (CaCl2) method. The results are usually reported in one of the following formats:
- If the water method is used, the results are reported as pHw, pH (water) or pH 1:5 water.
- If the calcium chloride method is used, the results are reported as pHCa, pH (CaCl2) or pH 1:5 CaCl2.
The ratio of 1:5 means that 1 part soil is mixed with 5 parts distilled water or calcium chloride solution before the pH reading is taken. Be aware of which method has been used in your soil test analysis.
The water method has been the test most commonly used in Victoria for over 30 years and more readily reflects current soil conditions than does the calcium chloride method. However, the water method is more subject to seasonal variations. The pH (water) value may vary by as much as 0.6 units over the year. For example, a soil may have a pH (water) value of 5.7 before the autumn break, but a pH (water) value of 5.1 after the break. This variation can affect recommendations for fertilisers, particularly lime.
The calcium chloride test is more useful for long-term monitoring of pH and is the one most agronomists tend to use for fertiliser and plant recommendations. The calcium chloride method is less subject to seasonal variations and better approximates field conditions. The fact that the calcium chloride method is less variable throughout the season makes it a more appropriate test to use when making management decisions regarding soil pH and lime applications.
When both test methods are used on the same soil sample, the pH (CaCl2) value is, on average, about 0.8 units lower than the pH (water) value. However, the pH (CaCl2) value can range from 0.5 to 1.1 units lower than the pH (water) value.
The pH readings from the two testing methods will be much closer if your soil contains high levels of salt. This is typical with soils that have a salinity problem or may be seen after a recent application of a fertiliser high in salt, such as muriate of potash.
Most of the major soil testing laboratories will present pH results for both testing methods. It is important to always be aware of which testing method has been used when discussing management options with an adviser or agronomist or when interpreting a soil test.
Portable pH testing kits can be used to measure pH. However, they are not as accurate and will only provide an indication of pH levels.
4.5.2 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 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.
- 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. Acidification caused by nitrogen is covered in detail in Section 12.7.
4.5.3 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.
- Suppression of rhizobia bacteria, which affects legume nodulation.
Suppression of root growth and the plant’s ability to take up water and nutrients.
4.5.4 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.
4.5.5 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 (Table 4.3), although they can tolerate levels below this.
Table 4.3 The optimum pH range of pasture plants
| Pasture Species |
pH (CaCl2) |
pH (water) |
|---|---|---|
|
Sub clover |
4.8 to 6.5 | 5.5 to 7.0 |
|
White clover |
5.0 to 6.0 | 5.8 to 6.5 |
|
Perennial rye |
4.3 to 6.0 | 5.0 to 6.5 |
|
Medic |
5.3 to 8.0 | 6.0 to 8.5 |
|
Lucerne |
5.2 to 7.5 | 5.8 to 8.0 |
|
Cocksfoot |
4.3 to 6.8 | 5.0 to 7.5 |
|
Phalaris |
5.2 to 7.3 | 6.0 to 8.0 |
|
Fescue |
4.3 to 6.4 | 5.0 to 7.0 |
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.
Figure 4.9 summarises the availability of plant nutrients at different pH (CaCl2) levels in mineral soils. (There are slight differences between mineral soils and organic soils in the availabilities of the various nutrients.) The widest sections of each bar symbolise a greater availability of that particular nutrient at the denoted pH levels. Note that the more essential plant nutrients are available in the pH (CaCl2) range 5 to 6.
Source: NRE (1999).
Figure 4.9 Effect of pH on nutrient availability (the wider the bar, the greater the nutrient availability)
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 4.9). 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. It is best to test molybdenum levels using plant tissue analysis of white clover or by applying a test strip.
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 4.9). Aluminium toxicity is particularly common in acid soils and restricts root growth in sensitive plant species.
A plant’s sensitivity to aluminium and manganese is a major influence on its ability to grow in acid soils. Lucerne, white clover and annual medics are sensitive to high levels of aluminium and manganese, whereas ryegrass and sub clover are more tolerant (Table 4.4).
Table 4.4 Susceptibility of some pasture species to aluminium
| Susceptibility |
Pasture Species |
|---|---|
|
Highly sensitive |
Lucerne |
|
Sensitive |
Phalaris, white clover |
|
Tolerant |
Sub clover, perennial ryegrass |
|
Highly tolerant |
Cocksfoot, tall fescue |
Most of the major soil testing laboratories will provide a measure of available aluminium in the soil, often expressed as a percentage of the soil’s cation exchange capacity (% CEC). When reading a soil test that shows the soil is acidic, it is helpful to check the percentage of exchangeable aluminium. The amount of exchangeable aluminium should be the lowest of all the cations, and it is desirable that it remains less than 5%.
4.5.6 The effect of pH on biological activity
Living organisms are an important component of the soil, and 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.
4.5.7 Soil pH across the dairying areas of Victoria
The pH (water) of the majority of the soils in the high rainfall areas of southern Victoria is in the range of 5 to 5.8 (Figure 4.10).
Source: State Chemistry Laboratory.
Figure 4.10 Soil pH (water) in Victoria
The soils in the Goulburn Valley of the Northern Irrigation area were slightly acidic to neutral (6 to 7) in their natural state. Those in the Kerang/Swan Hill areas were neutral to moderately alkaline (7 to 8). Northern dryland regions, such as the Kiewa Valley, had pH levels in the range 5.2 to 6 in their natural state. (Soil pH has decreased slightly due to agricultural practices, particularly in the northern areas of the state.) However, several soil types in southern and north-eastern Victoria have pH levels in the mid 4’s, and many soils in north-west Victoria are naturally alkaline.
The soils of the basalt plains of south-west Victoria and the sedimentary soils of far west Victoria generally have a soil pH between 5.5 and 6. In the higher rainfall areas (Otways, Central Highlands and to some extent the Dundas Tablelands), the soil pH is generally strongly acidic (below 5.5).
The sandy soils extending in a strip along the coast from Warrnambool to Nelson, the red-brown earths from Werribee to Geelong, and the lunette soils have a soil pH closer to neutral (pH of 7).
4.5.8 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.
How does lime work?
Liming materials consist of calcium and magnesium carbonates. When applied, the carbonates slowly dissolve in the acid soil solutions and replace the hydrogen ions with the calcium and magnesium ions. As the hydrogen ions are replaced by the calcium and magnesium ions, the pH of the soil rises. Figure 4.11 shows a simplified version of these chemical reactions.
Figure 4.11 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 also 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 clay loams, have a high pH buffering capacity. 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.
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 usually applied during summer or autumn because the heavy truck loads require firm ground, although it can be applied at any time of the year.
Lime application at sowing
Lime is relatively insoluble (does not dissolve easily). Thus, it is slow to react. For maximum benefit, it should be worked into the soil when resowing a pasture or sowing a summer fodder crop. Table 4.5 shows the recommended application rates of lime if applied to an area to be sown.
Table 4.5 Lime recommendations at sowing
| Existing pH (CaCl2) |
Existing pH (water) |
Recommendation |
|---|---|---|
|
5.1 and above |
5.7 and above |
No lime |
|
4.5 to 5.0 |
5.1 to 5.6 |
2.5 t/ha |
|
4.2 to 4.4 |
4.8 to 5 |
3.75 t/ha to 5 t/ha |
|
Less than 4.2 |
Less than 4.8 |
5 t/ha to 7.5 t/ha |
The pH of the soil will be the guide to the likely need for lime when re-sowing 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 by cultivation is recommended, depending on soil pH and sowing method.
Figure 4.12 indicates the amount of lime with a neutralising value (NV) of 100 required, by incorporation into the soil, to raise the pH level over a particular range on various soil types.
Source: Adapted from Seeliger (1973).
Figure 4.12. The relationship between pH and lime application rates at sowing (lime of NV 100) on different soil types For example, to raise the pH (CaCl2) of a sandy soil from 5.0 to 6.0 would require approximately 2 tonnes of lime per hectare. This graph has been modified to indicate possible lime requirements on clay loams. Since clay loams vary substantially in their proportions of sand, silt, and clay, estimations of lime applications based on this alteration must be used as a very rough guide only.
Note: Table 4.5 and Figure 4.12 are rough guides only. For a more accurate estimate of lime application rates, ask your laboratory to do a pH buffering test. Also, Figure 4.12 is based on a lime with a neutralising value of 100, not an effective neutralising value of 100.
Lime application as a topdressing
Lime can be applied as a topdressing (in other words, spread on 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 of a unit for each 1 tonne/ha of lime applied. More acidic the soil, show a greater the pasture response. Even on soils with a pH (water) of 5.8, responses were still measured. 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, a pasture response of 0.7% per tonne of lime applied is needed to breakeven. 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- to 10-cm range of the soil profile, an increase of 0.5 to 1.0 of a 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.
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 (Table 4.6). In the pH range of 4.3 to 4.6, either apply 2.5 t/ha 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 4.6 Lime recommendations on existing pasture
| Existing pH (CaCl2) |
Existing pH (water) |
Recommendation |
|---|---|---|
|
4.7 and above |
5.3 and above |
No lime |
|
4.3 to 4.6 |
5 to 5.3 |
Test strip or 2.5 t/ha |
|
Less than 4.3 |
Less than 5 |
2.5 t/ha* |
*Another 2.5 t/ha should be applied after 3 years on soils with a pH (CaCl2) less than 4.3.
Note: Table 4.6 is a rough guide only. For a more accurate estimate of lime application rates, ask your laboratory to do a pH buffering test.
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 via 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.
4.5.9 The effective neutralising value of lime and dolomite products
There are many sources of lime. They vary in their ability to change the soil pH and the speed at which this happens. This is referred to as their effective neutralising value (ENV).
Two factors influence the effective neutralising value of a lime product:
- The amount of calcium or magnesium as oxides or carbonates, expressed as a percentage.
- The fineness of the lime. The finer the product, the greater the surface area for the neutralising chemical reactions to occur.
Limes are ranked according to their ENV. The higher the ENV, the more effective the lime will be at increasing pH.
The Fertiliser Regulations 1995 have set the following standards for lime and liming materials (on a dry matter basis):
- Grade 1 lime must have a minimum ENV of 80.
- Grade 2 lime must have a minimum ENV of 65.
- Grade 3 lime must have a minimum ENV of 50.
Table 4.7 contains independently conducted analyses of limes and dolomites as supplied in Victoria and South Australia. (Not all lime companies are represented in Table 4.7.) This information has been compiled by the Australian Fertiliser Services Association in conjunction with the Victorian Limestone Producers Association.
Note that the ENV in Table 4.7 is calculated on an ‘as received basis’, so that moisture is taken into account. (In other words, the ENV has been calculated on a wet-weight basis.) Moisture level at the time of delivery should be checked with the supplier.
The lime analysis prepared for the survey is indicative of the lime quality at the time of sampling. However, because lime quality can vary due to changes in the area or depth mined or the degree of crushing, the analytical results of future lime samples from the same company may vary. If you are concerned about lime quality, samples can be submitted to the State Chemistry Laboratory for testing.
Table 4.7 Lime survey results (2001)
| Lime Company | CaCO3 | MgCO3 | Dry Matter % | NV | ENV As Received |
|---|---|---|---|---|---|
| Agricultural Limes | |||||
| Calcimo CalMag Lime | 48 | 15 | 90 | 61 | 45 |
| Calcimo Dried Lime | 75 | 2.5 | 98 | 84 | 67 |
| Calcimo Semi-Dried Lime | 53 | 2.1 | 96 | 62 | 50 |
| Codrington Lime | 70 | 2.8 | 97 | 79 | 72 |
| Couch Screened Lime | 77 | <2 | 89 | 84 | 62 |
| Darriman Lime | 53 | <2 | 87 | 56 | 39 |
| David Mitchell Buchan Lime | 88 | <2 | 99.5 | 97 | 72 |
| David Mitchell Lilydale Lime | 71 | 9.8 | 97 | 91 | 77 |
| David Mitchell Lilydale Lime (moisture added) | 68 |
15 |
98 |
93 |
71 |
| David Mitchell Lara Lime | 61 | 3.5 | 95 | 69 | 47 |
| David Mitchell Traralgon Lime | 92 | <2 | 99.8 | 99 | 84 |
| Gambier Earthmovers Lime | 89 | 2.1 | 93 | 93 | 70 |
| Gillear Lime 5 mm | 80 | <2 | 93 | 88 | 76 |
| Gillear Lime 3 mm | 77 | <2 | 91 | 86 | 76 |
| Green Valley Lime | 74 | <2 | 92 | 81 | 64 |
| Hillside Lime | 54 | <2 | 94 | 58 | 45 |
| Hillside Lime 2 mm | 57 | <2 | 92 | 61 | 53 |
| Kalari (Bridge Water) | 72 | <2 | 96 | 79 | 62 |
| Kalari (Heywood) | 70 | <2 | 88 | 81 | 61 |
| Kurdeez Dried Lime | 79 | <2 | 99 | 90 | 83 |
| Kurdeez Screened Lime | 72 | <2 | 87 | 80 | 57 |
| Lakeside Lime | 74 | <2 | 90 | 82 | 70 |
| Warrnambool Limeworks Lime | 74 | 6.6 | 99 | 90 | 75 |
| Dolomite | |||||
| Gambier Earthmovers Ag Dolomite | 60 | 36 | 96 | 99 | 82 |
4.5.10 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.
First, 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).
Second, 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.
Finally, 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 produce 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.
4.5.11 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 Victoria, 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.
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.
4.6 Summary
Apart from nutrients, many other soil-related factors (for example, slaking, dispersion, sodicity, compaction, salinity, waterlogging and soil acidification) can substantially limit plant growth.
Soil structure can be improved by increasing organic matter, improving drainage, and using gypsum on some soil types.
Applying lime to acidic soils will increase soil pH.
Limes vary in their ability to reduce acidity.
Limes should be applied on the basis of soil test analyses and purchased on the basis of effective neutralising values and cost
