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MODULE 6 : Healthy Soils
Tool 6.5
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Soil pH (acid soils)

A pH of 7.0 is neutral (above 7 is alkaline) so, strictly speaking, any soil with a pH below 7 is acidic. However, many plants prefer slightly acid conditions so the definition of acid soils is those with a pH in calcium chloride of less than 4.6 or less than 5.5 in water. Plant roots are often affected by aluminium (Al) and manganese (Mn) toxicities as these elements are more soluble in acid soils. Waterlogging of soils high in iron (Fe) can also cause toxicity (eg, in the Dundas Tablelands, western Victoria) and, over time, the formation of insoluble oxides can reduce drainage (eg, in the Adelaide Hills, South Australia) and cause a breakdown in soil structure (eg, the tablelands of New South Wales).

The table titled Preferred pH range of some common pasture species shows the preferred soil pH ranges for common pasture species, while the table titled Critical aluminium concentrations for growth shows the aluminium concentrations at which growth is affected for common introduced pasture species.

Native species, such as wallaby grass/white top (Austrodanthonia spp), weeping grass (Microlaena stipoides), kangaroo grass (Themeda triandra), redgrass (Bothriochloa macra), and windmill grasses (Chloris spp) are not included as they are rarely sown and limits are not as well defined. Weeping grass is able to tolerate soils with low pH. The many sub-species of wallaby grass have different preferred pH ranges. Kangaroo grass and redgrass grow on low to neutral pH soils, while windmill grasses prefer heavier neutral to alkaline soils.

Most plants and micro-organisms have defined ranges of pH for optimal growth. The optimal range for plants is generally between 5.5 and 8.0 (pH in CaCl2) whereas most soil organisms function best between pH 6.0 and 7.0. All species will grow outside their pH limits, but productivity and persistence may be less than their potential.

Preferred pH range for some common pasture species

Plant species Soil pH (water)A Soil pH (CaCl2)B
White clover 6.0–7.0 5.3–6.3
Sub clover 5.2–7.0 4.5–6.3
Perennial ryegrass 5.3–7.0 4.6–6.3
Annual ryegrass 5.3–7.0 4.6–6.3
Phalaris 5.7–7.5 5.0–7.8
Cocksfoot 5.0–7.5 4.3–6.8
Lucerne 6.0–8.0 5.3–7.3

A Acid Soil Action (NSW DPI).
B pH (CaCl2), in most soils, pHCa is normally about 0.7 (0.6 in severe acid soils to 1.0 in alkaline soils) units lower than pHw, eg, 5.0 pHw (-0.8) = 4.2 pHCa

Critical aluminium concentrations for growth

  Soil test level above which yields are reduced

 
Species

Al (% of Cation Exchange Capacity) 0.01M CaCl2 (mg/kg Sensitivity
Lucerne
Barley
Medics
Canola
5 2 highly sensitive
Red clover
Phalaris
Sub clover
Wheat
10 4 sensitive
Woolly pod vetch
Ryegrass
Some oats
Tall fescue
Cocksfoot
20 8 moderately tolerant
Oats
Triticale
Cereal Rye
Lupins
30 13 highly tolerant

Modified from Acid Soil Action, NSW DPI

Managing acid soils

For soils classified as acidic (pH in calcium chloride of less than 4.6 or less than 5.5 in water), about 1.5–2.5 tonnes of lime per hectare will be required to raise the pH in the top 10 cm by 0.5 of a pH unit. Soils with a lower cation exchange capacity (CEC), such as sands, will respond more quickly to lime than soils with higher CECs, such as clay soils. If your soil test gives exchangeable aluminium values, another rough guide is to multiply the value (cmol/kg) by 1.5 and that is the amount of lime required in tonnes/ha to limit AI toxicity issues, (e.g. 0.5 cmol AI/kg requires 0.75 t/ha lime).

Liming is expensive and before considering it, ask the following questions:

  • Is my pH above 5 (pasture growth response less likely)?
  • Is my subsoil acid (not usually practical or economic to overcome)?
  • Is the pasture aluminium tolerant (if yes, a lime response is unlikely)?
  • Is there something else (eg, phosphorus) that is the most limiting factor (again, if yes, then a lime response is unlikely)?

Because of the need for incorporation, lime application is generally not recommended for native pastures. Some native pastures are very acid tolerant. However, when sowing paddocks with high aluminium levels to phalaris or lucerne, to ensure good establishment, liming should be given higher priority than topdressing other established pastures. From an economic perspective, building soil phosphorus levels should come before liming.

Liming works best when the product is finely ground, the lime is incorporated into the soil (since it is relatively insoluble and so moves slowly down the profile) and where the soil surface is acidic, but the subsoil is not. Soils are best limed six months before sowing a new pasture or during a cropping phase when acid intolerant crops are grown, as these will give a quick return on the investment.

Liming to raise pH also increases the activity of soil organisms, which in turn benefits soil health.

See State Primary Industry Department websites for a range of resources by typing acid soils into the search bar.

Soil salinity

Soil salinity is usually assessed by measuring the electrical conductivity (EC) of the soil because conductivity is closely related to the level of salt in the soil solution. Most commonly, the EC is measured in a 1:5 soil/water solution (ECw). The interpretation of this test varies with soil type. Another method is to measure the EC in a denser, soil/water paste (ECe). With this method, the results are independent of soil type and, while it is less commercially available, it is sometimes calculated and reported in soil tests.

Salinity ratings of soils measured by the two methods ECe and ECw

    ECw (dS/m)      
Salinity rating ECe (dS/m) Sand Sandy loam Loam Clay
Low 0–2 0–0.15 0–0.18 0–0.2 0–0.3
Moderately saline 2–6 0.15–0.46 0.18–0.55 0.2–0.60 0.3–0.86
Highly saline 6–15 0.46–1.15 0.55–1.36 0.6–1.5 0.86–2.14
Extremely saline Over 15 >1.15 >1.36 >1.5 >2.14

 

Responsiveness of a range of pasture species to soil salinity

  ECe (dS/m)   ECw (dS/m)              
  No
effect
30%
red*
  No effect level
      30% reduction in growth
Species       Sand
Sandy
Loam
Loam
Clay
  Sand
Sandy
Loam
Loam
Clay
White clover 1.5 4.0   0.12 0.14 0.15 0.19   0.31 0.36 0.40 0.50
Sub clover 1.5 4.0   0.12 0.14 0.15 0.19   0.31 0.36 0.40 0.50
Perennial ryegrass 5.6 9.5   0.43 0.51 0.56 0.70   0.73 0.87 0.95 1.19
Annual ryegrass 5.6 9.5   0.43 0.51 0.56 0.70   0.73 0.87 0.95 1.19
Phalaris 4.6 8.5   0.35 0.42 0.46 0.58   0.66 0.78 0.85 1.07
Cocksfoot 1.5 6.3   0.12 0.14 0.15 0.19   0.49 0.58 0.63 0.79
Tall fescue 3.9 9.6   0.30 0.35 0.39 0.49   0.74 0.87 0.96 1.20
Lucerne 2.0 6.1   0.15 0.18 0.20 0.25   0.47 0.56 0.61 0.76

*red = reduction

Managing saline soils

Plants growing in saline soils may face the combined challenge of high salt levels, waterlogging that exacerbates the salinity impact, and extreme grazing pressure (because sheep have a strong preference for grazing salty areas). However, a range of pasture species is available (including saltbushes, tall wheat grass, puccinellia and, to a lesser extent, balansa clover and burr medic) that will grow well in saline land, and because the saline sites are usually wetter for longer, these pastures can be highly productive if the salinity levels are not too high. Out-of-season pasture production can be another advantage.

The science and technology for establishing and managing saltland pastures has advanced rapidly in recent years but, because saline sites are very variable, it is a good idea to seek local advice from experienced sheep producers or professionals before implementing a program to establish saltland pastures. Tool 5.9 in Protect Your Farm’s Natural Assets contains the current best practice guidelines for productive management of saline land, including some lower cost options for less affected areas.

Areas that are only moderately salt-effected are more financially viable to rehabilitate than areas that are severely effected. Highly saline soils are relatively unproductive even after rehabilitation.

For more information on saline soils see:

Managing Pastures in Saline Areas: read chapter 10 in the second edition of Greener Pastures for South-West Victoria edited by Z. Nie and G. Saul.

Saltland Genie: a website dedicated to the latest knowledge and tools for saltland management. Visit: www.saltlandgenie.org.au.

Soil sodicity

In technical terms, sodicity is a measure of the exchangeable sodium percentage (ESP), which indicates how much (percentage) of the cation exchange capacity is contributed by sodium. Sodic soils are unstable because the clays contain an excess of sodium, and soils with an ESP above 6% are classified as sodic. Gypsum (calcium sulphate) can reduce the dispersion of clay in soils by replacing some of the sodium with calcium. This can prevent surface crusting and so improve seedling emergence.

If your soil test indicates a high ESP (>6%), you can further test if your soil is likely to respond to gypsum by placing a soil aggregate (about 5mm in diameter) in water and leaving it for 24 hours. If the aggregate disperses and the water is cloudy then a gypsum response is possible. (Accurate prediction of effective rates for gypsum application to overcome sodicity is not yet possible.) Generally, rates of more than 5 tonnes/ha, incorporated into the soil, are used, which would almost never be economical in broadscale pasture situations.

Maintaining and increasing groundcover and organic matter levels are the keys to cost effective, long-term management of sodic soils. Gypsum is useful if you are going to cultivate the soil, but effectiveness is generally less than five years.

For more information on sodic soils see:

Sodic Soils: for information on management of sodic soils. Visit: agriculture.vic.gov.au/agriculture/farm-management/soil-and-water/soils/soil-physical-properties

Waterlogged soils

Waterlogging occurs when water fills the soil pores and does not drain away, thereby reducing oxygen availability, and reducing plant growth. Waterlogging can be caused by rising groundwater, or an impermeable layer, eg, where two soil horizons meet or a ‘hard pan’ from excessive cultivation. These are most prevalent on lower slope areas, on duplex and heavy clay soils.

There are no objective tests for impermeable soil layers other than digging a pit and looking:

1. For a bleached layer. This will indicate that the soil is likely to be waterlogged in winter. It is likely that this part of the soil will be poor in nutrients and most likely quite acid

2. At the clay layer and following the guidelines (below):

- A good rule of thumb is that uniform colour down the profile indicates uniform drainage characteristics

- Red soils are well drained, but if they are waterlogged for a period of time, the iron oxide (which is one of the components that gives soils their red colour) is converted to iron hydroxide which is yellow

- Mottling indicates fluctuating/changing drainage characteristics

- The progression (increasing waterlogging) is from red with yellow mottling to yellow with red mottling to yellow with grey/white mottling to grey/white with yellow mottling.

Plant production losses in waterlogged soils may also result from nitrate deficiency (lack of oxygen leads to soil nitrates being converted into a form plants cannot use) and fungal diseases (caused by plant roots in waterlogged soils being more susceptible to fungal attack). Waterlogging effects can be reduced by improving surface and subsurface drainage but you will need to carefully consider the feasibility and economics. Avoid grazing wet areas as pugging and further compaction will occur. Sowing waterlogging-tolerant species such as balansa and strawberry clovers can be a solution in some situations.

For more information on waterlogged soils see:

Managing Wet Soils. Go to http://agriculture.vic.gov.au/agriculture/dairy/managing-wet-soils 

Drainage: the WA DAF has a collection of publications on the costs, benefits and design of drainage systems for waterlogged areas. Visit: http://www.agric.wa.gov.au/ and use the search function to find articles on drainage..

Compacted soils

Soil compaction (an increase in bulk density) can occur as a result of grazing or cultivating wet soils. Routine grazing pressure often increases compaction in the surface 10-15cm. Hard pans can also occur as a result of repeated disc ploughing. To determine if a compaction layer exists, use a spade to examine the soil to about 30cm depth or use a backhoe to dig a deeper soil pit. Check for ‘j-rooting’ in tap-rooted species (where root growth is impeded by a hard layer) and if there is a mottled blue/grey layer, which indicates waterlogging.

Deep ripping is often proposed as a solution, but it is expensive, time consuming, can have variable results (particularly in sodic subsoils) and is unlikely to be economical for pasture production.

Well managed pastures (especially those with a perennial grass or legume content) will ameliorate compacted soils over time.

For more information on compacted soils: Visit: www.dpi.nsw.gov.au/land-and-water/soils/structure/compaction

Non-wetting sands

Water repellence in non-wetting sands is caused by plant waxes that coat the sand particles and prevent water from infiltrating the soil, particularly when it is dry. The result is poor germination and plant growth and, with large bare areas, it can greatly increase susceptibility to wind erosion. Techniques to combat the problem include direct drilling, sowing on the contour and sowing into the bottom of furrows with press wheels to improve soil–seed contact and establishment. Adding clay or cultivating to bring subsoil clay to the surface (delving) has been used successfully, but cost effectiveness depends on clay type and its incorporation into the soil, and on the distance that clay has to be carted from a pit to the paddock. If you think you have non-wetting sand you can approach a lab and ask for the MED (Molarity of Ethenal Dilution) test to assess severity.

Search the internet for more information on non wetting sands.