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Testing and interpretation of salinity and pH | AG0244 |
E. James, Knoxfield
March, 1995 |  |
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Salinity and pH are two chemical factors that should be given consideration when growing plants. They are involved in plant nutrition and soil fertility and can be manipulated to ensure that their effects do not limit plant growth. Measurement of pH and salinity is not difficult and many machines are available at a reasonable cost, so that routine measurements should be part of good nursery management.
Salinity
Salinity can occur naturally where drainage is poor, in inland areas that were once inundated by sea water (Murray-Darling Basin) or in areas with low rainfall and high evaporation. However, in the nursery soluble salts will most commonly be due to the use of fertilisers, most of which are salts (for example ammonium nitrate, potassium sulphate, etc). Salts are chemical compounds composed of two parts (ions) that have a positive or negative charge and so will carry an electrical current. Therefore salt concentrations can be determined by measuring the electrical conductivity of a solution. As the salt concentration increases, so does the electrical conductivity.
Symptoms of salt damage
These symptoms can be difficult to distinguish visually from soil-borne fungal diseases. Thus, specific tests are needed to determine the presence of salts.
Symptoms of salt damage include:
- brown stunted roots
- slow or no growth
- burning and dieback of young growth
- dead and brown sections on the margins and tips of older leaves
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Sometimes white encrustations of salt can be seen on the surface of soil or potting media or around drainage holes of pots. These indicate the presence of salt in excess of plant requirements and should be taken as an early warning of possible salinity problems.
Measuring salt levels
Salts can accumulate in either a soluble or insoluble form. Plants can only absorb soluble salts. Only soluble salts conduct electricity so that the conductivity of a solution gives a good guide to the salt content of water or growing media. Irrigation water can be measured directly, but the soluble salt content of soils or potting media can only be determined by extracting the salts into solution and then measuring the level of salt in the extract. By convention, electrical conductivities are measured at 25°C. Most conductivity meters have temperature compensation controls but, if not, corrections are always made to this temperature for an accurate result. Electrical conductivities are measured in deciSiemen/m. In practice, the quantities of salt measured are small so the results may be given as milliSiemen/cm (mS/cm) or microSiemen/cm (S/cm).
1 S = 1000 mS = 1 000 000 S. Older machines may give readings in mho/cm. This is equivalent to dS/m or S/cm.
Preparing sample extracts
Three different extraction methods are commonly used. It is essential when comparing results from different sources that the extraction method is known because it affects the interpretation of results.
- 1:5 extraction method - place 20 g of air-dried medium in a screw-top jar with 100 ml distilled water. Shake well (30 seconds every 5 minutes for a period of 60 minutes is suggested if no mechanical means are available). This is the most commonly used method in Victoria.
- Saturation extraction method - add distilled water to 250 g of air-dried sample until it has the consistency of a thick paste. Allow to stand for one hour, extract the soil water under vacuum and use the solution for measurement of electrical conductivity.
- 1:2 extraction method - similar to the 1:5 method except that the extract is made using a 50 g sample and 100 ml of distilled water.
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What do the results mean?
Table 1. For Irrigation water:
Conductivity reading
(mS/cm) or (dS/m)
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Progressively increasing danger of salinity problems. Sensitive plants will show symptoms at the lower end of the scale.
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To minimise problems where saline irrigation water is used:
- Use an open, well-drained mix and irrigate frequently to minimise increasing salt concentration as mix dries out and irrigate heavily to leach out any accumulated salt.
- Avoid light sprinkling of leaves with saline water.
- Attempt to keep growing conditions cool and humid.
- Find a better quality water source if possible.
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Table 2. For potting media:
Conductivity reading
(mS/cm)
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Very low. Potting medium needs fertilising.
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Usually safe. Higher values in this range may affect some species, particularly if drainage is poor. Seed germination may be affected.
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Possible salinity problems depending on species, potting mix and drainage. Specific ion effects likely. Check chlorine. Seedling growth likely to be affected.
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Plant growth likely to be affected. Leaf symptoms should be apparent.
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To minimise fertiliser salinity problems:
- Do not apply high concentrations of soluble fertilisers to force plant growth.
- If liquid feeding, use larger volumes of low concentration frequently rather than high concentrations infrequently.
- Occasionally leach pots with plain water to wash through any accumulated salts.
- Ensure that fertiliser is evenly mixed through the mix.
- Do not use potting mix that has been stored with controlled release fertiliser already mixed in (these may break down because of the heat generated during composting).
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pH
The term pH technically refers to the concentration of hydrogen ions in a medium. In practical terms it is a measure of the acidity or alkalinity of the growing medium and as such indicates the availability of nutrients to plants.
Most plants grow best in soils or soil-based media with pH values between pH 5.5 and 6.5. Artificial mixes usually require a slightly more acid pH of 5.0 to 6.5. Within these ranges all nutrients are readily available. At low pHs in soil-based media, aluminium and manganese can become toxic (although hydrangeas need high levels of aluminium to produce blue flowers) and calcium, magnesium and molybdenum may be limiting. As the pH rises to pH 7.0 and above, manganese, boron and particularly iron may become deficient. Plants such as azaleas and rhododendrons are sensitive to iron deficiency and therefore grow better at a lower pH. Where nutrients are continually supplied either through the irrigation water or slow-release fertilisers, the pH value is of lesser importance.
Most components of soilless media are acidic (for example, pinebark, peat, sawdust) so the problem is usually to increase the pH of the medium. Materials that can be used to increase the pH of soil or soilless media are slaked lime (Limil), ground agricultural limestone or dolomitic lime. It is preferable to use dolomitic lime as it is less caustic than Limil and provides the essential nutrient magnesium as well as the calcium that limestone supplies.
To lower the pH is much more difficult and expensive. A batch of potting mix with a pH higher than desired should be discarded. Check components to find the source of the high pH. In north-western Victoria, the inclusion of alkaline soils could be a factor. Scoria has a high pH, so its use in potting media should be in conjunction with components such as peat or pinebark to obtain a suitable pH.
Measuring pH
pH is measured most accurately with a pH meter containing a glass electrode. Most machines, like conductivity meters, will have temperature compensation controls. If not, adjustments must be made. Irrigation water can be measured directly but a soil:water extract needs to be made for media. The 1:5 extract method is most popular because the same extract can be used for both pH and electrical conductivity. Electrical conductivity measurements should be done before pH because the pH electrode contains potassium chloride, which can cause an over-estimation of the electrical conductivity.
What do the results mean?
Potting media that are very acidic (pH below 4.5) should be discarded or have the pH adjusted with dolomitic lime because of nutrient imbalances such as aluminium and manganese toxicity. Alkaline growing media should be discarded because they are likely to cause iron, manganese and boron deficiency in plants.
Conclusions
Growing media should have an electrical conductivity (1:5 dilution) of between 0.1 and 0.8 dS/m and a pH of between 5.0 and 6.5. The ideal levels of both pH and salt concentration will depend on the species being grown as well as the specific nursery practice.
Further Information
For further information, the following reading is recommended:
Peverill, K. and Lee, I. (1979), Salt - A growing nursery problem. Seed and Nursery Trader, October 1979, pp. 26-31.
Peverill, K. (1979). How much lime should I add to my potting medium? Seed and Nursery Trader, December 1979, pp. 29-31.
Lane, M. (1985). Salt damage in container plants. Australian Horticulture, January 1985, pp. 96-100.
Handreck, K. A. and Black, N. D. (1984). Growing media for ornamental plants and turf. University of N.S.W. Press.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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