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Measuring soil salinity in irrigated horticulture2010411225432

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					Measuring soil salinity
in irrigated horticulture


Soil salinity refers to the total amount of salts present      • Under-irrigation and inadequate drainage or
in the soil or water. In Australia salinity is dominated by      leaching of salt out of the root-zone.
sodium chloride salt. Soil and water also contain salts of     • Poor soil structure that limits drainage or leaching.
calcium, magnesium, potassium, carbonate, bicarbonate,
sulphate, borate and nitrate.                                  • Uneven uniformity across the irrigated area,
                                                                 resulting in local areas of inadequate leaching.
Some salts e.g. fertilisers, are useful but too much salt of   • Salinisation of the root-zone by high water tables,
any kind is detrimental to plants and other organisms.           which may bring salt from other areas or from the
                                                                 soil below.
Accumulation of salts in the root zone can have drastic
effects on the growth and yield of horticultural crops. A
                                                               Measuring soil salinity
very high concentration of soluble salts can kill plants.
                                                               Salt concentration in soil is measured by the electrical
Figure 1 shows a severely burnt grapevine caused by
salinity.
                                                               conductivity (EC) of soil water, or of a saturated paste
                                                               extract made from a soil sample. Theoretically, EC of
                                                               the soil solution or soil water (referred hereafter as
                                                               ECsw) is a better index of soil salinity than EC of the
                                                               saturated paste extract called ECe. This is because
                                                               the plant roots are exposed to the soil water; they
                                                               extract their nutrients from it, absorb other solutes from
                                                               it and they consume this water through the process of
                                                               transpiration. The most common unit of soil salinity is
                                                               deciSiemen per metre (dS/m). One dS/m is equivalent
                                                               to one thousand EC units.

                                                               Soil sampling for soil salinity
                                                               Standard soil sampling procedures should be followed to
                                                               obtain a representative sample from different soil types
                                                               or from specific problem areas on the property. Samples
                                                               should be taken from different depths in the root-zone,
                                                               e.g. 25, 50, 75 and 100 cm for perennial crops, and 10,
Figure 1. Grapevine leaf damage caused by salt.
                                                               25 and 50 cm for shallow-rooted annual crops.
Effects of soil salinity                                       In drip irrigated vineyards/orchards soil samples should
A high concentration of salts dissolved in soil water can:     be taken at the same distance from a dripper.
• Restrict a plant’s ability to take up water from the soil.   Samples should be taken on a regular basis at the
   Both water and dissolved salts are taken up by plant        beginning and end of the growing season, eg. in late
   roots. High concentrations of salt produce a high           Spring and again in Autumn. Trends should be followed
   osmotic pressure, which the plant must work against to      over time to ensure that soil salinity is not building up and
   extract water from the soil.                                that leaching practices are effective. Soil samples taken
• Have a direct toxic effect on the plant. Some ions,          to determine salinity levels can also be used to determine
   especially sodium, chloride and boron, if taken up in       nutrient levels for fertiliser recommendations.
   large amounts, have a toxic effect on plant metabolism.
• Adversely affect the physical structure of soil. High        Soil water sampling for soil salinity
   concentrations of sodium in relation to calcium and         Soil water can be extracted directly from the root-zone
   magnesium can cause clay particles in the soil to           using inexpensive equipment such as suction cups placed
   separate from each other. This produces tiny soil           at different depths. Samples can be tested in the field by
   particles that block soil pores and limit water, air and    using a hand-held electrical conductivity meter.
   root movement in the soil.
                                                               A suction cup is a custom designed ceramic cup glued to
                                                               a short length of casing, housing an extraction tube with
Causes of soil salinity                                        a two-way stopcock. It is used to extract soil water over
Salts are naturally present in all soils. However              a range of soil moisture conditions from wet (0 kPa) to
additional salts can build up in the soil root-zone by:        relatively dry (60 kPa). Once under vacuum, the suction
• High concentrations of salts in irrigation water or          cup draws moisture from the surrounding soil and stores
  fertiliser applied.                                          it in the inert ceramic cup. For drip irrigated vineyards


Irrigation and Salinity Fact Sheet number 1
August 2008
Measuring soil salinity in irrigated horticulture                                                          Irrigation and Salinity Fact Sheet number 1


suction cups are typically placed at 30, 60 and 90 cm in
a root-zone of 1 metre, and located about 15 cm from a
dripper along the line of a drip irrigation system.
The advantage of using suction cups is that they allow
growers to monitor salinity levels continuously, which gives
a better idea of the actual variation in soil salinity as it
occurs. This information can assist in making management
decisions and may help prevent potential salinity problems.
One example of a commonly used commercial unit in
Australia is the SoluSAMPLER™ (Figure 2) marketed
by Sentek Sensor Technologies (www.sentek.com.au) in
Adelaide, South Australia. A manual with installation and
operation instructions for suction cups has been developed
                                                                               Figure 2. The SoluSAMPLERTM, a real time soil water salinity
by the South Australian Research and Development
Institute (SARDI) and is available from the Cooperative                        measuring device.
Research Centre for Irrigation Futures or from Sentek.

How SoluSAMPLER™ ECsw readings relate to saturated paste extract ECe and soil suspension EC1:5
The relationship between ECsw and ECe was derived by taking SoluSAMPLER™ water samples and soil cores from around
the SoluSAMPLER™ from a number of properties in the Riverland of SA. In many cases EC measured in 1:5 soil:water ratio
(EC1:5) is used as a soil salinity indicator. SoluSAMPLER™ ECsw was found to be twice that of ECe and 30 times that of
EC1:5.

Interpretation of SoluSAMPLER™ readings for salt tolerance of horticultural crops
General salt tolerance levels measured for wine grapes and other horticultural crops, and expressed as SoluSAMPLERTM
ECsw readings, are reported in Tables 1 and 2. These values should be used as a guide only. ECsw values may require
adjustment depending on agronomic management, irrigation salinity, soil moisture content, variety, soil type and leaching
efficiency.

 Table 1. Rootzone salinity threshold for grapevines when measured in SoluSAMPLER™ solution.
 Crop sensitivity                    Varieties                                                                            ECsw at which yield
                                                                                                                          decline starts (dS/m)
 Sensitive to moderately             Own roots (Vitis vinifera): e.g. Sultana, Shiraz, Chardonnay.
                                                                                                                                    3.6
 sensitive                           Rootstocks: 1202C, Kober 5BB, Teleki 5C, S04
 Moderately tolerant to tolerant     Rootstocks: e.g. Ramsey, 1103 Paulsen, Ruggeri 140, Schwarzmann, 101-14,
                                                                                                                                    6.6
                                     Rupestris St. George.
Modified from Zhang et al.2002. Australian Journal of Grape and Wine Research 8: 150-156.
R Walker, CSIRO Merbein, personal communication.

 Table 2. Rootzone salinity threshold for tree crops when                           Authors: Tapas Biswas1 John Bourne2
 measured in SoluSAMPLER™ solution                                                  1
                                                                                     SARDI Water Resources & Irrigated Crops,
     Tree        Threshold ECsw               Threshold ECsw (dS/m) for             Adelaide SA 5001
    Crops           (dS/m) for               reduced yield levels (impact           2
                                                                                    DWLBC Irrigation Programme,
                maximum production                of higher salinity)               GPO Box 2834 Adelaide SA 5001
                       100% yield              75% yield       50% yield
   Orange                  3.4                      6.6           9.6               Further information
  Grapefruit               3.4                      6.6           9.6
                                                                                    SARDI Water Resources and Irrigation
                                                                                    GPO Box 397 Adelaide SA 5001
   Lemon                   3.4                      6.6           9.6
                                                                                    Phone: 08 8303 9400
   Apricot                 3.2                      5.2           7.4
                                                                                    Email: biswas.tapas@saugov.sagov.au
    Peach                  3.4                      5.8           8.2
                                                                                    www.sardi.sa.gov.au
Modified from Maas and Hoffman,1977. ASCE J.Irrig & Drainage
Div. 103: 115-134.


                                                     This research project is supported by




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