Salinity measures, units and classes

Salinity measurement, monitoring and equipment
To understand the 'saltiness' of a salinity reading, you must know if it was soil or water being measured, the way it was measured (or estimated in most cases), and the units being used. Always ask for the details from your laboratory or advisor. For example, moderately salty soil could be presented as 600 mS/m (ECe), 125 mS/m (EM-38 hor), or 60 mS/m (EC1:5 (w/v) loam).

• Salinity measures include electrical conductivity of a solution or soil and water mix, weight of salts in a given amount of water, and the quantity of molecules of salts in a solution. Each of these measures have particular uses. A popular tool to measure is the EM38 (ElectroMagnetic Induction Metre) which estimates electrical conductivity.
• Units includes grains per gallon, milligrams per litre, milliSiemens per meter, deciSiemens per centimetre and many more. Unfortunately, units used vary between States, and even between different organisations within States.
• Classes (for plants in this case) range from non-saline to extremely saline. On this page, soil salinity classes (from non-saline to extremely saline) refer to plant tolerances. These are now Australian standards that are also used in some international publications. Animal tolerances to water salinity are provided on another page.

Salt tolerance of plants is affected by other factors than the soil salt reading; waterlogging, soil type, sodicity, depth to watertable, salinity of the watertable, rainfall and other factors. Waterlogging interacting with salinity is probably the most important influencing factor on most sites.
Most 'measures' of salinity use electrical conductivity to estimate salinity of soil and water. These measures are cheap and easy to do, and can even be done (with some care) in the field.

• Soil salinity measures
• Soil texture and salinity estimates
• Water salinity measures
• Salinity classes for revegetation
• Soil texture and salinity class

Soil Salinity

EC1:5

Soil samples can be measured by the '1:5' w/v method - one part by weight (g) air dried soil to five parts by volume (ml) distilled water, which is agitated then allowed to settle, then the solution is measured for Electrical Conductivity (EC1:5).
This measure is usually interpreted to allow for soil texture differences. Sand particles will not hold as much salt from the soil water as will clay. Therefore, sand will give apparently lower readings than from clay, even though the soil water (which is the part affecting plant roots) is the same.
Some field surveys estimate an EC1:5 based on volume:volume (v/v). That is, one scoop of soil to five scoops of water. This can lead to large errors in reporting relative to the w/v method and between samples from the same site, especially for clay soils. It would pay to 'calibrate' any EC1:5 estimated from the v/v technique against the w/v estimated technique. The technique is not recommended for critical measurements.

ECe

The Electrical Conductivity of a saturated soil Extract (ECe) is the most useful and reliable measure of salinity for comparing between soil types. The 1:5 reading can be used to estimate the ECe. Conversion factors from EC1:5 to ECe are below. Note that these are guides - the actual conversion figure can vary quite widely for particular soils. Note also, that proper measurement of the ECe is a laboratory technique, and expensive.
A technique for converting reported ECe to EC1:5 can be used in the reverse direction; for converting EC1:5 (less than 350 mS/m) to ECe. The technique is based on a limited data set, and care should be taken extrapolating the technique.

EM38

Another, increasingly common estimate is with the EM 38 (see the image to the right) or EM 31 using Electro-Magnetic induction. EM readings are useful to compare within and between similar sites, but use EM readings with caution unless they are calibrated against soil salinities (ECe for preference) and other influencing factors. The EM38 is quite an expensive unit, but relatively easy to use in the field, and gives estimates of root zone.

Soil Texture and Salinity

Factor to estimate ECe from EC1:5 (w/v):

Soil texture	Multiply EC1:5 (w/v) by factor	Example
Sand sandy loam loam clay loam light/medium clay heavy clay	15 12 10 9 8 6	10 mS/m (EC1:5) = 150 mS/m (ECe) 10 mS/m (EC1:5) = 120 mS/m (ECe) 10 mS/m (EC1:5) = 100 mS/m (ECe) 10 mS/m (EC1:5) = 90 mS/m (ECe) 10 mS/m (EC1:5) = 80 mS/m (ECe) 10 mS/m (EC1:5) = 60 mS/m (ECe)

Water Salinity

Nearly all testing will use electrical conductivity. The results are reported in several units, although the standard in Western Australia is millisiemens per metre (mS/m). Other common, uncommon and historic units still used are 'Parts Per Million' (ppm), which can be thought of as the same as milligrams per litre (mg/L), osmotic potential (OP), % Total Soluble Salts and grains per gallon (gpg)

Conversions:

mS/m x 0.01 = dS/m (decisiemens per metre). For soil or water measures

mS/m x 5.5 = mg/L = ppm (assuming sodium chloride is the only salt present; this figure increases with other common salts and may be as high as 6). For water.

mS/m x 0.42 = gpg (Grains per Gallon. This conversion is rough, and the factor depends on salts present). For water.

mS/m x 0.0034 = % TSS in soil from a EC1:5 (w:v) measure (conversion varies based on salts present). For soil.

mS/m x 0.00055 = % TSS in water from an ECw (conversion varies based on salts present). For water.

mS/m x 10 = mmol/dm3 NaCl = mmol/litre NaCl. For water.

Salinity measures and ranges in the tables below are guides for revegetation. Some of these measures are more useful in predicting plant growth (for instance, EM38), others are cheap and easy (eg EC 1:5 v/v), and others are reliable standards.
The ECe figures are widely accepted by research and are 'standards', but the groundwater, EM38 and EC1:5 measures can vary between sites for the same apparent salinity. All of these measures are trying to relate plant growth to a salinity level. Units used vary between Australian States and even within local communities.
Ask local specialists to interpret results of measures.
Table 1

Salinity classes for plants. Note these are specific to soil or water measures.
	ECe (dS/m)	ECe (mS/m)	ECgw (mS/m)	EM-38 hor (mS/m)	NaCl (sol. mmol/l)	EC1:5 (w/v) loam (mS/m) approx	EC1:5 (v/v) loam (mS/m) approx	Total Dissolved Solids (TDS %)
	Soil (a)	Soil (b)	Water (c)	'Soil' (d)	Water (e)	Soil (f)	Soil (g)	Water
Non-saline	<2	<200	<500	<50	<20	<20	<40	<0.275%
Slightly	2-4	200-400	500-1000	50-100	20-40	20-40	40-80	0.275-0.55%
Moderately	4-8	400-800	1000-2000	100-150	40-80	40-80	80-160	0.55-1.1%
Very	8-16	800-1600	2000-3000	150-200	80-160	80-160	160-320	1.1-1.65%
Extremely	>16	>1600	>3000	>200	>160	>160	>320	>1.65%
(a) Based on USDA 1954 categories: Used by CSIRO Canberra and others in Australia. (b) Units used in Western Australia (c) Groundwater from within potential rooting distance of plant (bores). Suitability for 'tree' growth. (d) From D Bennett and R George, DAWA Bunbury. (e) 'Irrigation' water used in pot trials.

Soil texture affect on ECw/v measures and salinity class

This table is given because ECw/v measures are very commonly used to estimate salinity from field samples. The measure is usually done in a laboratory.
Note that these values are estimates of salinity in the plant root zone. When salinity is measured from the surface layer, the estimate of salinity at the root zone may be incorrect. Note also, that the EC1:5 is one part soil by weight to five parts of water by volume. Where the EC1:5 volume:volume technique is used in the field, it is prone to large errors.
Table 2

Soil salinity measurements using EC1:5(w/v) in mS/m
Salinity Rating	Sand	Sandy loam	Loam	Clay loam	L/Med Clay	Heavy Clay
Non-saline	<13	<17	<20	<22	<25	<33
Slightly	13-26	17-33	20-40	22-44	25-50	33-67
Moderately	26-52	33-67	40-80	44-89	50-100	67-133
Very	52-106	67-133	80-160	89-178	100-200	133-267
Extremely	>106	>133	>160	>178	>200	>267

Page last reviewed November 2006