Why measure EC?
Irrigated land accounts for 40% of our food supply, and salts impact yields on about one-fifth of those acres. All irrigation water contains at least some salt. If salts are allowed to build up around the root zone of a crop, they injure plants, reduce yields, and even change soil structure causing long-term damage to the land itself. In order to preserve the productivity of irrigated land, it is important to understand how to manage salts.
The steps to managing salts are:
- Measure how much salt is currently in the soil
- Determine how much salt is added through irrigation
- Monitor continuously during irrigation to manage salts
Electrical conductivity (EC) is the key to making these measurements. Pure water does not conduct electricity, but most water, even tap water, has enough dissolved salts to be conductive. Because the concentration of salts in water directly affects its conductivity, the measurement of electrical conductivity is a very effective way of measuring salt concentrations in soil water.
Salt and plants: What’s the problem?
Most people have had the experience of fertilizing too heavily, perhaps by accident, and killing grass or other plants. It is often said that the fertilizer has “burned” the plants, but generally, it isn’t the nutrients themselves that cause the damage. It’s often their effect on water. Plants take up water, but they don’t take up salts in any appreciable quantity. When salt is added to the soil through fertilization and irrigation, it becomes concentrated there. Salt may cause a variety of problems for plants. For instance, Na+ may reach concentrations that are toxic to plants, even though the plant isn’t taking up any appreciable quantity. Salt also attracts water and makes it more difficult for plants to take up water from the soil. Some plants are more sensitive than others to salt in the soil. Bean yield will be affected if soil saturation extract EC exceeds 2 dS/m, for example, while barley can be grown without yield reduction in soil saturation extract up to 16 dS/m. Ultimately, however, high salt content will affect all plants.
|Sensitive ||Moderately Tolerant ||Highly Tolerant|
|Red clover ||Wheat ||Date palm|
|Pea ||Tomato ||Barley|
|Bean ||Corn ||Sugar beet|
|Pear ||Alfalfa ||Cotton|
|Orange ||Potato ||Spinach|
Table 1. Salt tolerance in crops
Common units for EC
The SI unit for electrical conductance is the Siemen, so electrical conductivity has units of S/m. Units used in older literature are mho/cm (mho is the reciprocal of ohm). Soil EC was commonly reported in mmho/cm. 1 mmho/cm equals 1 mS/cm, but because SI discourages the use of submultiples in the denominator, this unit is changed to deciSiemen per meter (dS/m), which is numerically equivalent to mmho/cm or mS/cm.
- Electrical resistance – ohm
- Conductance – 1/ohm
- mho – now Siemens
- Old units – mmho/cm
- Modern units – mS/cm or dS/m
|USDA Class ||Saturation Extract |
|Salt in Soil |
(g salt/100g soil)
|Osmotic Potential |
|Crop Tolerance ||Example Crops|
|A ||0-2 ||0-0.13 ||0 to -70 ||Sensitive ||Bean|
|B ||2-4 ||0.13-0.26 ||-70 to -140 ||Moderately sensitive ||Corn|
|C ||4-8 ||0.26-0.51 ||-140 to -280 ||Moderately sensitive ||Wheat|
|D ||8-16 ||0.51-1.02 ||-280 to -560 ||Tolerant ||Barley|
Table 2. Salinity classes for soils (Richards, L.A. [Ed]. 1954. Diagnosis and Improvement of Saline and Alkali Soils, USDA AG Handbook 60, Washington DC)
More than one way to measure EC
There are three ways to measure EC in soils: measuring pore water EC, bulk EC, or saturation extract EC. All three are related, but there are tools to convert one into the other. In order to understand measurement data, it is important to know what type of EC is being measured.
Pore water EC: what many researchers assume they’re measuring
Pore water EC or soil water EC (σw) is the electrical conductivity of the water in the soil pores. Researchers often mistake the value coming out of a soil EC sensor for pore water EC. It would be ideal to simply measure the electrical conductivity of pore water in situ. Try to imagine how this would work, however. Tiny sensors would have to be inserted into microscopic water-filled pores. Obviously, it’s not possible to measure the EC of water on that scale. In fact, the only way to measure pore water EC is by extracting a soil water sample and measuring the EC of that sample.
Bulk EC (σb) is the electrical conductivity of the bulk soil (soil, water, and air). Soil moisture sensors installed into the soil all measure bulk EC. Empirical or theoretical equations can be used to determine pore water EC and saturation extract EC (σe) from measured bulk EC values. Bulk EC is the only EC measure that can be continuously monitored in situ.
Saturation extract EC: the traditional method
Saturation extract EC (σe) tells exactly how much salt is in the soil and can be converted to soil salinity. This is the traditional way to measure EC. It is measured by taking a soil sample, making a saturated paste of soil and deionized water, extracting the water, and then measuring the EC of the extracted solution. Published EC values reported in the literature are almost always saturation extract EC.
Converting bulk EC to pore water EC
As stated previously, in situ sensors measure the electrical conductivity of the bulk soil surrounding the sensors (σb). A considerable amount of research has been conducted to determine the relationship between σb and the conductivity of the pore water (σw). Hilhorst (2000) has taken advantage of the linear relationship between the soil bulk dielectric permittivity (εb) and σw to allow conversion from σb to σw if εb is known. The TEROS 12 sensors measure εb and σb nearly simultaneously in the same soil volume. They are well suited to this method. The pore water conductivity can be determined from (see Hilhorst, 2000 for derivation)