SOIL ELECTRICAL CONDUCTIVITY - Back Paddock

1 soil ELECTRICAL CONDUCTIVITY ELECTRICAL CONDUCTIVITY (EC) is an indirect measure of the total salt concentration of a soil , or salinity. It is measured as the ELECTRICAL conductance of a soil solution or slurry containing soluble salts. The soluble salts contain both cations and anions. Any single or combination of cations and anions, if in high concentration, can cause a high soil EC. High soil EC can restrict water uptake by plant roots, inducing a drought like effect, even if the soil has high water content. Plant cells are surrounded by semi-permeable membranes and are subject to osmotic flows of water. By osmosis, water moves from areas of low salt concentration to areas of higher salt concentration until equilibrium is reached. As a result of an elevated osmotic pressure of the soil solution, plants are required to expend larger amounts of energy to extract water from the soil solution, thereby limiting growth potential .

2 In more extreme cases, as the soil dries, the soil solution concentrates and water may be pulled out of plant tissue. High EC is most commonly associated with; poor drainage and/or a rising water-table, poor quality irrigation water, seawater and microenvironments in the soil influenced by fertilizer bands. Measurement When soils are saline, the most desirable soil water content is field capacity (FC) or the upper drained limit (UDL) as this is the most dilute soil solution likely to be encountered by a growing plant. For practical purposes it is difficult to extract sufficient water for analysis from a soil at field capacity so a more dilute extract is commonly used EC from 1 part soil and 5 parts water by weight. EC (1:5) is SoilMate NutriFact ECS-06 Water Salt Phone: 07 3220 2959 Email: Web: directly measured by a CONDUCTIVITY meter and results are usually expressed in deciSiemen per metre (dS/m).

3 Conversions to other units are: 1 dS/m = 100 mS/m = 1 mS/cm = 1 mmho/cm = 1000 S/cm = 1000 mho/cm dS/m x 640 = ppm = mg/L dS/m x = osmotic potential (OP) in bars dS/m x = % total soluble salts (TSS) in 1:5 mix EC (1:5) is a useful measure for assessing salinity but because the 1:5 soil :water ratio dilutes the salt concentration from that at FC by 5 to 40 times (depending on the soil texture), EC (1:5) doesn t accurately measure the concentration of soluble salts in a solution likely to be encountered by a plant root. For this reason the EC of a saturated extract (ECSE) is a better measurement for predicting plant response. Although not perfect, it more closely mimics what plants encounter in the field. ECSE is the ELECTRICAL CONDUCTIVITY of a solution extracted under suction from a soil sample that is just at saturation with water.

4 The water content when the soil is at saturation is related to the field texture and is generally considered to be 2 to 3 times the FC water content. Even though ECSE is probably the most dilute soil solution that plants would encounter, it is now the most commonly used standard for interpretation. Conventional laboratory methods for determining ECSE are impractical in a commercial laboratory so mathematical relationships involving EC (1:5) and other soil parameters such as clay content, air-dry moisture content (ADMC) and chloride have been found to be a practical substitute for direct measurement. The equation for converting EC (1:5) to ECSE is: ECSE (dS/m) = EC (1:5) (dS/m) x Conversion Factor where the Conversion Factor is derived from clay content. The texture conversion factors in Table 1 have been derived from field research conducted for Australian soils of average salt composition.

5 The equations used to calculate the Conversion Factors are from Shaw (1994) as detailed in the Salinity Management Handbook. In soils where chloride salts dominate, these conversion factors may lead to an underestimate of ECSE while in soils with high sulfate may be over=estimated Other relationships described in Salinity Management Handbook can be used to better estimate the effect of chloride on ECSE. SoilMate displays the EC value adjusted for soil chloride content as ELECTRICAL CONDUCTIVITY (se Cl adj). Phone: 07 3220 2959 Email: Web: Table 1. ECSE Conversion Factors vary with clay content. Texture Clay % Conversion Factor Coarse sand 5 Sand 5 Fine sand 5 Very fine sand 5 Silt 6 Loamy coarse sand 8 Loamy sand 8 Loamy fine sand 8 Coarse sandy loam 13 Sandy loam 13 Gritty sandy loam 13 Fine sandy loam 13 Very fine sandy loam 13 Silty loam 25 Gritty silty loam 25 Gravelly silty loam 25 Loam 25 Gritty loam 25 Gravelly loam 25 Coarse sandy clay loam 25 Sandy clay loam 25 Gravelly sandy clay loam 25 Fine sandy clay loam 25 Very fine sandy clay loam 25 Silty clay loam 33 Gravelly silty clay loam 33 Clay loam 33 Gritty clay loam 33 Gravelly clay loam 33 Coarse sandy clay 38 Sandy clay 38 Fine sandy clay 38 Very fine sandy clay 38 Silty clay 38 Light clay 38 Sandy light clay 38 Gritty

6 Light clay 38 Gravelly light clay 38 Light medium clay 43 Medium clay (sandy) 50 Medium clay (fine sandy) 50 Medium clay 50 Medium heavy clay 60 Heavy clay 70 Phone: 07 3220 2959 Email: Web: Another benefit of calculating ECSE is that there is a large interpretative database of likely responses of plant species. Crop species tolerance is generally grouped into the five categories in Table 2. Table 2. Salinity sensitivity ratings of crop species. Sensitivity rating Salt tolerance group ECSE at 90% Yield (dS/m) 1 Moderately sensitive species 2 Moderately tolerant species 3 Tolerant species 4 Very tolerant species 5 Generally too saline for agriculture > (from Shaw, 1988) More detailed information on some common agricultural species is shown in Table 3.

7 For information on a greater range of plants refer to the Salinity Management Handbook. Table 3. Plant species crop tolerance data (dS/m) (Salinity Management Handbook). The salinity threshold is the ECSE above which yield depression is likely. Other columns indicate reductions in yield with increasing ECSE. Common name Salinity threshold ECSE 90 % yield ECSE 75% yield ECSE 50 % yield Strawberry Bean Potato Linseed Oat Wheat Barley Lucerne Clover, white (These data assume that salinity is uniform and that any limitation occurs after seedling establishment) References Shaw R. J. (1988). soil salinity and sodicity. In Understanding Soils and Soils Data, (ed. I. F. Fergus). Australian Society of soil Science Incorporated.

8 Queensland Branch, Brisbane. Shaw R. J. (1994). Estimation of the ELECTRICAL CONDUCTIVITY of saturation extracts from the ELECTRICAL CONDUCTIVITY of 1:5 soil :water suspensions and various soil properties. Project Report QO94025, Department of Primary Industries, Queensland. Salinity Management Handbook, Queensland Department of Natural Resources. Phone: 07 3220 2959 Email: Web.