Soil electrical conductivity is expressed in μS/cm (or mS/cm), dS/m or ppm:
• 1 dS/m = 1000 μS/cm (= 1 mS/cm) = 640 ppm (when EC < 5 dS/m);
• 1 dS/m = 800 ppm (when EC > 5 dS/m);
Both the soil texture and CEC have an effect on EC:
• Soils that have a higher content of smaller soil particles (higher content of clay) conduct more electrical current than do soils that have a higher content of larger silt and sand particles (lower content of clay).
• Soils that consist dominantly of clay minerals that have a high cation-exchange capacity (CEC), can have higher EC than soils that consist dominantly of clay minerals that have a low CEC.
Based upon the EC value, a classification can be made for the degree of salinity:
Salinity
There are different methods to determine the EC: ECe (on a saturated soil paste extract), EC1:1 (on a mixture of soil over water mass ratio of 1:1), EC1:5 (on a mixture of soil over water mass ratio of 1:5), ...
The salt tolerance of a plant is the maximum salt level that plant can tolerates without losing its productivity or inhibiting growth:
• For example, the FAO has an extensive list of crop salt tolerance;
• Similar lists exist for landscaping plants;
HC
Soil hydraulic conductivity is the ability of a soil to transmit water, under saturated or nearly saturated conditions.
The hydraulic conductivity is a flow rate and hence expressed in a volume of water per unit of time.
Sometimes the hydraulic conductivity is equated to the infiltration rate, but from a scientific point of view this is not quite true:
• The hydraulic conductivity is a property of the soil and can vary according to the level of saturation:
hydraulic conductivity in the unsaturated zone is not the same as the saturated hydraulic conductivity (Ks) in the saturated zone;
• The infiltration rate is the inflow into the soil: how fast is water entering the soil.
However, just talking about infiltration rate might not be helpful. Imagine a small rain shower, sort of a drizzle. A lot of soils can soak that water in, no problem, giving an infiltration rate that is equal to rainfall rate. So instead, it is better to talk about infiltration capacity: the maximum rate at which water can enter a soil.
The infiltration capacity changes over the course of a rain storm (or irrigation event), so you can’t just measure infiltration capacity at any random point of time. Initial infiltration rate is high, but afterward it decreases. Finally it become constant. That steady-state rate is approximately equal to the saturated hydraulic conductivity. And it’s that steady state rate that we most often want to measure.
The saturated hydraulic conductivity Ks is expressed in mm/hr or cm/hr and is strongly related to soil texture and structure:
• Water moves more quickly through the large pores in sandy soil than it does through the small pores in clayey soil.
• When the soil is compacted and has little or no structure, the Ks will be even lower.
• Depending on the amount and type of clay minerals, some clayey soils develop cracks from shrinkage as they become dry. The cracks are direct conduits for water to enter the soils. Thus, clayey soils can have a high infiltration rate when dry and a slow rate when moist (cracks close).
Management practices that improve soil organic matter content, soil aggregation, and porosity can also improve infiltration.
Measuring saturated hydraulic conductivity Ks
Measuring saturated hydraulic conductivity Ks
Depending on the speed of infiltration, the following classification can be made:
Importance of infiltration capacity
• When the infiltration is too slow, ponding can occur, surface run-off and soil erosion.
• When the infiltration rate is too high, leaching of nutrients can take place.
Best management practices are needed to improve soil infiltration:
• Sports pitches ideally are sand-based to have a high infiltration and drainage rate allowing training practice and games to be played even during high rainfall.
• In agriculture, tillage needs to be done wisely as it can break up soil structure and cause compaction. Keeping the soil covered with cover crops will reduce the direct impact of raindrops and limit crust formation and soil erosion.
Learn more about soil on our Blog Parts 1 and Blog Part 2