Quick Estimation of Salinity in Field Soils and Irrigation Water with LandMapper ERM-02

Soil salinity is routinely evaluated in the labs from electrical conductivity of liquid soil saturation extract (ECe). The resulted total salinity is reported either directly in conductivity units (dS/m) or converted to TDS (total dissolved solids) concentration in ppm (parts per million) using formula: 1 dS/m = 1 mS/cm = 1 mmho/cm = 640 ppm = 640 mg/L= 0.64 g/L=0.064%

But now EC of soil and waters can be measured directly in the field using highly accurate method of four-electrode probe and Landmapper ERM-02 measuring device. Best of all, probes can be build to sense different soil layers down to 30 ft! Probes are simple and inexpensive to make from common materials available at any hardware store.

ECa or apparent (bulk) electrical resistivity measured with LandMapper in the field can be related back to ECe by multiplying ECa*Ktexture. The Ktexture varies from 3 to 6 for typical loam and clay loam soils, and can be estimated from soil clay content and a graph at the left. However, those Ktexture for recalculation of ECa to ECe were derived for relatively dry California soils and if one is measuring ECa in wet situations like after heavy rain in saturated or flooded soil (also possible with Landmapper ERM-02!), Ktexture multiplication is not necessary. Recent measurements on rice paddies in TX have shown that at ECa=1.5 dS/m at 6” and 16” depth rice is thriving under full flood.

Bulk soil electrical conductivity (ECa) is measured from soil surface to the depth in the big soil volume determined by the distance among four electrodes (ABMN) and therefore is more representative of field conditions than measurements in small soil sample or soil ECa insertion probe. The depth and volume of measurement may be varied by changing the spacing between electrodes. When the distance between the outside pair of electrodes (the current electrodes, AB) is small, the flow of electricity is shallower. The effective depth of measurement is about one-third of the distance between AB electrodes. The calculation of ECa from field measurements done with different size probes will be done automatically by LandMapper ERM-02 if geometrical coefficient Kg is set in device for specific probe. Kg can be calculated from distances among ABMN electrodes using formula below (input distances in meters):

Four-electrode probes supplied by us will have Kg printed on the probe and saved in LandMapper ERM-02 memory. Device can store nine Kg (K1-K9) coefficients to facilitate quick changes in the field for up to 10 probes for different depths. Default K0=1 and cannot be changed. K0 is used with a probe to the depth ~6” where AM=MN=NB=a=16 cm or for resistance/conductance measurements.