Salinity Assessment In The Laboratory

Electrical conductivity meansurements are the key to salinity evaluation.

Electrical conductivity (EC) is one of the most frequently measured soil parameters in arid and semi-arid regions. It is commonly grouped with soil fertility testing as part of a package or routine soil test since it is the primary means to evaluate salt problems. There are several methods that are followed in the laboratory to measure EC. When soil samples are received they are usually air dried and then ground to pass through a 2 mm sieve. For routine analysis, pH and EC can be measured on a 1:1 or 1:5 basis, where one part soil is mixed with one part or five parts water. The mixture is shaken on a rotary shaker for 30 min. with the sample stirred at 15 min. The sample is then removed from the shaker, allowed to settle for 5-10 min and then measured for pH and EC. When measuring EC, the conductivity electrode is immersed in the soil solution and a reading is taken. If the meter used for measuring EC automatically compensates for ambient temperature, the EC reading can be recorded as the conductivity of the sample. Some meters, however, do not compensate for temperature making it necessary to read a 0.01M KCl solution to determine the effect of temperature. Under ideal conditions at 25oC a 0.01M KCl solution should read as 1.413 dS/m using a conductivity cell (1 cm in diameter). Some meters have temperature probes that can be placed in the sample along with the conductivity cell to compensate for the temperature. However, it would still be a good practice to measure a National Institute of Standards and Technology (NIST) conductivity standard (available from Fisher Scientific) or soil standards available from the North American Proficiency Testing (NAPT) Program that is administered by the American Society of Agronomy. The Soil, Water, and Plant Testing Lab also has a check soil that is used on a daily basis to evaluate the lab’s EC readings and is available to those wishing to check their own EC meters. Another way to evaluate conductivity is to measure the EC of a soil paste extract. Approximately 50-100 g of ground soil is placed in a plastic cup and enough water is mixed with the soil to create a paste condition. The paste tends to glisten when enough water has been added and usually has the consistency of a thick cake batter. The saturated sample is allowed to stand four hours or overnight to help bring salts into solution. After standing for the necessary amount of time, water is added to the sample to bring it back to saturation, if necessary. The soil paste is then vacuum filtered and the soil extract is measured with the conductivity cell by pouring the extract into the cell (the holes in the cell are plugged to prevent the extract from flowing out). For routine soil analysis the 1:1 or 1:5 EC readings are faster and usually evaluate EC adequately. However, if the EC of a 1:1 or 1:5 soil mixture exceeds certain limits, it may be necessary to evaluate the EC on a paste basis, since there is the possibility that the salts could be high enough to warrant the analysis of a sodium adsorption ratio (SAR). Paste EC’s tend to be higher than EC’s done on soil to water ratios. At the Soil, Water, and Plant Testing Lab, the 1:1 EC is evaluated for each sample, and if the 1:1 EC exceeds specific levels (0.8 dS/m in a sandy soil, 1.2 in a sandy clay loam or clay loam, or 1.6 in sandy clay or clay soil), a paste is made and the EC is evaluated from the paste extract. If the EC from the paste exceeds 7 dS/m, then the sample is analyzed for calcium, magnesium, and sodium to calculate a SAR. The most recent unit for EC is dS/m (deciSiemans/meter), which is the same as mmhos/cm. However, many labs still express EC readings as mmhos/cm. The mho is actually the inverse of the ohm, which used to be the unit of measure on very old EC meters (yes, we do have one in the lab); however, the readings are very large and cumbersome to work with. While soil to water ratios are faster to measure and are suited to routine measurements, it is felt that the paste EC more accurately evaluates the salt conditions of the soil since it more closely simulates field conditions. Whether one method is used over the other, EC is one of the most common tests to evaluate soil conditions for plant growth and should be considered when attempting to reclaim salt affected soils or when purchasing topsoils or soil amendments. by Jim Self, Manager Soil, Plant, and Water Testing Laboratory

Mapping Soil Salinity with Electro-Magnetic Device

Non-invasive tool speeds up processing of spatially quantified soil salinity maps.

For years we have had to collect dozens of soil samples from different quadrants of our cooperators’ fields to obtain valuable information in determining salinity or sodicity. Hours of sampling, bagging, transporting to the lab, and waiting for the analyses to return was the norm. Then we questioned the results if the pH, electrical conductivity (EC), or sodium absorption ratio (SAR) values were not representative of the field or to a particular portion of the field. We thought that after we sampled 101 sites in 80 acres we could have an idea of what the salt levels were. Well, guess again! Even after 101 samples you would still not have a spatial relevancy for the samples taken and not be any further along in ascertaining the variability in field salinity levels. Besides that, no grower could afford that kind of sampling regimen. Dr. James D. Rhoades, retired research leader of the United States Salinity Laboratory (USSL) in Riverside, California, worked with several people in the geophysical measurement arena to devise a new approach to determine salinity levels to a depth of 60 inches. By utilizing conductivity measurements of the soil that coincide with the lab analysis of EC, Rhoades and his fellow scientists determined an accelerated method to measure conductivity in the field. This more recent approach to determining salinity with electromagnetic induction methods allows rapid measurements with an electro-magnetic (EM) tool, along with statistical sampling and lab analyses, to determine the spatial variability of salts across the field. How does this work? The scientist travels across the field placing the EM-38 tool (from Geonics Ltd.) on the surface in two positions. These two dimensions allow the tool to measure resistance or conductance of the soil medium. In the horizontal dipole position, the EM tool emits an electromagnetic frequency of 14,600 Hz into the soil medium approximately 30 inches. A receiving magnet reads the in-phase residual signal of apparent conductivity in mS/m. In the vertical dipole position the EM tool sends the electromagnetic signal about 60 inches, and the receiving magnet reads the remaining signal and displays it on the units readout in mS/m. These two readings, coupled with a GPS X-Y coordinate are recorded into a data logger or onto a log sheet for future manipulation into a computer model constructed by scientists at USSL. EM38 quickly measures conductivity in soil. In consultation with the producer, local conservationists, soil survey information, and topographical maps, a modified grid design of the growers’ field(s) is set. The landscape positions, size of field, water flow direction, irrigation type and soil delineations are considered in designing a grid spacing for the em survey. Each time that the EM tool is used, it must be calibrated. Locating a starting point and using a GPS receiver, scientists can walk or ride an all-terrain vehicle (ATV) from one X-Y grid coordinate to the next while recording data from the EM-38. It is important to note that one must be a minimum of 45-90 ft. from overhead electrical lines, and 12-15 ft. away from the ATV to minimize attenuated signals. Also when initializing the unit, keep clear of pickups, tractors, power lines, underground pipelines and electric fences. USSL scientists suggest a minimum of 50 data points from both the vertical and horizontal dipole positions of the EM tool to obtain a reasonable data set for the modeling software to offer statistically valid results. This dictates the grid spacing for any field(s) to the field scientist. After the field data is collected, it can be loaded into the ESAP software. The frequency and soil sampling locations for verification samples are determined and samples are taken for lab analysis. Alternatively, a Hach Chemical Co. SIM Kit can be used. The number of samples and location is very important to assist in the modeling process. Then the values are loaded into SURFER software (Golden Software Co.) and an isobar two-dimensional map is developed. The grower can use the map to determine the best course of action with the consulting agronomist. This methodology is reasonably quick; it offers growers very reasonable estimates of salt levels and the location of salt problems in their fields. Salinity is a growing problem in many irrigated areas of Colorado. Growers want to know why their crops are detrimentally affected. If salts are a yield limiting factor, this mapping device can help diagnose the extent of the problem. by Michael Petersen Area Resource Soil Scientist USDA-NRCS