Measuring the soil's charge
Soil electrical conductivity can quickly map soil variability.
The use of soil electrical conductivity mapping by orchardists and vineyardists is still considered to be in the "early adopter" stage, but that number is slowly growing as benefits are better understood, the service becomes more available, and knowledge about how to use technology improves.
Soil electrical conductivity measures how easily an electric current flows through the soil. Soil EC responds to the amount of salt in the soil and indicates the soil's composition of sand, clay, and organic matter. Soil EC is influenced by soil property interactions, such as water content, salinity levels, cation exchange capacity, soil depth, and temperature.
By using the soil EC to characterize soil differences within a field, vineyard, or orchard, soil maps can be generated to help design and lay out irrigation systems in new blocks and make corrections to existing systems. Measuring soil EC is a fast, inexpensive, and accurate way to map soil variability in a given field.
Craig Walters, owner of the agricultural consulting firm PACER (Professional Agricultural Consulting, Education and Research Corporation), estimated that less than 10 percent of the total farmland in Washington State has been mapped using soil electrical conductivity. But the number of farms mapped by soil electrical conductivity grows each year as growers see the benefits, the soil scientist added.
Two of the state's larger farm-service companies, Wilbur-Ellis Company and Simplot Grower Solutions, provide soil EC mapping to help guide growers' nutrient and water management decisions.
Soil electrical conductivity mapping is done with one of two types of devices. One measures the resistance of a direct current moving through the soil, and the other reads the strength of the electromagnetic field by using a current source passing over the soil (see "How Soil EC works").
To map a field, the electrical conductivity device is driven over the soil in a series of passes. A lightweight or all-terrain vehicle that pulls the device should be equipped with a Global Positioning System receiver to record the location of each soil measurement taken. The EC data logger is then interfaced with Geographical Information System software to develop maps that show low to high EC readings. Zones with high readings usually have higher clay and organic matter content than zones with lower EC readings.
"You're not taking a picture of the whole field," Walters commented. The EC measuring units are driven in a series of closely spaced passes, but the grower determines how wide to make the passes.
It's not necessary to drive up and down each orchard or vineyard row, he added. In an established tree fruit or grape block, the number of passes made in the block is influenced by the existing row width. In blocks with rows that are nine or ten feet wide, the EC devices might be driven down every third row; in orchards with wider spacings of 16 to 18 feet, the device might be used in every other row.
"Generally, with permanent crops you want precise data, so the passes need to be closer together," he said. For some field crops, pass spacings are up to 60 feet.
"The EC values are directly related to soil texture, especially if that's the main variable. It will help identify pockets of rocks and sandy zones."
He explained that a soil EC map helps pinpoint where the soil differences are located in a field, so growers know where to look for something that is different. "It is not like a soil profile photo. You still need to 'ground truth' the information and dig soil pits or take soil samples to find out if the variability is from a gravel or clay layer or caliche layer."
Costs of soil EC mapping vary, depending on the amount of acreage mapped, but they range from around $6 to $10 per acre, according to Walters.
The maps are usually sufficient for several years, as soil electrical conductivity patterns within a field do not change significantly over time unless there has been major soil movement such as land leveling or significant changes in water quality.
"The biggest benefit from soil EC maps is in water management," he said, and identifying nutrition issues is a secondary benefit. "Water and nutrition go together, and one explains a lot of the other."
For example, in an apple orchard that had great variability in tree growth, the soil EC maps helped to identify zones with different water-holding capacity. The smaller trees had the same amount of nutrients available as larger trees, but the water-holding capacity influenced nutrient and water uptake. The orchardist was able to change his irrigation scheduling based on the soil EC maps.
Soil EC maps can also identify zones that are susceptible to water stress caused by low water-holding capacity or have potential leaching and drainage problems and can help diagnose soil salinity problems.