Steven H. Moore and Maurice C. Wolcott

Soil electrical conductivity was measured in a production field at the Dean Lee Research Station and found to correlate with soil texture, organic matter, soil nutrients and crop yield. Research is under way to calibrate nitrogen needs in corn and cotton based on soil electrical conductivity, paving the way for site-specific fertilizer application.

Measuring electrical conductivity in soil has been investigated as a way to determine its fertility and productivity. This may help farmers make site-specific fertilizer applications. The end result is increased efficiency and reduced waste. A research project at the LSU AgCenter’s Dean Lee Research Station was initiated to determine how closely electrical conductivity corresponded to changes in soil texture, soil nutrient concentration and crop yield.

Electrical conductivity was measured in a 166-acre field of silt loam to silty clay loam soil at the station on October 27, 1998, using a Veris 3100 sensor cart (Figure 1). Coulters on this cart are equipped with electrodes through which an electrical current is passed. The Veris sensor cart can measure conductivity at two depths simultaneously, and in this study it was measured at 12- and 36-inch depths.

The measurements of electrical conductivity were taken by driving the Veris sensor cart over the fields at about 8 miles per hour, logging one data point per second. Costs for measuring soil electrical conductivity generally run between $4 and $9 per acre. Soil electrical conductivity is a relatively stable parameter, and a surface map may be useful for 10 years.

Conductivity measures soil texture

Soil texture affects the amount of fertilizer required to produce optimum crop yields. Usually clay soils require more fertilizer. One outstanding feature of electrical conductivity in this study was how well it corresponded to changes in soil texture.

Soil electrical conductivity is depicted in the surface map in Figure 2 using 400 square-foot grid cells. The darker the shade, the higher is the conductivity. Of note is the general increase in conductivity from west to east (left to right) across the field. This corresponds to an increase in heaviness of soil texture. The highest conductivity measurements in the eastern-most portion of the figure fairly accurately depict the denser clay soil in this area. The surface map may be used as a prescription map for fertilizer input based on soil texture.

In a separate study, soil samples were taken from eight specific electrical conductivity zones and analyzed for soil texture and organic matter (Tables 1 and 2). There was very high correlation between electrical conductivity and clay content (0.99) and between electrical conductivity and organic matter (0.99).

Conductivity measures nutrients

Soil nutrient concentration also affects the amount of fertilizer needed to produce optimum crop yields. Soils with high clay and organic matter usually have higher nutrient concentrations because there is more total surface area for nutrients to attach to, although the nutrients may not necessarily be more available to the plant. Soil electrical conductivity corresponded highly with soil nutrients in this study.

Nutrient concentrations were determined in soil samples taken from eight electrical conductivity zones (Table 1). Correlation coefficients between electrical conductivity and mean concentrations of nitrogen, phosphorus, potassium and zinc were very high, running from 0.95 to 0.99 (Table 2). Although electrical conductivity does not measure the actual concentration of soil nutrients per se, the two variables did rise and fall closely together. This finding paves the way for potential site-specific application of fertilizer, once base rates are established.

Conductivity measures crop yield

Soybean and corn yields for the past four years were correlated with electrical conductivity. To do the analysis, the field was divided into 400 square-foot grid cells, or rasters. Values from more than 15,000 rasters were used to derive the correlation coefficients in Table 3. Yield correlated significantly with electrical conductivity each season, although correlations were weak.

A somewhat puzzling finding was the negative correlation between electrical conductivity and yield in 1997. In the three other years, conductivity correlated positively with yield. The difference could be explained by rainfall patterns across the four seasons. Heavy-textured low areas of a field tend to do better in dry years and worse in wet years. The 1997 season was fairly normal, whereas the 1998, 1999 and 2000 seasons all had uncharacteristically hot, dry periods. The implication is that the relationship between electrical conductivity and yield will differ each season. Fertilizer rate recommendations in site-specific agriculture will be based on average response, just as in conventional agriculture.

Correlation of means between electrical conductivity and corn yield in 1999 from the zones in Table 1 was much higher (0.97). Correlating means often results in higher coefficients.

Measuring conductivity shows promise

Soil electrical conductivity was found to highly correspond to changes in soil texture, organic matter, nutrient concentrations and crop yield. Using electrical conductivity as a prescription map for site-specific fertilizer application appears to be a promising technology. Field experiments are now under way at Alexandria to determine crop response to specific nitrogen rates within different electrical conductivity zones. If the experiments are successful, producers may be able to determine site-specific fertilizer applications using electrical conductivity measurements on their own farms as early as the 2002 or 2003 season.

Acknowledgment Louisiana Soybean and Grain Research and Promotion Board for its support.

(This article appeared in the summer 2001 issue of Louisiana Agriculture.)