Introduction

There are many different measures that farmers can use to provide insight as to what is happening in their soil. Collectively, these measures give insight as to how healthy the soil is. Soil health, broadly, has three main components: life; fertility and structure. All three of these components must be optimal in order to have a fully healthy soil, which effectively supports plant growth. In order to understand each component, there are specific measures that provide insight, and depending on the levels in a specific soil, a farmer can know how far the soil is from optimal health. One of the insightful measures which we use as an indicator of soil fertility is electrical conductivity.

In soil, electrical conductivity (EC) is a measure of the ability of the soil to conduct an electrical current. Most importantly to fertility, EC is an indication of the availability of nutrients in the soil. The higher the EC, the more negatively charged sites (clay and organic particles) there must be in the soil, and therefore the more cations (which have a positive charge) there are that are being held in the soil.  Sodium (Na⁺), ammonium (NH₄⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), hydrogen (H⁺), iron (Fe²⁺), aluminium (Al³⁺), copper (Cu²⁺), zinc (Zn²⁺) and manganese (Mn²⁺) are some examples of these cations that are beneficial to plants.

As with most things in the soil, it is important that the EC does not get too high either, as too many of these nutrients, especially Na and Mg, can be detrimental to soil health. Optimal EC levels in the soil therefore range from 110-570 milliSiemens per meter (mS/m). Too low EC levels indicate low available nutrients, and too high EC levels indicate an excess of nutrients. Low EC’s are often found in sandy soils with low organic matter levels, whereas high EC levels are usually found in soils with high clay content.

As has already been alluded to, soil EC is influenced by numerous soil properties. In order for EC to be used as a soil health indicator, and therefore inform farm management, it is important to understand the interaction between EC and other soil properties.  Texture, salinity and moisture are soil properties which most significantly influence EC levels, and each of them will be discussed in detail below.

Texture

Even though the texture of the soil cannot be changed by any management action, it is still important to take note of how it interacts with EC. Sand has a low EC (1-10mS/m), silt has a medium EC (8-80 mS/m) and clay has a high EC (20-800 mS/m). This means that sandy soils have a poor capacity to store and hold onto cations and lose nutrients easier than silty and clayey soils. Silty and clayey soils thus have a much better ability to store and hold onto cations, and the loss of nutrients would be much less so than in sandy soils. Understanding this, something to take into account  – relative to the specific soil texture of a camp – is that less fertiliser can/should be applied on soils with higher EC’s, as less of what is applied will be lost through leaching. In sandy soils, improving the organic matter levels can lead to an improvement in the ability of the soil to hold cations, thereby improving the EC levels. Another interesting dynamic to point out is that in areas where the soil texture is unknown, EC can provide a good idea of the texture, based on the norms referred to above.

Salinity

Soil EC is often used as a measure of salinity. Salinity is an indication of the amount of salts in the soil. The general rule of thumb for salinity is that very high EC values (>1600 mS/m) are an indication of strong saline soils. Low EC values (0-200 mS/m) on the other hand, are an indication of non-saline soils. The cation that most significantly influences salinity is sodium, especially when it is in excess of 100 mg/kg in the soil. Sodium can be leached from the soil, thereby reducing the EC levels, through the addition of elemental sulphur or gypsum (read the blog: The impact of Gypsum on sodic soils for more info).

Moisture

It is important to note, when measuring in-field EC, that the more moisture there is in a soil, the higher the EC reading at that time will be. This is because the more moisture there is, the more cations are in solution in the soil. In basic terms, water is a good electrical conductor in itself, therefore the more water there is in the soil, the better the soils ability to conduct an electrical pulse.

Conclusion

Soil EC, much like pH, is a good overall indicator of soil fertility. It can be used to show the capacity of the soil to store nutrients, as an indicator of soil texture and as an indication of an excess of soil nutrients (e.g. excessive sodium levels leading to salinity). Good soil fertility management practices will contribute to maintaining optimal EC levels. The two most important things to take into account are that in low EC soils, nutrient shortages should be addressed and in high EC soils, nutrient build up should be addressed.

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