Introduction
Plant growth and development are incrementally linked to environmental conditions and the available nutrient supply. This means that optimisation of nutrient application is essential for enhancing crop production. Poor management of nutrient solutions, whether due to excessively high or low concentrations or imbalanced ion compositions, can inhibit plant growth through toxicity or nutrient-induced deficiencies. Electrical conductivity (EC), an index of salt concentration and an indicator of electrolyte concentration in the solution, reflects the availability of ions to plants in the root zone. Various factors, including temperature, fertiliser quantity, salinity, moisture levels, irrigation practices, and soil types, influence soil EC values. Monitoring nutrient contents via EC is crucial, as soil EC is directly influenced by nutrient availability. Higher EC levels can impede nutrient uptake, increase osmotic pressure, waste nutrients, and lead to environmental pollution, while lower EC levels may adversely affect plant health and yield. While fertiliser application typically enhances crop production, excessive fertiliser use can elevate soil salinity and increase EC levels, leading to salinity build-up1,2,3.
Methods for Measuring EC in Soil and Nutrient Solutions
EC is often expressed in units such as millisiemens per centimetre (mS/cm) or decisiemens per meter (dS/m).
Sensor-Based Methods
- Electrical Resistivity: Measures resistance to current flow;
- Electromagnetic Induction: Utilises induced currents to assess EC;
- Time Domain Reflectometry (TDR) & Frequency Domain Reflectometry (FDR): Analyse signal reflections to determine EC;
- Electrical Resistivity Imaging (ERI): A geophysical method that estimates soil EC by measuring soil electrical resistivity using electrodes4, 5.
Commercial EC Sensors
- EC probes and meters provide direct readings of soil or nutrient solution EC values.
Soil EC Measurement
- Handheld EC Meter: A simple and easy-to-use tool that measures the EC value of soil extract using a probe inserted into the soil extract, with readings displayed on a digital screen;
- Laboratory Analysis: Soil samples can be sent to a laboratory for EC analysis using a conductivity meter, providing accurate results but potentially time-consuming and expensive;
- Soluble Salt Test Strips: Paper strips dipped into a soil extract change colour based on the EC value, allowing comparison to a colour chart for EC determination;
- Soil Moisture Sensor: Used in smart agriculture for real-time and remote monitoring of soil EC to ensure optimal plant growth.
The optimal EC value for plant growth is usually between 0.8-1.8, and should not exceed 2.56.
The following table is the adaptation range of EC value of some common crops for reference
The Importance of EC in Plant Growth
EC doesn't have a direct impact on plant growth; however, it serves as an indirect indicator of nutrient availability and salinity levels within the growing medium. As the EC value increases, more negatively charged sites are present in the soil, allowing for the retention of positively charged nutrients (ions). Consequently, soils with higher EC values typically exhibit a greater capacity to retain and provide essential nutrients to plants. Conversely, soils with low EC values may suggest a deficiency of available nutrients, potentially impeding optimal plant growth6. Maintaining balanced EC levels is essential to prevent nutrient deficiencies or toxicities, which can occur with excessively low or high EC levels7. It's essential to understand that while electrical conductivity offers insights into nutrient availability, it doesn't directly quantify the concentration of individual nutrients in the soil6. Osmotic potential, determined by EC, affects water movement into plant roots.
High EC levels increase osmotic potential, causing water to move away from roots, potentially leading to water stress. Therefore, balanced EC levels contribute to optimal yields and the development of fruits, flowers, and foliage.
Factors Influencing EC
Soil composition and minerals play a significant role in determining EC. The inherent mineral content of soil, including ions like potassium, calcium, and magnesium, influences EC levels, with soils rich in these ions typically exhibiting higher EC values. Additionally, different soil types, such as sandy, loamy, or clayey soils, vary in EC due to their mineral composition. Soil texture also impacts EC, with sandy soils having larger particles and lower EC, while clay soils with smaller particles tend to have higher EC. Loamy soils fall in between. Organic matter, such as humus, affects EC as well; when organic matter decomposes, it releases ions contributing to EC, and adding organic amendments like compost or manure can alter EC levels. Soil moisture content is another crucial factor affecting EC, with increasing water content typically leading to higher EC levels. Optimal soil moisture levels, ideally around 10–25%, support plant growth and correlate with higher EC. Fertilisation practices significantly influence EC by introducing ions from fertilisers into the soil solution. Over-fertilisation can elevate EC, potentially harming plants. Climate and environmental conditions also play a role, with temperature affecting ion mobility, humidity impacting soil moisture, and factors like bulk density, soil structure, timing of measurement, and electrolytes in soil water further contributing to variations in EC levels8.
Conclusion
In summary, optimising nutrient application is crucial for crop production, as poor management can hinder plant growth. Electrical conductivity (EC) serves as a key indicator of nutrient availability and salinity levels in the soil, indirectly influencing plant health. Balancing EC levels is essential to prevent nutrient deficiencies or toxicities and ensure optimal plant productivity. Factors such as soil composition, texture, organic matter, moisture content, fertilization practices, and environmental conditions significantly impact EC. Monitoring EC through various methods enables efficient nutrient management for maximum crop yields and sustainable agriculture.
