Agricultural Irrigation Water Quality Control: How Does a Water Quality Analyzer Detect Hardness and Conductivity to Ensure Crop Growth?
Introduction
Do you know about the “invisible drought” for crops? High-salinity water, as indicated by high conductivity, is osmotically inaccessible to plants. Even with a visibly wet soil, the plant cannot utilize the water for its growth. Similarly, water with high levels of hardness, which means above normal levels of magnesium (Mg2+) and calcium (Ca2+), can cause chalky limescale deposits on soil in drip irrigation, causing uneven watering. In water spray, deposits on leaves and fruit can reduce their photosynthetic capacity, resulting in lower yields and crop growth.
Monitoring the water quality and controlling its parameters is absolutely vital to ensure proper crop growth. Modern sensors enable small-scale agricultural activities to maintain water quality without the need for sophisticated lab testing. This article is about understanding water hardness and conductivity and their effect on crop growth. Moreover, it explains how we can control water quality for agricultural irrigation using online analyzers.
Understanding Water Hardness in Irrigation
Definition of Water Hardness
Hardness is the measure of divalent cations concentration, which are primarily magnesium (Mg2+) and calcium (Ca2+). They are essential macronutrients for plant growth through cell wall structure and chlorophyll production.
Sources that Increase Water Hardness
The land is full of naturally occurring stones. In particular, limestone (calcium carbonate) and dolomite (calcium-magnesium carbonate) contribute to the rise in divalent cation concentrations in groundwater. Rainwater has lower hardness, but the concentration may increase as water evaporates.
Ideal Ranges for Irrigation Activities
In practice, agricultural irrigation activities require vigilant monitoring of water quality and maintenance of parameters such as hardness and conductivity. Water with 0 to 150ppm of CaCO3 results in optimal crop growth. It provides sufficient magnesium (Mg2+) and calcium (Ca2+) without the risk of scaling or clogging equipment.
The Impact of Water Hardness on Crop Growth
High Hardness and Its Impact on Crop Growth
The major problem is the degradation of the irrigation system. Scaling can block emitters in drop irrigation systems. Spraying high-hardness water on crops can cause foliar deposition, which can reduce photosynthetic efficiency. High concentrations of Ca2+ can interfere with the uptake of other essential cations, such as magnesium and phosphorus. It can lead to stunted growth and leaf yellowing due to a lack of magnesium. The osmotic stress can reduce crop yield by 20-30%.
Low Hardness and Its Risks
If the concentration of micronutrients is below 50 ppm, it may lead to plant disorders such as blossom-end rot in tomatoes due to low calcium ion concentrations in the water. Crops sensitive to micronutrient concentrations can use soft water with 0 to 60 ppm, but require careful nutrient management through fertilization.
Management and Soil Interaction
To control high temporary hardness in water, a controlled injection of sulphuric or phosphoric acid can be used. It reacts with bicarbonates in water to form soluble compounds. However, it should be done in moderation to present the adverse effects of acidification. High hardness/salinity (2250-3150 ppm) cuts maize yield by 50% due to root stress. High hardness also correlates with elevated manganese, causing hidden toxicity in vegetables.
Understanding Electrical Conductivity in Irrigation
Definition of Electrical Conductivity (EC)
It is the water's ability to conduct electricity. Typically, it's proportional to the concentration of ions present in the water. These can be Na+, Cl-, NO3-, K+, and Ca2+. These are measured in µS/cm (micro siemens per centimeter) for drinking water or dS/cm for irrigation. Typically, all readings are standardized at 25 °C to ensure the relevance of the results.
Sources and Optimal Range
There can be reasons for the electrical conductivity to increase beyond the normal value or decrease below it. Let's have a look at them:
- Sources of High EC: Fertilizer runoff, evaporation in arid zones, and contaminated groundwater can increase electrical conductivity by adding ions.
- Source of Low EC: water purified by reverse osmosis (RO), distilled water, and rainwater, with no contact with mineral deposits, can have low EC.
- Ideal Range for EC: To ensure that there is a high yield of crops, control the EC based on the following:
○ General Crops: Below 1.5 dS/m.
○ Sensitive Plugs/Seedlings: Under 1.0 dS/m.
○ The Risk: Values above these values increase the risk of osmotic stress
The EC is linked with the total dissolved solids (TDS). As a thumb rule for easy conversion:
TDS (mg/L) ≅ 640 x EC (dS/m)
The Impact of Electrical Conductivity on Crop Growth
Consequences of High Electrical Conductivity in Irrigation
Plant physiology and growth are directly dependent on electrical conductivity. Generally, a 3 mS/cm or 3 dS/cm is considered too high and can disrupt irrigation activities. It can lead to osmotic stress, which makes it challenging for plants to draw water. Excessive salinity can cause toxicity and lead to wilting, reduced transpiration, and significant yield reductions in sensitive crops such as lettuce.
Impact of Low Electrical Conductivity
Finding the balance is key. Even with the low EC, the plant's growth is affected. Below 0.6 mS/cm can slow photosynthesis and reduce biomass, typically in hydroponics, where nutrient delivery depends on water alone.
Management and Mapping
To reduce the EC, use the dilution technique by mixing low-EC water with high-EC water to achieve the optimal EC level. Some may even use the excess water technique to move surface salts below the roots to ensure good soil conditions for plants.
Modern techniques such as Electrical Resistivity Imaging (ERI). It offers a unique, non-destructive way to monitor electrical conductivity across a field. It allows growers to use precision irrigation and leaching strategies.
How Water Quality Analyzers Detect Hardness
Some methods rely on isolating and quantifying the specific calcium (Ca2+) and magnesium (Mg2+) ions in the sample.
Ion-Selective Electrodes (ISE)
There is a membrane that surrounds the electrode. The membrane then selectively allows the ion to be detected and reach the electrodes. Typically, the membrane is made from polymer matrices containing specific ionophores, especially for calcium (Ca2+) and magnesium (Mg2+) ions.
● The Ion-Selective Electrode (ISE) has a selective membrane and an internal reference electrode, typically Ag/AgCl.
● External Reference Electrode (RE): A stable electrode (like Ag/AgCl) that provides a constant, known electrical potential baseline.
As the ion reaches the ISE, it creates a voltage difference. Measuring the voltage difference gives the concentration of ions present in the water. For this, the Nerstian relationship is used:
Where:
E: The measured potential (voltage).
E0: The standard potential of the cell (a constant).
R: The Universal Gas Constant.
T: The temperature in Kelvin.
z: The charge number of the ion +2 for Ca2+.
F: Faraday's Constant.
a: The activity (effective concentration) of the ion in the sample solution.
Note: The voltage changes logarithmically with the ion concentration.
Chemical Methods (EDTA Titration)
Ethylenediaminetetraacetic acid is the titrant. It is used in the classic method of detecting ions in water. We add a complexing agent, such as EDTA, and allow it to form stable complexes with calcium (Ca2+) and magnesium (Mg2+) ions. We do that until all the ions are bound.
M2++EDTA4- → [M-EDTA]2-
Here M2+ represents either Ca2+ or Mg2+
A special dye called a metallochromic indicator is used to visually provide the point at which all ions are bound. When all the free ions are bound, the further addition of EDTA causes the indicator to displace, returning to free color, signalling the completion of the reaction. The color changes from wine-red to blue, which is very sharp and makes the indication prominent.
Indirect and Field Estimations
Another method to detect hardness is to do so indirectly via EC for a quick approximation. Since hard ions in water contribute to EC, we can generate a general correlation.
EC of 1.4-2µS/cm ≅ 1ppm of CaCO3
Most portable meters allow direct field measurement, making it the most convenient method to detect hardness in water.
How Water Quality Analyzers Detect Conductivity
Practically, the EC probe is inserted into the sample water for quality analysis. When the probe is in water, voltage is applied across these electrodes. The current is conducted through the ions present in the water. Measuring the current flow is directly proportional to the concentration of dissolved ions.
Warm water conducts electricity more readily than cold water, so there needs to be standardization. EC probes have processors calibrated for different temperature conditions. The feedback from the temperature compensation module provides real-time temperature conditions and fine-tunes the EC live value. However, it's important to maintain the probe's cleanliness and perform periodic calibration.
Practical Applications in Farmland and Greenhouses
- Live EC Adjustment in Farmlands: In farmlands, analyzers serve as a key control mechanism to control the water source. They can dilute the high-EC groundwater with low-EC rainwater to optimize water quality. It makes the water suitable for targeted broad-acre crops.
- Drip System Integration: They support precision agriculture by integrating with drip systems. The system detects EC and hardness and adjusts pump speed or issues start/stop commands based on EC thresholds.
- Drainage Management: EC confirms that salt leaching is doing its job, pushing harmful salt concentrations below the roots to revive crop health.
- Crop-specific tuning: Using lower EC thresholds during sensitive stages, such as seedlings or plugs, minimizes osmotic stress and significantly enhances survival and early growth rates.
- Research and Adaptation: Analyzers reveal seasonal EC spikes in water sources, informing adaptive irrigation plans to manage high-salt periods in river-fed agricultural systems.
Conclusion
Managing EC and water hardness for crop growth is non-negotiable. There should be extensive monitoring of water quality to ensure that crops receive the right nutrients and avoid osmotic stress and nutrient toxicity. For example, under saline conditions (EC 4.5-6.5 dS/m), maize suffers severe growth impairment, with 50-75% reductions in height, seed weight, and overall yield.
Water quality analyzers make it possible to monitor and control water nutrient levels through a system of pumps, valves, gates, etc. These analyzers use precise sensors, such as Ion-Selective Electrodes (ISE) and conductivity probes, that feed real-time data to controllers for automated, accurate nutrient adjustment. If you are looking for some high-end, precise, and agriculture-oriented water quality analyzers, then consider Shanghai BOQU Instrument Co., Ltd. Their offerings include the AH-800 for hardness, ECG-2090Pro, DDG-2080Pro, SJG-2083CS, and DDG-2080X for conductivity. Visit https://www.boquinstrument.com/.
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