Fluoride testing is crucial in agriculture to ensure soil and water quality.
This article covers methods for measuring fluoride levels in water, reviews popular fluoride meters, and explores the importance of monitoring and mitigating excessive fluoride in soil and irrigation water to protect plant health and optimize crop yield.
How to use fluoride meter: step-by-step guide
There are various fluoride testing solutions on the market, ranging from compact pocket meters to advanced lab-grade gas chromatographs, with prices ranging from ~$70 to over $50,000.
For non-lab professionals, fluoride measurement can be done by colorimetric method (where a photometer and reagents are required for each test) or by potentiometric method (Ion Selective Electrode – ISE).
Colorimetric method: Hanna HI97729 Fluoride Low Range Photometer
Potentiometric: HORIBA LAQUAtwin F-11
We’ll take a closer look at the Horiba fluoride meter, a relatively new addition to the market in 2024.
The Horiba LAQUAtwin F-11 is a pocket fluoride meter that uses an ion-selective electrode to measure soluble fluoride concentrations up to 990.0 ppm.
Unlike colorimetric methods, it doesn’t require reagents. However, it does need daily calibration using the provided 1 ppm and 10 ppm calibration solutions.
Once calibrated, the LAQUAtwin F-11 provides an almost instant fluoride concentration reading as soon as a sample is placed on the sensor.
How to measure fluoride with LAQUAtwin F-11:
Calibrate your meter at two points (1ppm and 10 ppm)
Apply your sample to the sensor (0.3 ml is enough) or immerse the meter directly into the water.
Wait for the reading to stabilize - a smiley face will appear on screen once it's ready.
Rinse the sensor under running water to clean it after each use.
What is the Normal Fluoride Level in Irrigation Water?
Fluoride occurs naturally in groundwater and freshwater due to the dissolution of fluoride-rich minerals present in the soil.
Some industrial activities (ceramic & coal industry), can also contribute to high fluoride levels in groundwater.
Fluoride concentration found in fresh water depends on the geographical location and source, typically ranging from 0.01 ppm up to 100 ppm. 1
For drinking water, the recommended concentrations (often added by municipalities) is below 0.70 ppm, and studies suggest that a similar limit applies to irrigation water. 2
If the concentration of fluoride is elevated, you can use the following purification methods:
· Reverse osmosis
· Deionization
· Filtration through activated carbon (can reduce fluoride levels but does not remove it completely)
Fluoride in Soil
Soil fluoride levels are largely influenced by its composition.
The normal range of total fluoride in dry soil is between 150 and 400 mg/kg. 3
Sandy soils in humid climates tend to have the lowest fluoride concentrations, while heavy clay soils can reach levels as high as 1000 mg/kg. 4
Even in soils with high fluoride concentrations, most fluoride remains unavailable to plants. This is because the majority of fluoride in soil is insoluble due to strong ionic bonds with elements like calcium, forming stable compounds such as calcium fluoride (CaF₂).
Low pH can also increase fluoride solubility in the soil solution, which in turn raises the potential for fluoride uptake through plant roots. 5
Fluoride in Fertilizers
Certain fertilizers, such as phosphate fertilizers, can contain significant amounts of fluoride because they are made from phosphate rock, which naturally contains a lot of fluoride. 6
Here are some examples of fluoride concentrations in fertilizers:
When phosphate fertilizers are applied to soil, the fluoride from these fertilizers tends to bind with clay minerals and oxides, making it less available for plant absorption.
Long-term use of phosphate fertilizers can lead to a accumulation of fluoride in the soil over time.
Toxicity and Limitations of Fluoride in Agriculture
In agriculture fluoride is classified as an undesirable salt, similar to sodium.
Excessive fluoride can be toxic to plants and its toxicity impacts several processes including:
· Germination,
· Plant growth and crop yield
· Oxidative and respiratory function
· Reproduction
· Amino acid and protein metabolism,
· Photosynthesis and chlorophyll production 7
Regular monitoring of fluoride levels in irrigation water is crucial to maintain crop health and productivity.
Studies show that long-term exposure to soluble fluoride in water (more than 1.5 ppm) can have detrimental effects on plants. 8
Mitigating Fluoride Effects
Irrigation water and soil solution testing
Regularly monitor fluoride levels to address any sudden increases before you see detrimental effects in your plants.
Soil washing
Soil washing is a potential remediation technique for soils contaminated with fluoride. 9
Sodium nitroprusside application
Sodium nitroprusside (SNP) is a water-soluble salt that can help plants tolerate salinity and fluoride contamination.
Maintaining soil pH
Fluoride is not soluble above pH 6 - ensure to maintain your soil or media solution pH above this level to prevent most fluoride toxicity problems. 10
Treating soil with neutralizing substances such as lime, loam and charcoal. (liming was shown to be most effective and charcoal as least effective)
The easiest way to ensure your plant is not absorbing fluoride is to keep your soil pH intact, however this may not be possible if you culture thrives in lower pH environment (i.e. blueberries).
If you found this article useful, sign up for our email newsletter to receive free nutrient guide and other curated crop nutrition advice.
References:
Aoun et al "The Fluoride Debate: The Pros and Cons of Fluoridation", Preventive Nutrition Food Science, 2018
University of Massachusets Amherst
Bhat et al, "Assessment of Fluoride Concentration of Soil and Vegetables in Vicinity of Zinc Smelter, Debari, Udaipur, Rajasthan", Journal of Clinical Diagnostic and Research, 2015
Fuge and Andrews, 1988
Anshumali, 2014
Ramteke et al, "Study of fluoride content in some commercial phosphate fertilizers", Journal of Fluorine Chemistry, 2018
Garrec et al, 1981
Sharma et al "Fluoride toxicity triggered oxidative stress and the activation of antioxidative defence responses in Spirodela polyrhiza", Plant Interact, 2019.
Moon et al "Soil washing of fluorine contaminated soil using various washing solutions". Bulletin of environmental contamination and toxicology, 2015.
University of Massachusetts Amherst
Comments