Agricultural soil is often contaminated with a wide range of compounds, such as pesticides and fertilizers. Soils are also polluted via indirect inputs such as flooding and atmospheric disposition. Soil testing using various chemical reagents helps analyze the nature of the soil, its composition, and health. However, many of the chemical reagents are linked with health hazards. Recently, optical sensors have helped to identify and quantify soil contaminants as well as mineral components of soil. Optical sensors have rapidly achieved this feat at a relatively low cost and without using harmful chemical reagents.
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Soil Composition and Contaminants
Healthy soils are composed of sand, silt, clay, organic matter, soil microbes, macro and micronutrients, water, and air. The elements that constitute major (or macro) and micronutrients exist in different forms of availability and stability. Soil organic matter (SOM) is associated with processes that control nutrient supply and storage in soils. Therefore, if SOM is depleted, the organic components of the soil are reduced. Healthy soil is the basic requirement for an environmentally sound agriculture system.
Common soil pollution sources are pesticides, mineral fertilizers, organic fertilizers, wastewater irrigation, and rural waste. Although chemical fertilizers enhance the yield, they deteriorate the soil system. For example, they cause soil hardening and reduce soil organic matter.
Chemical fertilizers also change the soil’s pH level after prolonged application and decrease soil productivity substantially. A solution to this problem might be replacing chemical fertilizers with organic fertilizers, which would help reduce some of the detrimental effects of the chemical fertilizers on the soil and ultimately on the environment, while enhancing crop productivity.
Pesticides protect the crop plant from harmful pathogenic infestations and weeds. Pesticides include a wide range of compounds, such as insecticides, fungicides, herbicides, molluscicides, rodenticides, nematicides, and plant growth regulators.
Farmers often overuse chemical fertilizers and pesticides to enhance their agricultural yield. Pesticide overuse (like any other chemical) in the soil has detrimental effects on the soil. They bring about chemical changes in the soil pH, its characteristics, and mineral content. There have been instances where scientists have found traces of chemical pesticides residues even after 30 years.
Common Chemical Reagents used for Soil Testing in the Laboratory
Many chemical reagents are used to determine the chemical content of the soil. For example, the fertility of the soil is predominantly analyzed by assessing the nitrogen (N), phosphorous (P), and potassium (K) content of the soil. Potassium dichromate is the main chemical that is used to detect and quantify the presence of N. Several studies have shown that when an individual is exposed to potassium dichromate, they can experience shortness of breath, wheezing, coughing, and asthma-like reactions. This chemical can also cause serious eye damage in the case of accidental contact.
For the detection of P, potassium bicarbonate and ammonium molybdate are commonly used. A risk assessment of potassium bicarbonate showed that this chemical can cause irritation to the eyes, skin, and respiratory tract. Previous studies have shown that ammonium molybdate is extremely harmful if swallowed. Furthermore, if this chemical meets the skin and eyes, it could cause serious irritation.
To detect the concentration of K, ammonium acetate, ethanol and sodium tetraphenylnboron are used in the laboratory. If ammonium acetate is accidentally ingested, it causes gastrointestinal ailments. In the case of inhaling, it causes respiratory problems and irritation when in contact with the skin or eye. Sodium tetraphenylnboron is highly flammable and is extremely harmful upon inhalation. Ethanol is a common laboratory reagent that is regarded as a highly flammable liquid and vapor.
Three chemicals used to assess the pH content of the soil are bromothymol blue, chlor phenol red, and bromo cresol green. Previous studies have shown these pH indicators are also associated with health hazards. For instance, if bromothymol blue is accidentally ingested, it causes gastrointestinal irritation with nausea, vomiting, and diarrhea. Upon inhalation, it causes respiratory irritation. Chlor phenol red, when in contact, is capable of causing serious eyes and skin irritation.
Optical Biosensors in Soil Testing Reduced the Use of Chemical Reagents
Optical techniques determine concentrations of analytes in soil.
Important optical properties used for the detection and quantification of soil samples are adsorption, fluorescence, polarization, refractive index, and wavelength. In 1965, methods based on optical properties were developed for soil analysis. These methods proved to be less costly, rapid and avoided the use of harmful reagents for soil analysis. Spectroscopy helped quantify the organic matter content of the soil by using reflected energy.
The introduction of near-infrared (NIR) and mid-infrared (MIR) wavelength on a soil sample produces an image or spectra, which is unique to each soil component.
Each of the soil composites absorbs light at a specific wavelength and provides data in terms of spectral peaks that are used to identify and quantify the soil components.
Advances in laser and optical techniques, coupled with machine learning and statistical models, have recently enabled scientists to quickly obtain and study multiple physical and chemical properties. Therefore, this method has helped reduce the use of hazardous chemical reagents for soil analysis. The use of optics in soil analysis is also a step towards practicing “green chemistry”.
References and Further Reading
Daly, K. (2021) The dark mysteries of the soil beneath our feet. [Online] Available at: https://www.rte.ie/brainstorm/2021/0913/1246357-secrets-soil-alchemy-dirt-mysteries/
Muenchen, K.D. et al. (2016) Pesticide Detection in Soil Using Biosensors and Nanobiosensor. Biointerface Research in Applied Chemistry 6(6). pp.1659 – 1675. https://www.researchgate.net/publication/323092257
Aktar, M. W. et al (2009). Impact of pesticides use in agriculture: their benefits and hazards. Interdisciplinary toxicology. 2(1). pp. 1–12. https://doi.org/10.2478/v10102-009-0001-7
Genu, M.A. and Damatte, M.A.J. (2011) Prediction of soil chemical attributes using optical remote sensing. Acta Scientiarum Agronomy 33(4). pp. 723-727. http://dx.doi.org/10.4025/actasciagron.v33i4.7975