The use of IR spectroscopy is based on the fact that the spectra store information about the fundamental composition of the soil - its organic and inorganic parts. This is a fast and relatively cheap method.
Chemical analysis of soils is increasingly becoming a necessity for farmers, and for farms with an emphasis on precision farming it has become an obligatory element of analytics.
However, the main problems of the traditional analysis method are slowness, significant influence of human factors and high cost. Prices for surveying one hectare start at an average of 200 UAH. According to the Belarusian startup OneSoil, Ukraine has 35.9 hectares of arable land, divided into 2 million plots with an average area of 17.9 hectares.
In order to conduct a chemical analysis, the field is usually divided into sections (squares), the area of which will depend on the farm budget and the size of the field itself. However, each field may have several different soil properties, and their location and boundaries cannot be squeezed into geometric figures of the correct shape. For example, a plot of 3 hectares, depending on the heterogeneity of the soil cover, may have 3 - 5 different soils from which only one sample will be taken. The analysis of such a sample, in the opinion of the candidate of agricultural sciences. Sciences, chief researcher at the Ukrainian Center for Soil Ecology LLC Valery Grekov, will not characterize even one soil.
However, in addition to the analytical research method, there are also less well-known, but no less promising instrumental ones, in particular the method of infrared (IR) spectroscopy. There was active talk about it in Ukraine with the entry into the market of the Dutch startup AgroCares, which wants to simplify and reduce the cost of field inspection procedures. So far this is the only such solution on the domestic market, so using its example it is more convenient to understand the operating principle of similar systems in general.
Remember something from the physics course...
The principle of IR spectroscopy is the absorption of IR radiation by chemical substances with simultaneous excitation of vibrations of molecules or their individual fragments. In this case, the wave passing through the sample weakens. Molecules, depending on their composition, adsorb and reflect this radiation in different ways and convert it into kinetic energy. Absorption does not occur of the entire spectrum, but only of those parts of it whose energy corresponds to the energy and excitation of the molecules under study, that is, the wavelengths (frequencies) at which maximum absorption of IR radiation is observed. This may indicate the content of certain functional groups or fragments in the molecules. As a result, the atoms inside the molecules move faster, bending, lengthening and rotating side by side. The more radiation is adsorbed, the less it will be reflected.
Although the amount of energy quanta absorbed will depend on the specific type of chemical bond in the molecules, it is also influenced by the chemical matrix and environmental factors, such as the type of functional group, neighboring molecules, etc. (Miller, 2001). This makes it possible to identify a range of molecules that may contain the same type of bonds. The same molecule can give rise to multiple overtones (a phenomenon that occurs during vibrations) and Raman bands in the NIR region with decreasing intensity and increasing order of the overtones, and therefore absorption bands in the NIR region may overlap.
Encyclopedic data regarding the use of this method specifically for the chemical analysis of soils is very ambiguous and, moreover, it is quite small. For example, Russian scientists (Krischenko et al.) studied this topic more than 20 years ago. They noted that the moisture content in the soil can be determined quite accurately from absorption bands at 1400, 1900 and 2200 nm. At the same time, the content of organic matter and clay particles affects the reflectivity of the soil. The diffuse reflectance spectra of soils in the near-infrared region differ primarily in the overall level of absorption. A number of experiments have proven a direct relationship between the degree of absorption and the amount of humus in the samples - the more humus, the less infrared radiation was absorbed in this part of the spectrum. Scientists noted that if the value of the optical density in a certain sample at a specific wavelength is greater than in another, then this relationship remains the same for other wavelengths. This, in particular, masks the functional relationship between the optical density of the soil and the humus content in it. They also proved a statistical relationship between optical density and the pH value of the salt extract, hydrolytic acidity, content of exchangeable calcium, magnesium, etc. At the same time, this relationship was poorly traced in the case of mobile forms of phosphorus and potassium.
Absorption in the NIR region between 780 and 2500 nm results from vibrations of OH, NH, CH, SO and CO compounds, which have a large dipole moment (Stenberg et al. 2010).
Despite the complexity and heterogeneity of chemical and physical properties, spectral soil data are quite uniform. For example, the 30 USDA soil types can be grouped into only five spectral classes.
Malley (2004), Niederberger, Todt and others (2015) studied that mineral fractions that dominate the soil are poorly determined by this method. Due to the low dipole moment between phosphorus and oxygen, phosphates are not excited by infrared radiation and are therefore not directly detected by IR spectroscopy. However, P can be detected indirectly through organic connections or some correlation with other soil properties, for example, the ratio of C to N. In their work, Niederberger and Todt indicate that the quality of predictions of phosphorus content in organic fractions is higher than in inorganic ones. In any case, it depends on the variability of phosphorus content and soil properties.
Destroyers or sources of myths?
Since the IR spectroscopy method involves the use of special prediction models, a significant number of soil samples are required to calibrate them. Each variation in the interaction of soil and climatic factors creates unique conditions for the growing season of agricultural crops.
AgroCares considers its strength to be its calibration database, which, according to its representatives, is constantly being updated. Before examining the soils of a certain country and adding them to this database, the startup team first analyzes its market in detail, first of all assessing potential partners in this country, their level of knowledge and experience, the demand for such solutions and the awareness of farmers themselves about the importance of agrochemical soil analysis. Now the priority for the startup is product development in Poland, Hungary and Ukraine. Local partners help the startup in selecting samples; then the selected material is sent to the company’s central office in Wageningen (the Netherlands). In Ukraine, such a partner was the Institute of Soil Science and Agrochemistry named after. A. N. Sokolovsky. Each sample is analyzed using classical methods and then using spectrometers. Spectral and analytical data are subsequently used to create regression models, which should include the properties of soil samples already tested on SoilCares equipment. The company says that for each country it is enough to select from 400 to 1,500 samples, depending on the variability of soil cover, the area of the country and the presence of the same soils in neighboring countries where research has already been carried out. Currently, the SoilCares global soil database contains about 14,000 samples from more than 15 countries. The startup's soil scientists are confident that by collecting 30,000 samples, they will cover the spectral range of all the world's soils.
There are over 3000 soil varieties in Ukraine. But the AgroCares team collected only 1,350 samples. Selection was carried out not only on farms, but also on virgin lands in order to obtain the maximum range of variations for each element.
A scanner (spectroscope) is an optical device that uses a natural or artificial light source to create appropriate vibrations. In SoilCares, these are X-ray, near-infrared and mid-infrared. The result of the device is an absorption curve of a characteristic shape, which is used to analyze and predict soil properties. It represents the frequencies at which radiation is absorbed.
The device is connected via Bluetooth to the AgroCares mobile apps, which instruct the user on how to calibrate and scan, and are also linked to a database. The factors that most influence the spectral properties of the soil are humidity, temperature and its chemical composition. The depth of sampling for scanning depends on the planned crop, most often from 25 to 35 cm. It is very important to mix the sample thoroughly, as this can affect the result, it should not be too wet and not too dry. Samples are taken with a GPS reference, so that based on the results it will be possible to construct a map of the content of nutrients, etc.
After scanning, the resulting image should be sent for further processing by the system. Therefore, the scanner is only a technical component of the solution. Processing takes a few minutes. From all the calibration tests, the system selects the most similar ones, combines them and uses this combination to make predictions.
After this, the user will be able to familiarize himself with the results, such as: the amount of total nitrogen and phosphorus, exchangeable potassium, pH, organic matter content, cation exchange capacity, physical clay content (as one of the elements of the granulometric composition) and recommendations regarding these parameters.
But there is also a “but”. For example, total (gross) phosphorus in the soil cannot serve as an indicator of the availability of phosphorus for plants, because they consume only water-soluble dihydrogen phosphates and, to a lesser extent, hydrophosphates, the concentration of which in the soil solution is insignificant, because they quickly turn into slightly soluble and/or inaccessible forms.
Therefore, the scanner is intended only for operational basic diagnostics.
— Using this device, you can study many different soil indicators: the presence of macro- and microelements, physical properties, and the like. To collect data in Ukraine, the AgroCares team spent 500,000 euros over three years. Thanks to this, we can now compare the results obtained and find relevant values,” comments Drone.UA co-founder Valery Yakovenko, exclusive distributor of AgroCares in Ukraine. — The operating principle of this device cannot be compared with other methods of soil diagnostics, because this is a rapid assessment that is needed to quickly determine zones of heterogeneity in the field and detect their nature. On the agricultural market, opinions about the new method diverge into two camps - some accept it with joy, while others are quite cautious and skeptical. However, manufacturers are mainly interested in this product. One generated report with recommendations will cost farmers approximately 10 - 12 euros. It will contain the results of measuring soil pH, organic matter content, total nitrogen and phosphorus, exchangeable potassium, particle size distribution, cation exchange capacity.
Instead of a conclusion
Any of the currently known methods of soil analysis has its advantages and disadvantages. The classical method is lengthy, complex and the number of samples studied is very limited. To quote a textbook on agrochemical analysis, “it is often impossible to draw a conclusion about the reliability only from the final analytical results, since the analysis itself is multi-stage and each stage introduces its own error into the conclusions.” The main chemical research method is extraction. Others involve chemical or thermal oxidation. However, extractants are selected in such a way as to isolate only those compounds that can be absorbed by the plant. This, for example, concerns phosphorus. Extraction of potassium, calcium and magnesium is the main way to remove soluble and exchangeable forms of these elements from the soil. And that's a plus.
The IR spectroscopy method determines the content of elements at the molecular level. Some scientists have emphasized that the accuracy of this method is higher for organic fractions and the elements contained in them than for mineral ones. Luleva et al. investigated the possibility of determining potassium nitrate and came to the conclusion that the texture of the soil and its granulometric composition, namely the clay content, interferes with the development of a universal model for calculating the amount of potassium. However, this method is mobile. At the atomic level, the results may be more accurate, but the question of which of them are accessible to plants and which are not remains open.
What is clear is that the use of a scanner should be in conjunction with other elements of precision farming, so that the farmer can determine as much as possible what primarily requires his attention.
Expert commentary
Sergey Plishchak, agronomist-consultant of the Dneprovsky RSP UkrAgroCom LLC:
— I paid attention to this method of analysis because I constantly work with clients in the field, and in fact I want to get a device that will always be at hand and help make more accurate recommendations. Last year, after presenting their solution, AgroCares invited me to join a professional research team to provide expert advice. During the testing period, I carried out about 500 analyzes and at the end of it I knew for sure that I wanted to continue using this scanner. I usually work with clients who have 15-20 thousand hectares under cultivation. Despite the compactness of the device and the specific method of its operation, the results obtained were reliable. In my work, a scanner helps to diagnose faster and be able to better explore the field. Now I can diagnose more often with instant results.
Elena Ninua, Agroexpert (Ukraine)
