Sustainable Soil Management and the Role of Spectroscopy

Today’s agricultural research aims to feed tomorrow’s global population. Every year, farmers try to produce more food out of the same resources. However, the quickly changing climate conditions only increase the pressure on the world’s food supply.

Changes to weather patterns mean that extremes of heat and cold swing wider, floods are more frequent and droughts deepen. Inevitably more must be produced from less. Growing seasons become shorter, resources such as clean water are depleted, and even the soil itself can become exhausted.

Scientists are currently working hard to develop the technologies and tools that will make the future of farming possible.

For many technologies and tools currently in development, spectroscopy is a key enabling technology. Spectroscopy is everywhere, from innovative research to integration in sensors and analytics.

Avantes is also having this widespread impact. Trusted in field research outposts, production lines, and labs, Avantes instruments bring about proven results globally.

Soil Management

Soil is a compound mixture of liquids, gases, minerals, organic matter, and even living organisms. As well as supporting plant life for crop production, soil also functions to transport, store, and purify water.

It serves as a habitat for a variety of organisms and helps to moderate the atmosphere that we depend on. As well as being vital for future food production, sustainable soil management is vital for life on Earth.

Soil health is a fundamental aspect of sustainable land management, and farms of all sizes should take it into consideration. Anything from contamination to erosion, soil compaction, loss of biodiversity, and everything in between, can be damaging to crop production and a farm’s viability.

Several technologies and studies are in development that aims to analyze and manage soil health and Avantes is at the head of research and technology development aimed at protecting the future of the world’s food supply.

Moisture Measurements

Measurements of soil moisture have historically employed a device called a tensiometer, a hollow tube with a gauge, and a porous reservoir of water on top. The tube is implanted in the soil and draws water from the cup into the soil, generating a vacuum until equilibrium is reached.

Irrigation Systems. Image Credit: Avantes BV

Using the gauge, the user gathers a reading based on that vacuum that correlates to the soil matrix’s water-carrying potential. With this data, farmers can determine the need for irrigation, but there are downsides. The tensiometer is limited in its scale of use due to being slow, requiring a period of time for water to reach equilibrium.

Since 1978, researchers have been drawn to the effects of soil moisture first on the visible spectrum, and as spectroscopy techniques improve, the near- and mid-infrared spectrums².

In 2014, researchers in Hungary attempted to calibrate spectral data intended to develop algorithms enabling fast, field-scale soil moisture content measurements³.

The development of these algorithms that will someday enable field-scale deployment of spectroscopy-based moisture measurements starts with data collection in the laboratory.

Soil samples obtained from orchards around a region with varying soil characteristics were initially kiln-dried to consistent aridity in the lab. Then, water was reintroduced slowly, with spectra collected with every 2.5 ml until fully saturated.

The wavelengths 1450-1460 nm and 1920-1930 nm were identified as the most sensitive for quantifying soil moisture.

The AvaSpec-NIR256-1.7-EVO and AvaSpec-NIR256-2.5-HSC-EVO allow the sensitivity and range for this type of application in the laboratory. However, future developments in compact NIR spectrometers could lead to the tensiometer being replaced with handheld field instruments.

This would enable quick assessment of moisture content, integration with irrigation systems, and calibration and authentication of (ground truth) airborne hyperspectral imaging technology³.

Soil Characterization

Soil is not a homogenous mixture. Differences in organic matter, particle size, and minerals are a few examples of factors that can alter numerous soil characteristics⁴. All sorts of factors from irrigation schedules to types of crops likely to prosper are determined by soil type.

Soil samples for classification. Image Credit: Avantes BV

Twelve types of soil are recognized by the US Department of Agriculture, with clay and sand at opposite ends of the spectrum⁵. There are predictable characteristics, including color, for each type.

The most commonly used method to determine soil type depends on subjective (and fallible) personal experience for comparison against the Munsell Corporation’s specially designed color chart².

Other soil classification methods require chemical processes that can adversely affect data interpretation³. They also require more advanced technical skills to carry out and are more time-consuming. On the other hand, optical spectral sampling requires no harsh chemicals and little to no sample preparation, and many parameters can be analyzed using the same spectral data.

UV/VIS/NIR spectroscopy was investigated by researchers in Hamadan, Iran, to analyze a number of soil parameters, including pH, color, total nitrogen, available organic carbon, moisture content, electrical conductivity, and exchangeable cations.

Minerals such as calcium, magnesium, iron, potassium, sodium, and titanium oxides were also identified³. After being collected, randomized, and kiln-dried, the samples were ground and sieved, then divided into calibration and validation sets. There was a full battery of chemometric data collected from the set of validation samples.

Using the spectrometer to cover the UV-VIS range from 200-1100 nm at 1 nm resolution, an average of over 24 broadband spectral scans were collected per sample in the calibration set.

For the 1000-2400 nm range, measurements were in the NIR. Spectral analysis carried out on the calibration set enabled a series of operations for statistical analysis based on least partial squares regression analysis (LPS-R) and principal component analysis (PCA) to link spectral reflectance measurements to observable soil properties.

Looking at these results, the researchers determined that soil classification could be sufficiently validated using a fraction of available variables.

Soil Compression and Bulk Density

The compression of spaces between soil particles that would normally hold water or air is called soil compaction. Soil compaction leads to oxygen deficiency, poor root development, and other deficiencies. Ultimately, these factors reduce crop yield and quality.

Effects of Soil Compaction. Image Credit: Avantes BV

Compaction can be the result of natural processes or man-made, but it is a serious agricultural and environmental problem¹. Repeated plowing to a uniform depth, extensive use of heavy machinery, and use by large animal populations are all common causes.

Soil compaction must be managed by sustainable agricultural systems, starting with the measurement of key parameters related to soil compaction.

The ratio of volumetric moisture (Θv) content to gravimetric moisture (ω) [expressed as BD = Θv/ω] is represented by bulk soil density (BD). BD is positively correlated with soil compaction, in other words, the higher the soil density, the greater the degree of compaction¹.

The penetrometer is the current technology for soil compaction measurements. This is a simple instrument that is essentially a pointy stick supposed to mimic a growing plant root. It includes a pressure gauge on top of a graduated driving shaft tipped with a wider 30-degree stainless steel cone.

In 2018, research from the journal Computers and Electronics in Agriculture revealed a prototype measuring system that combined a fiber-coupled NIRS sensor with a traditional penetrometer¹.

The system worked in situ, combining measurements of soil penetration resistance and frequency domain reflectometry to analyze volumetric moisture content. Near-infrared diffuse reflectance was used for the gravimetric moisture content measurement. This allowed for the calculation of bulk density (BD) with a single, easy to operate probe system.¹

In this novel probe design, a penetrometer for the measurement of soil penetration resistance was combined with a hollow shaft housing optical fibers. These fibers were coupled to a sapphire window in the shaft body at one end, and on the other, to the 20-watt halogen lamp for input. It was connected to the AvaSpec-NIR256-2.5 spectrometer for NIR reflection data output and capture.

Time-domain reflectometry measurements quantify volumetric moisture content (Θv) achieved by integrating an electrode in the shaft of the probe in the form of a copper ring insulated from the probe body.

An artificial neural network (ANN) was trained from hundreds of measurements to model the fusion of data, producing encouraging results for the development of bulk density solutions for fast results in field applications in the future¹.

Avantes Rises to the Challenge

The need for efficient, reliable, and affordable data does not come without its challenges. Many systems that are available are not appropriate for field deployment. Operator errors and inconstancies can also compromise data collection.

NIR Spectroscopy in the Laboratory. Image Credit: Avantes BV

A considerable amount of work that goes into designing all-in-one spectroscopy-based solutions seems to focus on eliminating inconsistencies in deployment or opportunities for user error². System design engineers can find solutions to numerous field deployment challenges by collaborating with an experienced partner in spectroscopy.

Avantes combines 25 years of experience working directly with equipment manufacturers and researchers to design systems that match and surpass measurement requirements. Contact Avantes to test drive one of its instruments with its exclusive demo program and discover the Avantes advantage.

References and Further Reading

1. Al-Asadi, Raed A., and Abdul M. Mouazen. "A Prototype Measuring System of Soil Bulk Density with Combined Frequency Domain Reflectometry and Visible and near Infrared Spectroscopy." Computers and Electronics in Agriculture. Elsevier, 30 June 2018. Web. 19 Nov. 2019. https://www.sciencedirect.com/science/article/pii/S0168169917316277
2. Han, Pengcheng, et al. “A Smartphone-Based Soil Color Sensor: For Soil Type Classification.” Computers and Electronics in Agriculture, vol. 123, 2016, pp. 232–241., doi:10.1016/j.compag.2016.02.024. https://www.sciencedirect.com/science/article/pii/S0168169916300618
3. Monavar, Hosna Mohamdi. “34th International Conference on Food and Agricultural Engineering.” The Ires, Determination of Several Soil Properties Based on Ultra-Violet, Visible, and near-Infrared Reflectance Spectroscopy, 12 Mar. 2016, https://www.researchgate.net/publication/297322303_Determination_of_several_soil_properties_based_on_ultra-violet_visible_and_near-infrared_reflectance_spectroscopy “Soil.” Wikipedia, Wikimedia Foundation, en.wikipedia.org/wiki/Soil.
4. “Soil.” Wikipedia, Wikimedia Foundation, https://en.wikipedia.org/wiki/Soil.
5. USDA. “Natural Resources Conservation Service.” The Color of Soil | NRCS Soils, US Department of Agriculture, https://www.nrcs.usda.gov/

About Avantes BV

Avantes is a leading innovator in the development and application of miniature spectrometers. To meet our customer’s application needs, Avantes continues to develop and introduce new instruments for fiber optic spectroscopy. Avantes instruments and accessories are also deployed in a variety of OEM applications and a variety of industries in markets throughout the world. With more than 18 years of experience in fiber optic spectroscopy and thousands of instruments in the field, Avantes is eager to help our customers find their Solutions in Spectroscopy^®.

Techniques supported

• UV-VIS/NIR spectroscopy
• Process control
• Absorbance/transmittance/reflectance
• Laser-induced breakdown spectroscopy
• CIE color spectroscopy
• Portable spectrometers
• Fluorescence spectroscopy
• Custom applications
• lrradiance
• Raman spectroscopy
• OEM application development

Major products/services

Low-cost. high-resolution, miniature fiber optic spectrometers: System solutions and OEM instruments for applications from 185 nm to 2500 nm. Detector choices: PDA, CMOS, CCD, back-thinned CCD, and lnGaAs.

Optical benches with focal lengths of 45, SO, or 75 mm; revolutionary new ultra-low straylight optimized optical bench (ULS) and a new high sensitivity optical bench.

Other features

• 14 and 16 bit AID converters
• TE cooling
• multi-channel instrument configurations enabling simultaneous signal acquisition
• USB2 communication support for multiple instruments from a single computer
• 14 programmable digital I/O ports

Standard application solutions

• lrradiance and LED measurements
• gemology
• hemometric analysis
• thin-film measurement
• color
• fluorescence
• laser-induced breakdown spectroscopy (LIBS)
• Raman spectroscopy
• process control

Light sources

• Tungsten-halogen
• Deuterium
• LED
• Xenon calibration sources for wavelength and irradiance

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