New infrared instrumentation for soil analysis field applications

The most common spectroscopic method used to measure multiple soil properties is infrared (IR) spectroscopy, using either the near or mid infrared wavelengths (NIR and MIR) (Soriano-Disla et al., 2014). Several new low-cost, portable IR spectrometers are now available off-the-shelf. We assessed these spectrometers for their ability to predict a range of agronomically useful soil properties, important for grains production in Australia. Experiments were carried out firstly in the laboratory to assess the spectrometer performance on a set of standard soils under similar conditions, and currently in the field for in situ soil analysis. For the instrumental assessment under standardised conditions, 80 complete soil profiles separated into 5-6 depth horizons (n = 458) were selected from the CSIRO National Soil Archive, with soil data sourced from the Agricultural Production Systems sIMulator (APSIM) project. 

These samples represented the nine soil orders most commonly used for cereal cropping in Australia. Most were classified as Calcarosols, Chromosols, Sodosols and Vertosols (in this order). Minor contributions of Dermosols, Kandosols, Kurosols, Ferrosols and Tenosols were observed. 

The soil samples were scanned with five hand-held or miniature IR probes and compared with scans from a laboratory instrument. Multivariate prediction models for a number of key soil properties were developed for each of the soils as follows:

• Extractable boron (B).
• Exchangeable bases, Ca²⁺, Mg²⁺, K⁺, Na⁺.
• Cation exchange capacity (CEC).
• Air-dry moisture (ADM) content.
• Electrical conductivity (EC).
• pH.
• Chloride.
• Organic carbon (C).
• Particle size distribution (sand, silt, clay).
• Bulk density (BD).
• Drained upper limit (DUL) (0.1 BAR) moisture.
• Lower limit (15 BAR) (LL15) moisture.
• Saturated Moisture (SAT).

The instruments tested are detailed in Table 1. 

The best (R² values above 0.70) predictions (relationship between measured and predicted soil property) were obtained for total and organic C and nitrogen (N), pH, CEC, exchangeable sodium percentage (ESP), clay/sand, exchangeable Ca, magnesium (Mg) and sodium (Na), and DUL and LL15.

The two best performing IR devices were the UV-Vis-NIR device (SM-3500 OreXpress by Spectral Evolution, MA, USA) and the handheld MIR device (4100 ExoScan by Agilent A2 Technologies, CA, USA) (Figure 1). 

Table 1. Infrared instruments and main features — note the Frontier instrument is the non-portable laboratory ’benchmark’ instrument. 

Instrument	Frontier	ExoScan	FlexScan	SM-3500	NIRscan	SCiO1
Brand	Perkin Elmer	Agilent	Agilent	Spectral Evolution	Texas instruments	SCiO
Spectral range	7800-370 cm-1	6000-650 cm-1	6000-650 cm-1	350-2500 nm	900-1700 nm	740-1070 nm
Used range	4000-450 cm-1	4000-750 cm-1	4000-750 cm-1	400-2450 nm	950-1650 nm	740-1000 nm
Scanning time	15 s	15 s	15 s	30 scans	30 scans	1.5 s
Weight	34kg	3.2kg	3.4kg	4kg	100g	35g
Size	520x600x300 mm	171 x 119 x 224 mm	340×220×160 mm	216x305x89	62x58x36	19x40x68 nm
Portability	Bench-top	Hand-held	Hand-held - cord	Hand-held - fibre cable	Miniature	Miniature
Back freq	60 m	60 m	60 m	30 m	30 m	5 m
Sample	Manual holder	Stainless cups	Stainless cups	Petri dish	Glass vials	Petri dish
Price	~$80,000	~$60,000	~$60,000	~$80,000	~$1,000	<$1,000

¹Due to poor performance and technical issues, only n = 215 samples scanned
²Standard normal variate
Compared across all soil property predictions, the cheaper instruments performed poorly overall (Table 2). 

Table 2. Median R² over all soil analyses (relationship between predicted and measured soil properties) for each instrument.

Instrument	R2
ExoScan	0.73
FlexScan	0.73
Frontier	0.70
SM-3500 full range	0.66
NirScan	0.49
SM-3500 (NirScan range)	0.48
SM-3500 (SCiO range)	0.42
SCiO	0.22

Figure 1. ExoScan MIR probe (left) and SM-3500 OreXpress UV-VIS-NIR probe (right).

The two best performing instruments are currently being deployed in field campaigns in order to assess their performance under field conditions and for intact soil core analysis. Soils for infrared analysis are generally air-dried and sieved to <2mm incurring significant cost, but the use of field-moist, intact cores would result in time and cost savings. The main aim here is to see if these instruments could be integrated into commercial field soil sample/soil core collection equipment. 

The major limitation related to field conditions is the effect of sample heterogeneity that could adversely affect the performance of soil analysis. This heterogeneity is mainly due to variable moisture content (Janik et al., 2016) and particle size variability. This project set out to identify the best performing instrument under environmental conditions and to develop novel approaches (through both physical and analytical manipulations) to tackle this variability. This work is in collaboration with the Department of Agriculture and Food through its GRDC project ’Managing sodic and magnesic soils - western region’ and with Precision Agriculture Pty Ltd. 

Conclusions

Both portable Vis-NIR and hand-held MIR spectrometers used in this project showed great potential for the assessment of soil properties in situ in the field. The cheaper instruments performed poorly due to a restricted spectral range and are not useful for soil analysis. The ExoScan MIR probe showed the best performance, while the SM-3500 Vis-NIR probe was less affected by soil moisture than the MIR probe. Current work is examining the utility of these instruments for deployment in the field.

References

Janik, L.J., Soriano-Disla, J.M., Forrester, S.T., McLaughlin, M.J., 2016. Moisture effects on diffuse reflection infrared spectra of contrasting minerals and soils: A mechanistic interpretation. Vibrational Spectroscopy 86, 244-252.Soriano-Disla, J.M., Janik, L.J., Viscarra Rossel, R.A., MacDonald, L.M., McLaughlin, M.J., 2014. The performance of visible, near-, and mid-infrared reflectance spectroscopy for prediction of soil physical, chemical, and biological properties. Applied Spectroscopy Reviews 49, 139-186.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support. 
The authors also wish to acknowledge Dr Ed Barrett-Lennard (Department of Agriculture and Food WA) for the supply of intact soil cores.

Contact details 

Mike McLaughlin
CSIRO Land and Water, Waite Campus, Waite Road, Urrbrae SA 5064
08 8303 8433, 0409 693 906
Mike.McLaughlin@csiro.au, michael.mclaughlin@adelaide.edu.au