Soil spectroscopy might soon offer faster, cheaper soil testing. Soil analysis is an essential cost in strategic crop nutrition management.
Spectroscopy uses mid infrared (MIR) or near infrared (NIR) radiation. Spectrometers fire radiation at a sample and then measure what’s reflected back. Different absorption and reflectance patterns can measure soil properties. Values measured by wet chemistry techniques calibrate spectroscopy in specific soils.
Advantages for cheaper, faster soil testing:
- quick sample preparation
- instant results
- portable equipment
- minimal training to use in field – ‘point and shoot’ – although not so simple to interpret.
- measures many parameters at the same time
- some current hand-held devices cost hundreds, not thousands, of dollars.
Soil spectroscopy is unlikely to fully replace lab analysis. It could make it easier to define different soil zones. This can reduce the number of soil samples that need full lab tests.
What are the limits?
Trials funded by the GRDC compared the results from available portable soil spectroscopy equipment. Acceptable precision was not achieved for all soil properties. The best predicted parameters were:
- Total and organic carbon and N
- Cation exchange capacity (CEC)
- Exchangeable sodium
- Exchangeable calcium and magnesium
Exchangeable boron and bulk density were less accurate. At this stage, infrared spectroscopy cannot measure available nutrients such as Colwell P or mineral N.
Handheld devices measure a few microns into the soil, analysing only the surface of the sample so some properties may not be accurately predicted. Moisture content and particle size affects results.
Researchers are learning which equipment will work reliably in Australian conditions. Work has started on calibrating spectroscopy results for southern Australian soils. Some commercial labs are already using spectroscopy to predict soil properties. Soil testing using spectroscopy in paddocks is still a few years away.