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New metrology to support agriculture, food production and the climate by protecting vital soil moisture levels

Healthy soil is a backbone of life on Earth. When soil is moist, it acts as a filter during the water cycle, helps biodiversity to flourish and is a natural sink for carbon.

Conserving soil organic carbon stocks is a key objective of the European Commission’s ‘Soil Deal for Europe’, which aims to improve soil health to support the European Green Deal and protect the environment. Soil moisture also has a huge role to play in agriculture, with 70% of the world’s water being consumed for irrigation and at less than 50 % efficiency.

To protect soil and reduce water waste, networks are being developed to monitor soil moisture levels. Large-scale measurements across landscapes are made using remote satellite sensing, while point-scale sensors are deployed for highly localised measurements. To bridge the gap between them, cosmic-ray neutron sensing (CRNS) has emerged as a promising technique. Cosmic-ray neutron sensing (CRNS) detects naturally occurring neutrons produced when cosmic rays interact with atoms in the atmosphere and at the land surface. Hydrogen—mainly in soil water—slows and scatters these neutrons, so wetter soil results in fewer neutrons being detected above the ground. This allows soil moisture to be estimated over a broad area.

To support soil moisture monitoring across these different scales, Partnership project ‘Metrology for multi-scale monitoring of soil moisture’ (21GRD08, SoMMet) has developed new metrology. This includes transfer standards for point-scale measurements, procedures to improve the harmonisation of local and remote sensing, and new calibration and validation practices for CRNS.

New calibration service

Project partner the Danish Technological Institute  (DTI), a Designated Institute based in Denmark, has developed a new calibration service for soil moisture meters based on the work of the project. The new service can calibrate using local (uncontaminated) soil samples, with water content up to 40 %, and so can be tailored to users’ needs without requiring precise information on soil composition. The work responds to national reference needs in the area of soil moisture measurements and complements the calibration service for wood moisture meters already available from DTI.

New measurement systems

During SoMMet, project partner TÜBİTAK UME (the National Metrology Institute of Türkiye) developed two new systems to make SI-traceable measurements of soil moisture and soil water content. While the majority of soil moisture comes from water, some comes from other organic materials, especially in the uppermost parts of the topsoil. This distinction is important but cannot be made using conventional ‘loss-on-drying’ methods.

The first system developed to address this problem was an Evolved Water Vapor System (EWS). This involves drying a sample in a hermetically sealed container under a flow of dry gas. This gas picks up water and other volatile compounds and can be analysed using a chilled-mirror hygrometer and other parameters to determine the water content of the sample. The total mass loss of the sample is used to determine the total moisture content, allowing the two values to be collected simultaneously. The system was then validated through EURAMET Project No. 1685, under the Technical Committee for Thermometry.

Although this EWS provides accurate measurements (with uncertainty of under 0.2 %), the measurement process can be slow and destroys the sample under test. Spectroscopic instruments existed but were often bulky or expensive, and calibrated only to a specific soil type. To address this, the partners at TÜBİTAK UME developed a low-cost LED-based spectroscopic system which can be used in-situ and non-destructively. The system has been designed as a transfer standard and calibrated against the EWS as well as gravimetric reference methods. Machine-learning models were additionally used to improve its applicability across different soil types, achieving uncertainties of below 0.5 % in laboratory experiments and below 3.5 % in field tests.

Adapted moisture meters

These two measurement systems have now been taken up as part of Partnership project ‘Metrology for standardised moisture / water content measurements in plant-origin bulk materials in support of International and European food safety and trade’ (23RPT03, GrainMet). The EWS has been adapted to measure water and moisture content in plant-origin materials including barley, corn and wheat flours. These measurements are critical throughout production, storage and processing of food to ensure safety and quality, and for fair trade. The spectrometer, meanwhile, has been adapted for rapid moisture measurements in sumac.

Together, these two new systems have helped to establish a traceable pathway from primary-level reference measurements to portable, low-cost devices which can be used in-situ. The adaptations made in the GrainMet project have further extended this pathway, ensuring that accurate moisture measurements are available from soil to product to storage to sale.

Miroslav Zboril (PTB), coordinator of the SoMMet project, says:

“Soil moisture is measured at scales ranging from individual sensors to entire landscapes, but these measurements can support sound decisions only when they are accurate, traceable and comparable. SoMMet has strengthened the metrological links between laboratory standards, field measurements and larger-scale observations and developed new tools to improve the harmonisation of soil moisture measurement methods. In the longer term, this progress will contribute to more efficient water use and climate-resilient agriculture.”

Zuzana Pálková (CMI), coordinator of the GrainMet project, says:

“GrainMet shows how results from one Partnership project can be taken up and extended in another. By building on the systems developed in SoMMet, we were able to bring traceable moisture measurements from the field all the way to food production and trade much faster than any single institute could have done alone.”

This Metrology Partnership project has received funding from the European Partnership on Metrology, co-financed by the European Union Horizon Europe Research and Innovation Programme and from the Participating States.