Particle size distribution (PSD) is fundamental to a soil’s biological and ecological functioning. For 50 years or more, the reference method for analysing PSD involved taking a series of measurements manually, over a 24-hour period. Known as the pipette method, this is not only time-consuming and prone to human error, but it also disturbs the sedimentation process in the sample every time the pipette is inserted.
Commonly used alternatives are the hydrometer method that is based on the same physical principle and, likewise, disturbs the sedimentation process, and the far more expensive laser diffraction method that suffers from inaccuracies due to the flat nature of clay particles.
In 2017 METER Group launched the revolutionary PARIO system for automatic PSD analysis, also referred to as the PARIO soil texture analyser. As with the pipette and hydrology methods, the PARIO is based on the principle that larger, heavier particles sink faster in water – as per Stokes’ Law for the drag exerted on spherical objects moving through viscous fluids. Over time, as particles sink, the suspension pressure exerted at a given depth reduces correspondingly. In the PARIO system, readings are taken from a high-precision pressure transducer located in the sample at a fixed depth; at the same time, the temperature is measured so all measurements are temperature-compensated.
The 10-second interval between readings is much shorter than the hourly intervals typically used in the pipette method, meaning sufficient data is collected for the accompanying software to generate a quasi-continuous plot of PSD. The process is automated, less susceptible to human error and completed in around 8 hours. A paper by Durner et al  concluded that the PARIO’s Integral Suspension Pressure (ISP) method yielded results that were in excellent agreement with those obtained using the pipette method.
Realising that accuracy and speed could be improved still further, METER has now developed the Extended Integral Suspension Pressure Method (ISP+). Compared with the ISP method, ISP+ improves the precision for the clay fraction of samples and cuts the time required from 8 hours to 2.5 hours. The principle behind the ISP+ method is the same as for the ISP method except that an additional step is introduced. After 2.5 hours, when the measurement phase is complete, part of the sample is drained from above the pressure transducer by means of a tap in the side of the cylinder.
This portion of the suspension, which only contains fine material, predominantly clay, is oven-dried and the weight of the dried suspension measured. A modified algorithm in the software utilises this weight when generating a refined plot of cumulative particle size distribution. As a result, the ISP+ method has an extremely low estimated error of just +/-0.5%, which compares extremely favourably with the PARIO Classic’s +/-3.0%. A paper by Durner and Iden  showed significantly improved precision of the ISP+ results in the range of silt and clay particles and very good agreement with the pipette method. Furthermore, the researchers concluded that reducing the analysis time from 8 hours for the ISP method to 2.5 hours for the ISP+ method did not compromise the accuracy. Unlike some other automated analysis methods, PARIO Plus does not need soil-specific corrections to be made with transfer functions.
METER’s PARIO Plus system comprises a sample cylinder, the PARIO instrument and Windows 10 software for data acquisition, imaging, visualisation, evaluation and export. The particle size measurement range for the PARIO Plus is 2-63µm. Estimation of clay content is derived from integral particle mass in the effluent, while sand content is estimated from sieve data. Importantly, existing PARIO Classic analysers can be upgraded to the PARIO Plus specification simply by purchasing a new sample cylinder.
As the PARIO Plus is part of METER’s LABROS system, it can be combined with the HYPROP, KSAT or WP4C to enable a soil’s physical and hydraulic properties to be characterised fully.
Labcell is the sole UK distributor for METER Group. For more information, a data sheet and to request copies of the papers mentioned above, go to , or contact Labcell by emailing or calling +44 (0)1420 568150.
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