Determination of water content at permanent wilting point in a large set of soils

Master thesis of Lorenzo Ceriscioli, conducted at the Università degli Studi Mediterranea di Reggio Calabria (Italy) together with Agroscope (Switzerland) Summary Modern agriculture faces challenges that no longer concern only the sphere of economic sustainability of production per se, but rather socio-environmental issues that claim to place it at the centre of an increasingly urgent ecological transition. In this context, the sustainable management of irrigation is a crucial element. To this purpose, the essential starting point is the most precise knowledge of the hydrological properties of the soils in which one operates. In fact, if the calculation of the irrigation volumes to be administered is made on the basis of the reserve of ‘useable water’ from the plants, knowing precisely where the Permanent Wilting Point (PWP) and Field Capacity (FC) lie are unquestionably the cornerstone on which irrigation technique and strategy should be based. In order to obtain these parameters to date, the Pressure Plate Apparatus (PPA) has been used, an instrument that although it offers accurate measurements in the ‘wet’ soil range (up to − 1.0 𝑀𝑃𝑎 ) seems to lose reliability in the ‘dry’ soil range (around − 1.5 𝑀𝑃𝑎). The instrument that can make up for this shortcoming is the Dewpoint Potentiameter (WP4), which measures the water potential of soil samples and guarantees maximum performance precisely in the dry range. In order to revisit the measurements of the water content at the PWP carried out so far by the laboratories with the PPA in this study, paired measurements with both instruments were carried out on the same soil samples, enriched with data of the same nature extracted from other publications and finally a statistical analysis was carried out to investigate which factors most influence the difference between the pairs of values. In addition to the purely pedological characteristics (clay, silt, sand, bulk density, organic carbon and specific surface area), the methods used by the various laboratories conducting the analyses were taken into account to investigate whether the differences in the pairs of measurements were inherent solely to the physicochemical properties of the samples or also to the practices implemented during the analyses. The results of the analyses showed that the most influential physical characteristic in the difference in measurements was specific surface area (SSA), organic carbon (SOC) correlated positively with water content at PWP for both instruments, and bulk density was not significant. With regard to the analytical procedures, the effect of hysteresis and thus the re-equilibration time given to the rehydrated sample prior to analysis in WP4, as well as the choice of rehydrating or not the samples were not significant as an assumption. Instead, the days spent within the PPA had a positive correlation with the difference in values, which is unexpected and deserves further investigation. The positive correlation between the ring height of the sample (Ring Height PPA) in the PPA and the difference between the measurements was confirmed and the use of a solution (Wetting Solution PPA) to progressively reduce (as SSA increases) the differences between the measurements. In conclusion, in order to be able to carry out this type of analysis and comparison and obtain more robust results independent of the influences of various methodological factors, it would be necessary to make use of a dataset of samples representing a broad spectrum of textural combinations, where preferably clay type would also be taken into account, and this should allow SSA to be estimated more accurately. Finally, for validation at the inter-laboratory comparison level, it would be appropriate to test the procedures adopted by the different laboratories to get a precise view of the possible contribution of methodology in influencing the analytical reading.