Monitoring salinity of the soil solution in a citrus orchard: the 4P technique revisited

Abstract

During a 6-year salinity experiment in a citrus (Shamouti) grove, 3 salinity levels of irrigation water (0· 8, 1· 3, and 1 ·7 dS/m) were used. Measured salinity levels depended on the monitoring technique, aqueous extract (Ext) v. electrical resistance (4 electrode method, 4P), and consistently differed by up to 50%. The main advantages of the resistivity method over sampling are: specific consideration of the actual water content at time of measurement, minimal disturbance of the natural soil structure and chemical composition, and a reduced spatial variability due to the possibility of repeated measurements at one spot. Despite massive salt inputs (12×10³ -36×10³ mmol charge(+)/ha), the final salinity levels, even according to the higher estimating resistivity method, were moderate, and during the 6 years had no significant effect on yield. It could thus be categorised as being either below or above the yield-reducing threshold value, depending on the measuring technique.