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SOIL PHYSICS

Salinity Effects on Soil Moisture Measurement Made with a Capacitance Sensor

^a Dpto. Producción Vegetal, Univ. de Almería, La Cañada, 04120 Almería, Spain
^b Research Station of the Cajamar Foundation, Autovía del Mediterráneo, km. 416.7, 04710 El Ejido, Almería, Spain
^c Dpto. Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, 5 de Febrero 818 Sur, Cd. Obregón, Sonora, México

^* Corresponding author (rodney{at}ual.es).

Three field experiments examined the effects of soil salinity on volumetric soil water content (SWC) measured with a capacitance sensor (CS). They were conducted in field-grown vegetable crops fertigated with complete nutrient solutions. Experiment 1 compared nutrient solutions with electrical conductivities (EC_ns) of 6.5 dS m^–1 (+SAL) and 2.4 dS m^–1 (control), applied in equal volumes, following fertigation with EC_ns of 2.4 dS m^–1. Once +SAL commenced, SWC (0–20-cm depth) increased rapidly and then remained approximately 30% higher than in the control. Soil matric potential (SMP, 10-cm depth) was consistently very similar in both treatments. In Exp. 2, increasing EC_ns from 2.1 to 5.5 dS m^–1 in irrigation treatments receiving 100% of crop evaporation (ET_c) and 25% of ET_c caused SWC (0–20 cm) to respectively increase appreciably and maintain relatively constant values. Experiment 3 examined the effect of increased salinity and whether normalizing sensors with higher EC_ns alleviated this effect. Treatments were equal volumes of: (i) EC_ns of 5 dS m^–1 with sensor normalization at EC of 5.2 dS m^–1 (SAL-N5.2); (ii) EC_ns of 5 dS m^–1 with normalization at EC of 1.9 dS m^–1 (SAL-N1.9); and (iii) EC_ns of 1.9 dS m^–1 with normalization at 1.9 dS m^–1 (control). Previously, the three treatments were fertigated with EC_ns of 1.9 dS m^–1. The SWC (5–15 cm) increased by approximately 10% in both SAL-N5.2 and SAL-N1.9, and maintained relatively constant values in the control. The SMP (10 cm) was consistently very similar in the three treatments. Normalizing the CS at 5.2 or 1.9 dS m^–1 had no effect on the response to salinity. In the three experiments, changes in SWC generally paralleled changes in EC of soil water (EC_sw); relative increases were 4 to 7.5% in SWC for each 1 dS m^–1 increase in EC_sw.

Abbreviations: CS, capacitance sensor • EC, electrical conductivity • EC_e, electrical conductivity of saturated soil extract • EC_ns, electrical conductivity of nutrient solution • EC_sw, electrical conductivity of soil solution • EC_w, electrical conductivity of water • ET_c, crop evapotranspiration • PVC, polyvinyl chloride • +SAL, treatment of standard nutrient solution with added salts • SAL-N1.9, treatment of 5 dS m^–1 nutrient solution where capacitance sensors were normalized in a solution of 1.9 dS m^–1 • SAL-N5.2, treatment of 5 dS m^–1 nutrient solution where capacitance sensors were normalized in a solution of 5.2 dS m^–1 • SMP, soil matric potential • SWC, volumetric soil water content