Saturday, 15 July 2006

A new way to measure soil salinity comes with a conversion factor.

Tina Dalby, Agronomy and Soil Science, University of New England, Armidale NSW 2351, Australia and Peter Cull, ICT International Pty Ltd, PO Box 503, Armidale NSW 2350, Australia.

Soil salinity is an environmental threat and problem worldwide. Soil salinity can occur naturally, or when human intervention disturbs the natural ecosystem. Subsequent changes in the hydrological cycle remobilise salts in the production zone of the soil and where salt accumulates salinity occurs. Salinity is quantified in terms of the total concentration of soluble salts such as sodium, magnesium, calcium, potassium sulphate and nitrate and can be practically measured by the closely related electrical conductivity of the solution. Ceramic soil salinity sensors measure the combined concentration of the salt mixture in the soil are ideal for efficient in situ field monitoring of soil salinity with minimal disturbance to the soil profile.

The soil salinity sensor consists of a pair of electrodes embedded in a fine-textured ceramic electrolytic element, 1.5 mm thick and 6 mm in diameter, with a thermistor behind the element. The assembly is permanently sealed in epoxy with only the element contacting the soil. Moisture saturates the ceramic element and comes into equilibrium with the soil water to give constant salinity measurements. An electrical cable, 1.2 m long, connects the sensor to a salinity meter. The electrolytic element is fabricated from a very precisely constructed ceramic with an extremely small and uniform pore size. This small uniform pore size ensures that the ceramic stays saturated throughout the soil suction range of 0-15 bars, the full plant growth range, to provide consistent salinity measurements.

Ceramic sensors were first produced in the 1960s and readings were made using a salinity bridge. A purpose-built, handheld salinity meter was manufactured in 2005 to provide portability and ease of measurement. One or more sensors can be used with the handheld salinity meter. A set of sensors can also be used with a data logger that is built into a field station. Using this system, dynamic changes in soil salinity at selected depths can be detected and logged at regular intervals.

Soil salinity is commonly measured using a 1:5 soil:water suspension and an electrical conductivity (EC1:5) reading of the supernatant is taken using an electrical conductivity meter. EC1:5 measures the quantity of soluble salts for a mass of soil and is useful where information about salt balance is required. Although this method is relatively simple, it can bring more salts into solution thus increasing the reading and does not allow for soil texture. Paste extraction (ECse) methods provide a measure of the concentration of salts for a volume of water and can be used to assess the effects of salinity on plant growth. However, ECse procedures are often considered time consuming. The saturated soil sample is transferred to a filter funnel and the water is extracted by suction and this requires a vacuum pump or centrifuge. An ECse reading of the extracted water sample is then obtained with an electrical conductivity meter. The ceramic soil salinity sensor can measure temperature-corrected salinity in situ without the need to take soil samples that need to be transported back to the laboratory for testing. Electrical conductivity of soil water (ECw) is measured by electrical signals from porous-matrix sensors. Used with the handheld meter or data logger, salinity can be monitored over time.

With the introduction of the ceramic soil salinity sensor, comparisons between ECw measurements and those of EC1:5 and ECse were required. ECse estimates from EC1:5 have been presented and a conversion factor formulated, but a method for comparing ECw is not available. This paper offers a simple equation to convert ECw to EC1:5 and ECse.

To obtain data for the conversion factor, six soil samples of the same soil (a Yellow Podsol) were prepared. The samples were air dried, ground to pass 2mm and moistened (6 mL moisture per 10 g soil) with varying rates of saline water from 0.005 to 0.2 M KCl ( 0.72 - 24.82 dS/m). Three ceramic salinity sensors were set up in each soil. The sensors were logged until equilibrium was reached and the salinity for that sample recorded. The soils were then destructively sampled and routinely tested for EC1:5 and ECse. A linear regression was completed for the EC1:5, ECse and ECw data to determine the conversion.

The data were tested in a second experiment. Here, soil salinity sensors were set up in each of six relatively undisturbed soil samples to model field conditions. Soil textures ranged from sandy to clay and the samples were accepted at moisture received. Data were logged at room temperature, 0C and 30C until equilibrium and the salinity values recorded. The samples were then routinely tested for EC1:5 and ECse to verify the conversion formula.

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