Why Monitor Soil Moisture and Nutrients?

The moisture and nutrient status of your soil are key pieces of information for every grower or greenkeeper. Armed with this knowledge, you can precisely tailor your irrigation and fertilisation program to suit your crop's requirements – this leads to less wastage in terms of costs of inputs, and achieving optimum quality and yields.

Soil Moisture (Volumetric Water Content)

The volumetric water content (VWC) is the ratio of the volume of water compared to the total soil volume. At saturation, the volumetric water content (expressed as a percentage) will equal the per cent pore space of the soil.

In-field soil moisture content will range from air-dry to saturation. However, plants cannot extract all the water in a saturated soil and can extract none of the water in an airdry soil. Instead, two other moisture content levels, field capacity and permanent wilting point are often used to indicate the upper and lower limit of plant available water.

'Field capacity' is defined as the condition that exists after a saturated soil is allowed to drain to the point where the pull of gravity is no longer sufficient to remove any additional water. Water draining from a soil profile cannot, in general, be taken up by plant roots. On the opposite end of the spectrum, permanent wilting point is the highest moisture level at which an indicator plant cannot recover turgor after being placed in a humid environment.

Irrigation should be scheduled somewhere between these two extremes. One rule of thumb is to apply water when half the plant available water has been depleted. However, individual circumstances may dictate a more conservative or liberal approach.

Monitoring Soil Salinity

The salinity of the soil solution, irrigation water or fertiliser solution is an important parameter affecting the root zone environment. Any of these factors can have a significant effect on plant growth and physiology.

The easiest way to monitor salinity is by measuring the electrical conductivity (EC). EC is strongly correlated to the salinity of the soil solution. EC measurement is also affected by temperature and, to a lesser degree, by soil moisture content. The WaterScout SMEC 300 Soil Moisture/EC/Temperature Sensor measures EC with a pair of carbon ink electrodes which provide a large contact surface with the soil solution. Temperature is measured with a thermistor potted in the sensor molding.

Electrical Conductivity (EC) of Soil

Electrical conductivity (EC) is a measure of how well a solution conducts electrons. Pure water does not conduct electricity at all. However, as the concentration of dissolved ions increases, the electrical charge carried by those ions of the solution will increase as well. This is reflected in a higher EC measurement for that solution.

In soil solutions, the sources of these ions are minerals in the soil (such as sodium or calcium) and applied fertilisers. The EC of a liquid at a given temperature is governed mainly by the amount of dissolved salt ions. However, when a direct measurement is being made in a wet soil, the degree of saturation also impacts the measured EC. This is because as water content decreases, the conductive path becomes increasingly tortuous, thus decreasing the EC even though the concentration of salt in the remaining liquid is increasing. Such an EC measurement is sometimes referred to as bulk EC because it reflects how well electrons in the bulk soil are conducted. The SMEC 300 measures bulk EC.

An alternative measure of EC is pore EC. This is the value that would be measured if the soil solution alone were extracted from the soil and measured. Pore EC will almost always increase as soil moisture content decreases. The EC sensor for the SMEC 300 consists of a pair of carbon ink electrodes that are integrated onto the sensor surface. The EC electrodes measure the conductivity of wet soil in contact with the sensor. The carbon ink electrodes provide a large surface area which minimises the effect of localised dry spots or loss of contact in the soil.

The WaterScout SMEC 300 Soil Moisture, EC and Temperature Sensor

This clever unit combines three sensors into one: soil moisture, EC, and soil temperature. The SMEC 300 brings affordability and accuracy to a sensor that is easy to install and can be a key part of your SpecConnect remote crop monitoring system.

The soil moisture sensor consists of two electrodes that function as a capacitor, with the surrounding soil serving as the dielectric. An 80 MHz oscillator drives the capacitor and a signal proportional to the soil’s dielectric permittivity is converted to the output signal. The dielectric permittivity of water is much greater than air, soil minerals and organic matter. So, changes in water content can be detected by the sensor circuitry and correlated to the soil’s moisture content.

Track soil changes over time by connecting the WaterScout SMEC to a WatchDog® data logging weather station. SpecWare software enables you to view your data in graphical and tabular form as well as run reports customised to your application. Alternatively, the unit can be used with a FieldScout® Soil Sensor Reader to take fast and accurate spot readings.

Find out more about the WaterScout SMEC 300 Soil Moisture, EC and Temperature Sensor.

Soil Sensor Placement

Sensors should be located in the effective root zone and at locations that will give a representative picture of the salinity and soil water status of the area being measured.

For outdoor applications, consider areas of the field planted to different types of vegetation. This could be fairways vs. greens or landscaped areas planted with trees as well as those planted with flowers. Areas with significant differences in factors such as topography or soil type should be considered unique environments. For greenhouse applications, select one or more representative containers. Selecting a site which receives the least amount of water from the irrigation system will tell you when that area becomes critically dry and needs attention. Sites that have the greatest fluctuation in soil moisture will exhibit the greatest level of EC variability as well.

Typically, one or two sensors should be installed in the root zone. A single sensor should be placed in the middle of the root zone. When two sensors are installed at a single site, it is recommended to place one sensor at the top of root zone and a second at the bottom.

An advantage of installing multiple sensors is it allows you to see how well irrigation and rainwater is moving through the soil profile.

See our full range of soil moisture meters & sensors.

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