Amrit Gautam & Sujan Pandey
A soil water sensor is an instrument which, when placed in a soil for period of time, provides information related to the soil water status of the soil (Cape 1997).
Unlike many natural resources that are gifted like water, air, forest, mineral ores etc. to us, Soil is one of the most important natural resource. With the rapid growth of population but the finite natural resources on the Earth, it has become compulsion to find the best and effective way to utilize these resources wisely. Soil, which has been overlooked in the past, is now the matter of most concern for the maximum productivity of the crops. Various organizations are actively monitoring the health of the soil and its interaction with the physical environment. It is critical to understand the health of the soil in order to increase its productivity of crops in natural way.
With the advancement in the field of science and technology, various methods to monitor the soil status has been purposed. Instantaneous measurement of the soil moisture content, salinity, temperature etc. helps to track down the health of the soil. These are essential factors which every small scale farmer, which are going to increase the crop yield, should focus on. Different kind of soil measurements and sensor are available with affordable price which every conscious farmer should use to trigger crop production.
Technologies for measuring soil water status
Suction measurement systems
- Porous media instruments
These instruments are made from materials that are porous to water i.e. materials through which water can move and be stored in the pores. Water is drawn out of the porous medium in a dry soil, and from the soil into the medium in a dry soil
It measures the soil moisture tension or suction.Porous ceramic tip attached at one end is inserted into the where the majority of the active root system is present. The vacuum gauge at the another end of the device measures the soil moisture tension or suction. It measures how much effort the roots must put forth to extract water. The higher the reading, the less moisture that is available and the harder roots must work to extract the water.
Figure 1: Tensiometers
2. Electrical resistance blocks
The resistance blocks use electrical resistance instead of vacuum pressure to determine how much water is in the soil. Resistance blocks or "gypsum blocks" are made by imbedding two electrodes in a block of gypsum. They are then buried in the soil and a current of electricity is sent through them. They measure the electrical resistance between the electrodes and the resistance is directly related to the soil's water content.
Figure 2: Electrical Resistance blocks
These detectors are soil moisture switches that are buried in the soil when switches on when soil moisture reaches above a set point. When the soil dries below the set label, the detector is activated sending out signal that water has reached a given depth. All types of soil moisture sensors can be used as Wetting-front detectors, however a special product is designed suitable for only information required when the wetting front arrives at a set depth in the soil.
Volumetric water content measurement systems
- Soil Dielectric
Senors based on dielectric method works by measuring the dielectric constant of the soil. The dielectric constant is a measure of the capacity of a non-conducting material to transmit electromagnetic waves or pulses.The dielectric of dry soil is much lower than that of water, and small changes in the quantity of the soil have large effects on the electromagnetic properties of the soil water. Dielectric sensors use two different methods to measure soil moisture without measuring electrical conductivity.
- Time Domain Reflectometry (TDR Sensors)
- Frequency Domain Reflectometry (Capacitance Sensors)
Figure 3: Frequency domain (FD) probes a) capacitance (plates embedded
in a silicon board);b) capacitance (rods); and c) FDR (rings)
It is based on technique on measuring fast-moving neutrons that are slowed ( thermalised ) by an elastic collision with existing hydrogen particles in the soil. Hydrogen is present in the soil as a costituent of soil organic matter, soil clay minerals and water. Water is the only form of hydrogen that will change from the measurement to measurement. Therefore, any change in the counts recorded by the Neutron moderation is due to change in water.
Figure 4: Neutron moderation
Sensors working with this principle, measure the heat dissipated in a ceramic medium. The amount of heat dissipated is directly proportional to the amount of water contained within the ceramic's void spaces. The more water that is contained in the ceramic, the more heat is dissipated and lower the sensor reading. This equivalents to high content of water in the soil. This type of sensor has no effect of soil type or salinity since they are completely based on thermal conductivity.
Figure 5: Heat dissipation Sensor
In this page, I am going to explain about measuring soil moisture using Soil Dielectric using Frequency Domain Reflectometry (Capacitance Sensors) technology.
Frequency Domain Reflectometry (Capacitance Sensors)
This sensor uses capacitance to measure the dielectric permittivity of the soil by calculation of change in frequency. The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric of water (80) is much greater than the other constituents of the soil like mineral soil-4, organic matter-4, air-1. When the amount of water is changed in the soil, it also affects the capacitance (from the change in dielectric permittivity).
The sample (moisture contained soil) for testing is placed between two test plates that form a capacitor connected into an LC- oscillating circuit. The dielectric permittivity of the soil is measured by calculating the change in frequency originated by the oscillator before and after the sample is placed. Frequency is affected by soil moisture. The greater the soil moisture content, the smaller the frequency.
Figure 6 : Capacitive moisture sensing system
The best way to reduce variations attributes to environmental conditions, such as temperature and room humidity, is the use of a differential technique.
That is frequency shift,
Change in frequency (delta f) = f0 - f1
where f0 = frequency produced by empty container and f1 is frequency produced with the sampled material
Transfer function of Capacitive sensor and a system
Figure 7 : Transfer function of a capacitive sensor and a system
- Its accuracy is poor when measuring moisture below 0.5.
- The sample must be clean of foreign particles having high dielectric constants - examples are metal and plastic objects
- Packing and fixed sample geometry must be maintained.
Soil Moisture (SM300)
Delta-T Devices Ltd
The soil moisture sensor SM-300 is the sensor which is used to measure soil moisture content and temperature. As a hardware its body is sealed with plastic which is attached to 2 sensing rods as shown in figure . The sensing rods are directly inserted into the soil to take reading or experimenting the soil.The reading is logged to the data logger through use of waterproof cables which carry output analogue DC voltage. It can be calibrated for specific soils.
Figure 8 : SM300 Soil Moisture Sensor
Features of SM-300:
- The measurement accuracy of this model of sensor is 2.5% .
- The temperature measurement of the soil is from 0.5 °C to 0-40 °C .
- It is rugged, waterproof and can be buried inside the soil.
- Not only in performance it is also stable device.
- It has good electrical immunity.
- The device uses minimal soil disturbance
It has optional cable system option.
Dimensions of Sensor
How the SM300 works ?
The figure below is the screenshot of the Users Manual 1.2 which shows the basic principle of how these sensors function.
Figure 9: Basic working principle
After inserting SM-300 into the ground or soil power is supplied to the sensor. When it receives the electrical signal it creates a waveform of 100MHz which is similar to FM radio frequency. Secondly, the waveform is transmitted to a 2 stainless steel rods which are rigged inside a soil. This convert electronic energy to electromagnetic field and this wave get transmitted into the soil. There is some amount of water content in the soil. When the rods in the soil detect it the constant of proportionality that the relates the electric field in a soil & water content to the electric displacement in the material. Which is also the dominants of permittivity. Sensor recognize the permittivity, which act as strong influence on the applied field. Then the result is shown as voltage output by the sensor. And those voltage is converted into numerical value percentage to make easier and understandable to everyone.
Advantages of SM-300
- It helps to measure both soil moisture and temperature
- It provides high absolute accuracy
- It is stable in high temperature and salinity.
- Easy installation at the depth
- Simple connection to the data logger (0-1v)
- Minimum soil disturbance
- It can be used in plant pots
Temperature response of soil moisture reading:
The Sensor reading could be effected by temperature of the particular soil, it could be backed by various conditions, for instance,
The sensitivity of the sensor is capable of ascessing very low temperature which could make the slight difference in the over all knowing of sensitivity.
The refractive index of water which produces a negative temperature response particularly in soils or substrates with high water content.
The percentage of bound water decreases with temperature and this produces a positive temperature response particularly in clay soils at lower water contents.
Figure 10 : Temperature dependence of SM-300 in clay soil
Above figure illustrate the temperature response of soil moisture reading. As soil temperature changes its degree but there is no any change in the apparent soil moisuture.
Conductivity response of SM300:
Figure 11: Conductivity response graph
Salinity can be measured in the given basic,
SI units for ionic conductivity are mS.m-1 , where,
S = Siemens,
The unit of electric conductance = ohm-1
1 mS.m-1 = 0.01 dS.m-1
= 0.01 mS.cm-1
= 10 μS.cm-1
Chart above shows how the salinity affects the output of the soil moisture sensor at various sensor moisture levels. Blue line indicates the water level. Wheras, salinity is classified on the basic of basic marking scale which sclales the extreme, moderate and non saline values. When the water level is at lowest point salinity is measured moderate when the condutivity is 600 mS.M-1 and SM output is 1V. Similarly when the water level is at high point the salinity measures the 2 dS.m-1
and conductivity is 0 and the SM output voltage is maximum at 1.6V.
- User manual for the SM300, Soil Moisture [ONLINE] Available at: http://www.delta-t.co.uk/product-downloads.asp?$=Product%20Manuals
Accessed on: 12.4.2015
- Field Devices for Monitoring Soil Water Content [ONLINE] Available at : https://www.bae.ncsu.edu/topic/go_irrigation/docs/Field-devices-monitoring-.pdf
Accessed on: 12.5.2015
- Irrigation needs and how to test for them [ONLINE] Available at: http://www.ndsu.edu/pubweb/chiwonlee/plsc211/student%20papers/articles08/tyrelcale/Irrigation.html
Accessed on: 12.05.2015
- Soil Water Monitoring [ONLINE] Available at: http://www.irrigationfutures.org.au/imagesDB/news/soilwatermonitoring2ed.pdf
Accessed on: 12.05.2015
- Jacob Fraden. Handbook of Modern Sensors. Physics, Designs and Applications ; Fourth Edition: pp. 445 - 457