Author: Das, Subir
Date published: September 1, 2011
Soil moisture refers to the quantity of water contained of soil. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from zero (completely dry) to the value of the materials porosity at saturation. The soil moisture is a very important factor in the field of agriculture. If moisture goes below the wilting point (wilting point (WP) is defined as the minimal point of soil moisture the plant requires not to wilt) or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.
The soil moisture sensor is connected to irrigation system controller that measures soil moisture content in the active root zone before each schedule irrigations that is soil moisture sensors measures real time soil moisture. For the growing of crop three factors are most important first the soil nutrients and soil properties; second the properties of the seeds that used to grow and most important is the soil moisture level. If the soil moisture goes down despite having good seeds and full nutrient soil we loss the crop. Knowledge of the spatial distribution in soil moisture content for the top 1 to 2 m of the earth's surface, known as the root zone, is important for under-standing meteorological, hydrologic, and agronomic processes. However, soil moisture is highly variable in space and time because of soil, topography, land cover, evapo-transpiration, and precipitation heterogeneity. Moisture takes some heat thus it helps to balance the temperature of the ground also it loses the soil clods which helps in penetrating the roots of the crop. The micro organism activity increases as the moisture content increase which balances nutrients in the soil. Different workers [1-11] have worked on different technique of soil moisture sensors. A soil moisture transmitter based on gypsum block sensor has been developed by Irrometer Company, Riverside , which is mainly fine aggregates mixed with gypsum buffer, held inside a permeable membrane and a perforated stainless steel sleeve. Electrodes are embedded in the granular matrix, and the electrical conductance between them is the parameter measured. Conductance increases with increasing soil moisture. The purpose of the gypsum is to buffer the measurement from ions that are found in uncontrolled amounts in the soil. Majid et al.  design soil moisture capacitive sensor interface circuit based on phase differential technique, according to them the circuit has been designed and fabricated using MIMOS' 0.35"m CMOS technology. Simulation and test results show linear characteristic from 36 - 52 degree phase difference, representing 0 - 100 % in soil moisture level. Test result shows the circuit has sensitivity of 0.79 mV/0.10 phase difference, translating into resolution of 10 % soil moisture level. A Wireless soil moisture sensor based on Fringing Capacitance has been designed by Wobschall et. al. , which based on mixtures of dielectric particles which are conducting. Dielectric constant is dependent on moisture content by volume. University of Florida  worked on the development of a soil moisture sensor with an objective to measure soil moisture at the root zone and regulates the existing conventional irrigation timer, resulting in considerable water savings when installed and used properly. Khan et al.  designed a high accuracy measurement circuit, employing a bridge amplifier, an integrator, and a comparator, for detecting the moisture content of soil. This circuit has the advantage of detecting soil resistance with better accuracy and with wide-range linearity. Davis, Johnson and Huberts  jointly studied and invented a soil moisture sensor which includes a processor to derive soil moisture values and a memory store associated with said processor to store measured values on a periodic basis, wherein the processor scales the stored moisture values to establish a moisture range for the sensor that can be used to calibrate each new reading.
The artificial application of water is called irrigation which is essential for growing of high yield crop. So measurement and analysis of soil moisture content is important factor in the field of agriculture. The current moisture measurement techniques are costly and out of reach of the poor farmers. Here a low cost device for measurement of soil moisture level has been proposed. The detailed research work is under progress and the observation from the preliminary work has been reported here. This circuit has the advantage of detecting soil moisture with better accuracy and linearity when compared with other works. Also cost of this proposed meter will be less.
2. Theoretical Background
Thermal conductivity of soil measures the heat flow through the particles that comprise the soil. It is regulated mainly by water content and there is much variation for different types of soil. In soil physics, the thermal properties reflect how heat is transferred through the soil by conduction, convection and radiation. Thermal conductivity is measured in watts (W) per Kelvin per meter.
2.1. Factors Influencing Thermal Conductivity in Soils
A variety of factors influence the heat flux through soil. The primary influences are the soil's moisture content and its dry density. Other parameters that have a secondary effect are soil type or mineral composition, dry density of the soil, temperature, texture and time. Soil is composed of weathered rock particles, water, and void spaces. The void spaces are very important in thermal conduction. They are not really empty like a vacuum but always contain either air or water. Air is a poor conductor of heat because the gas molecules are spaced far apart and do not physically contact one another. Water is an excellent thermal conductor because it's a liquid and there's more particle contact in the void spaces between soil particles and water. That heat energy is passed along through this medium in the voids until it contacts solid soil particles which absorb and move the thermal energy.
2.1.1. Soil Moisture and Dry Density
The consolidation of the soil determines its dry density. As soil becomes more compacted, the void spaces are reduced and air is squeezed out. With further consolidation, water flows into the smaller void spaces and the soil becomes more saturated as its density increases. As the soil moisture or the density of the soil increases, its thermal conductivity also increases. Saturation describes the amount of water present in the soil. An unsaturated soil still has air in the void spaces and its thermal conductivity is less than that of saturated soil. The dry density parameter refers to the mass of soil particles per unit volume and reflects a measure of the particle contact in the soil.
2.1.2. Moisture Content of the Soil
As the saturation levels of a soil increase, the thermal conductivity of the soil also increases. Moisture only partially coats the soil particles at low saturation levels. With more water, the void gaps between the particles begin filling and the thermal conductivity continues to rise accordingly. As the one hundred percent saturation level of the soil is approached, the voids fill completely and the heat flux or the thermal conductivity reaches its highest flow for that soil sample. A saturated soil has a thermal conductivity level near that of pure water.
2.1.3. Soil Types and Composition
There are several different soil types: gravels, sands, silts, clays, and soil containing organic materials or peat. Whether or not these soils are in a frozen or a thawed state will also affect their thermal conductivity. As the organic content of the soil increases, the thermal conductivity generally decreases because organics don't carry a heat flux very well. However, the peat is decomposing and giving off heat in that process so peat bogs can be very warm.
Even if a soil has high moisture content, it may not necessarily mean that the soil will warm up faster than a dry soil. Evaporation plays a role in removing much of the solar energy before the soil can warm up. So dry soils warm up faster under the sun but they cool more quickly at night. Soils with high moisture content evaporate the water and the soil doesn't warm as fast during the day but it cools more slowly at night because of the higher moisture content.
2.1.4. Other Factors
In agriculture, the soil's microclimate which nourishes seeds is determined through the thermal conductivity of the soil and its moisture content. This determines the early growth and development of a crop. The microclimate influences seed germination, seedling growth and establishment of the crop. Research has shown that increasing the percentage of organic content in the soil decreases the thermal conductivity. In hot areas, adding mulch or other organic components to the soil will help prevent seeds from baking under intense temperatures.
3. Experiment Set-Up
In a circular shape hollow pipe made of PVC material and tightly closed at one side a CL-100 transistor with emitter base shorted is connected as a source of heat and a temperature sensor (AD590) has been connected, 1.27 cm (arbitrarily selected) distance apart as a detector of heat as shown in Fig. 1, Fig. 2 and Fig. 3. Now hollow pipe has been filled up with sample (soil) for which moisture level has to measure. The output of the temperature sensor (AD620) that is output from the detector is then converted to voltage using a current to voltage (I-V) converter. The output from the I-V converter is send to an amplifier circuit for proper amplification and acquisition. The output of the amplifier circuit is calibrated in such a way that lmV is equal to 1°C. The experiment has been performed for several times and good results have been obtained in each time.
4. Experiment and Results
Several experiments has been performed on locally available soil (mainly clay type) and found a good results comparing with the slandered available method. The output current signal from the sensor is then converted into voltage signal using an I-V converter. The millivolt signal from the I-V converter is recorded. The output is calibrated in such a way that 1° C is equivalent to 1 mv.
The soil has been selected arbitrarily from the local area and experiment has been performed on this soil sample. By using the standard available method the soil moisture has been calculated on dry basis and after that soil has been tested by newly proposed soil moisture meter. The experimental data has been recorded for a fixed time for different soil sample in a regular time interval. Time verses output voltage (from the sensor) plot has been drawn for each and every soil sample and almost a linear curve has been observed. The experimental graph has been shown in Figs. 4-7. Finally percentage of soil moisture verses measured voltage graph has been drawn and a good linearity has been found as shown in Fig. 8.
The results obtained by the proposed measuring circuit has been presented, which demonstrate that the change in voltage from the sensor output is mostly linear with the moisture content. Output voltage is increasing with the increase of soil moisture. This is due to the fact that when moisture content is increasing in the soil the void space is filled up and thermal conductivity is increasing accordingly. The cost of the total system is very low, so that it can be easily affordable but the whole assembly has to be tested and proper calibration has to be done with more samples and more observation. Here we use only one particular soil sample. We have presented only very preliminary results; the research on this line is in progress.
The authors are thankful to Bidhan Chandra Krishi Viswa Vidyalaya and Murshidabad College of Engineering and Technology for providing the facilities to carry out this research work.
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1Subir DAS, 2Biplab BAG, 3T. S. SARKAR,
4Nisher AHMED, 5B. CHAKRABRTY
1,2,3 Department of Instrumentation Engineering, Murshidabad College of Engineering & Technology,
West Bengal, India
4 Al Habeeb College of Engineering & Technology, Hyderabad, India
5 Faculty of Agricultural Engineering, B.C.K.V., Mohanpur, West Bengal, India
E-mail: firstname.lastname@example.org email@example.com firstname.lastname@example.org
Received: 23 July 2011 /Accepted: 19 September 2011 /Published: 27 September 2011