WSN Sensors for Precision Agriculture

Ravi Kishore Kodali, Nisheeth Rawat and Lakshmi BoppanaDepartment Electronics and Communication Engineering

National Institute of Technology, WarangalAndhra Pradesh, INDIA

Abstract—The application of technology in the field of agri-culture has increased the effectiveness and efficiency of thefarmers. The application of Wireless Sensor Network (WSN) inprecision agriculture assists the farmers to know about their fieldsin statistical manner, which helps them in making better andaccurate decisions. There are various type of sensors that canbe used to calculate the statistical parameters of an agriculturalfields, which convert the event or a phenomenon into an electricalor measurable quantity. This paper provides an elaboration ofthe basic principles of some of the sensors and their relatedspecifications of few commercial products.Keywords:- WSN, Sensors, Precision agriculture

I. INTRODUCTION

A Wireless sensor network (WSN) comprises of manysensor nodes with radio communication capabilities. A WSNnode consists of sensors, analog to digital converter (ADC),micro-controller, radio frequency electronics, dc to dc powerconverter and a tiny power source. A node can support onesensor or more using which various physical phenomenarelated parameters can be converted into electrical signals.These signals are digitized and the digital data can be for-warded towards the base station (BS). The WSNs have variousapplications including agriculture. In agricultural applications,it is necessary to measure and monitor various soil relatedparameters such as soil moisture content, soil water content,soil temperature, soil electrical conductivity, salinity level ofthe soil and climatic condition parameters, which include solarradiation level, ambient temperature, wind speed, rainfall. Bymeasuring soil electrical conductivity, soil moisture contentand soil water content, it can be ensured that optimum amountof water is used for irrigation purposes.Soil characteristics can be identified based on the soil electricalconductivity values. The soil moisture content provides thewater tension level within the soil which dictates the amount ofeffort needed by the plant root system to absorb water from thesoil. The soil water content provides an approximate estimateof the water to be applied to the agriculture field. The soiltemperature over a time duration assists in determining thecrop type suitability. The salinity level of the soil determinesthe type and the quantity of the fertilizer to be applied tothe field. Precision agriculture is a farming practice whichintegrates the technology and agronomy principles to manageassociated spatio-temporal variabilities with the purpose ofcrop performance improvement. The alerts generated by theWSN can be forwarded in the form of messages to the farmersusing the Internet and appropriate action such as applicationof water to the fields can be initiated by the farmers.

II. RELATED WORK

Three sensors namely, a pH sensor, an electrical conduc-tivity (EC) sensor and a temperature sensor are integratedtogether on a single Si chip with an area of 25 mm2. Thesensor chip is deployed in the soil and the EC readings aretaken for 6 days continuously [2]. An attempt using the sensorsin agriculture and forestry has been made to improve yield andreduce pollution levels [3]. An autonomous vehicular systemdesign with global positioning system (GPS), has been given[4]. The measured values can be mapped along with spatiallocations. In this, the soil samples are collected at variouslocations of the field and soil characteristics are computed andsample locations are also stored. An opto-electronic sensorfor weed detection has been used [5]. In this instrument,optoelectronic sensors are used to detect weeds against thesoil background. The system is evaluated in three maizefields in the early spring. Various digital images are collectedthroughout the field and the weed quantity is estimated basedon these using image processing software and the same hasalso been compared with the ground truth.

III. SENSORS FOR AGRICULTURE

A. Soil water content sensor

Soil water content is the ratio of the amount of waterpresent in the test soil to the total amount of test soil. Thewater content is determined by a change in the capacitancevalue and this depends on the dielectric constant of the soil.The measurement methods are soil type dependent and hencethe sensors need to be calibrated for each soil type.

1) Basic principle to calculate soil water content: Thedielectric permittivity of the soil is calculated by using a probe,which is deployed at a depth of 1

′

to 3′

. The dielectric constantof water is higher than those of the other soil constituents, forexample, dielectric constant of water = 80, dielectric constantof organic matter = 4, dielectric constant of vacuum and air =1. Thus, any change in the water content of the soil can havea significant change in the capacitance of the soil. The sensorutilizes high frequency oscillations to measure the soil watercontent in different soils.

2) Mathematics involved: The capacitance is measured infarads given by equation (1).

C = εrε0A

d, (1)

where

C - capacitance,A - overlapping area of the plates,

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εr - the dielectric constant of the material inside theplates,ε0 - the dielectric constant,d - distance between the plates

The potential developed between the plates depends on thecapacitance and is given by equation (2).

C =q

V, (2)

where q is the charge on the plates and V is the voltagebetween the plates.

3) Deployment of water content sensor- eS1110: TwoeS1110 sensors are deployed at different depths: one is in-stalled near the upper fibrous root zone and another at thelower part of the root zone. The output of the sensor near theupper root zone provides information when to apply water tothe plants, whereas the sensor deployed at lower root zonedecides when to stop irrigation.

Fig. 1. Soil Water Content Sensor

4) Specifications of water content sensor- eS1110 : Thesensor, eS1110 manufactured by Decagon EC-5 as shown inFigure 1 has been used and its specifications are provided inTable I.

TABLE I. SPECIFICATIONS OF SOIL WATER CONTENT SENSOR

Sensor model eS1110Sensor Manufacturer Decagon EC-5Range 0-0.3 m3 / m3 water contentAccuracy 0.03 m3 /m3 all soil up to 8 dS/mCable Length 16ftConnector 6 pin SwitchcraftSize 3.5” × 0.7” × 0.275”Weight 0.3 lbs

B. Sensor for Soil Moisture Content

1) Introduction: This sensor measures the soil water ten-sion or suction, which is a direct indication of the quantum ofeffort to be put in by the plant root system while extractingwater from the soil. The drier the soil, the more effort isrequired to extract water from the soil.

2) Basic principle to calculate Soil moisture content: Inthis sensor, two electrodes are mounted into a block of gypsumand the resistance between these electrodes is measured. Thegypsum block is used to measure soil moisture content. Thesensor is inserted into the soil whose moisture content needsto be measured. When the soil is dry, the water present withinthe gypsum block is less and the resulting resistance betweenthe electrodes is high and vice versa. The soil moisturecontent measured in centi-bars is directly proportional to theresistance. While calibrating the sensor, the calibration curvesof commercial gypsum blocks are used.

3) Mathematics Involved: The sensor converts electricalresistance between the electrodes in the gypsum block to acalibrated reading of soil moisture content. The resistance andtemperature maintain a linear relationship up to a specific limit[6]. The measured resistance is normalized to 0C by usingequation (3).

R21 =Rs

1− (0.018× dT ), (3)

where

R21 - Resistance at 210C,Rs - measured resistance,Ts - Soil Temperature,dT = (Ts − 21)

The soil moisture content is then calculated using equation(4).

Soil moisture content = (0.07407×R21)− 0.03704 (4)

4) Deployment of moisture content sensor- es1100: Thesensor as shown in Figure 3 is normally deployed at a depthof 3

′

to 4′

nearer to the plant root system.

Fig. 2. Soil Moisture Sensor [7]

TABLE II. MOISTURE CONTENT SENSOR

Sensor model eS1101Sensor Manufacturer WatermarkRange Water Potential 0-200 cbarAccuracy +/- 5%Cable Length 16ftConnector 6 pin SwitchcraftSize 3.25” × 3/4” (diameter)Weight 0.6 lbs

5) Specifications of Moisture content sensor- es1100: ThiseS1100 soil moisture sensor from Watermark is used to carryout the measurements of moisture content of the soil. Table-II provides the specifications of the sensor. The sensor isconnected to an eKo node. The eKo node as shown in Figure3, has 4 ports and up to four sensors can be connected.

C. Soil Electrical Conductivity Sensor

The electrical conductivity (EC) of soil helps to differenti-ate and map various soil variables. It is used to evaluate the soilsolute concentration while assessing the soil salinity hazard.By measuring EC, many physical properties of the soil can bededuced. Traditionally, the salt content of a soil is measuredby making use of soil electrical conductivity sensor. Table -IIIprovides typical EC values for various soil types and water.

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Fig. 3. eKo node EN2100 and gateway [7]

TABLE III. TYPICAL RANGES OF ELECTRICAL CONDUCTIVITY

Material EC (mS/m)Irrigation water (0.7% salt ) <1500Potable water (0.25% salt ) <550Clay 10-1000Sand 0.01-1Loam 20-200Topsoil 5-20Sandstone 0.1-50Limestone 0.4-20Crystalline rocks <0.001-1

1) Basic principle to calculate soil electrical conductivity:It uses the Faraday’s law, which states that the rate of changeof electro magnetic flux(EMF) linkage with respect to time isdirectly proportional to the induced EMF in a conductor orcoil. EC is expressed in units of milli- Siemens/meter. Thecontact method is used to measure soil EC in a field. In thismethod, the electrodes make contact with the soil as shownin Figure 4. In this, three pairs of electrodes are used. Onepair of electrodes applies electrical current over the soil, whilethe remaining two pairs of electrodes are used to measurethe voltage drop between them [8]. The location information,latitude and longitude along with the EC values are stored ina database [9].

Fig. 4. Principle of operation for the contact type EC sensor

2) Deployment of the Sensor: The small size of the sensormakes it easy to install in fields. This robust probe should be

pushed directly into undisturbed soil to ensure good accuracy.The following soil properties affect the EC property of soil:

1. Porosity: The EC value increases with the increase in claycontent (high porosity) and decreases with the increase insandier soil (low porosity). Compactness increases soil EC.

2. Water Content: EC value increases with the increase in thewater content of the soil.

3. Salinity Level: Increased concentration of electrolyte or saltin soil water increases the EC value.

4. Temperature: At the freezing point, the EC value decreasesslightly but below the freezing point the pores becomehighly insulated and the EC value decreases rapidly.

5. Cation Exchange Centre: Organic matter and clay mineralssuch as illite, vermiculite have high ability to retain posi-tively charged ions (such as Ca, Mg, K, Na). The presenceof these positively charged ions enhances soil EC.

Fig. 5. Soil Electrical Conductivity Sensor

3) Specifications of the available Sensor- 40557: Thesensor as shown in Figure 5 uses a 70 MHz frequency,which minimizes salinity and textural effects, making thesensor accurate in most soils [10] and Table -IV provides thespecifications of the EC sensor.

TABLE IV. SPECIFICATION FOR ELECTRICAL CONDUCTIVITY SENSOR

Sensor model 40557Sensor Manufacturer DecagonAccuracy 10% from 0 to 7 dS/m,

calibration needed above 7 dS/mResolution 0.01 dS/m from 0 to 7 dS/m,

0.05 dS/m from 7 to 23.1 dS/mRange 0 - 23 dS/m (bulk)Operating Environment −40

oC to 50oC

Cable Length 5 m standardSensor Dimensions 10 cm × 3.2 cm × 0.7 cm

D. pH Sensor

The pH values range from 0 to 14. The solutions havingpH values less than 7 are considered acidic and the solutionshaving pH value greater than 7 are considered basic. Inagriculture, soil pH values outside the range of 5.5 to 6.5 areconsidered as non optimum as it reflects lack of availabilityin soil nutrients. The pH value is regulated in agriculture bythe application of alkaline or acidic fertilizers, which in turnimproves crop production. The pH value of soil varies withinthe field, thus it is profitable to apply the fertilizer accordingto the spatial variation of soil pH.

1) Basic Principle to calculate pH: A pH sensor hasthree major components: a measuring electrode, a referenceelectrode and battery. The positive and negative terminalsof the battery are connected to the measuring electrode andthe reference electrode respectively. The measuring electrodedevelops a potential, which is directly related to the hydrogenion concentration of the solution. The reference electrode

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provides a constant potential against which the measuringelectrode can be compared. When immersed in a solution,the reference electrode potential does not change with thechanging hydrogen ion concentration [11]. At a pH of 7, theelectrodes produce 0 volt between them. At a pH value lessthan 7, a voltage of one polarity is developed, and at a pHvalue greater than 7 a voltage of the opposite polarity can beseen. One of the electrodes is made up of special glass, whichis doped with Lithium ions to create the ion-selective barrier,which screens out the hydrogen ions from all the other ions inthe solution. Glass is an insulator, so the circuit path from oneelectrode contact through the glass barrier, the solution to theother electrode and back to the other electrode’s contact is ofvery high resistance. The second electrode is made up of thechemical solution of neutral pH, which allows the exchangeof ions with the solution through a porous separator forminga relatively low resistance connection to the test liquid. Thewet chemical interface with the measure solution is necessaryto avoid creating a very large voltage, which may be falselyinterpreted by any measuring device being indicative of thepH [12].

2) Mathematics involved: pH value is the negative loga-rithm of the molar concentration of hydronium ions aH+ inthe solution and is given by equation (5).

pH = − log(aH+) (5)

In general, the potentiometric determination of the pHvalue is defined by equation (6).

pH(X) = pH(S) +E(S)− E(X)

R× T/F log10, (6)

where

R - the gas constant,T - the thermodynamic temperatureF - the Faraday constantpH(S) - known pH value of the standard solutionpH(X) - unknown pH value of the solutionE(S) and E(X) describe the electrode potential of a pH cellcontaining a standard solution with known pH value, pH(S)and pH(X) a solution with unknown pH value

The measuring principle is based on the electrochemical reac-tion that takes place on the surface of the sensing electrodeplunged into the solution of unknown pH. This reactiongenerates a potential at the sensor electrode, E(X), whichis proportional to the logarithm of the hydrogen ion (H+)activity in the unknown solution. While the potential E(S) atthe reference electrode is held constant, the pH value is derivedfrom the difference [E(S)− E(X)].

3) Deployment of pH sensor: The row cleaners clear thecrop residue after which the loose soil is compacted by usinga firming wheel. A hydraulic cylinder lowers the cutting shoeusing which a soil sample is collected into the sampling trough.The hydraulic cylinder raises the trough with the soil sampleagainst two pH electrodes. During each cycle the cutting shoeis cleaned by a scraper and pH electrodes are washed usingtwo 150 psi nozzles and the pH data is recorded.

4) Specifications of the Sensor: The soil pH value of anagricultural field may vary from place to place. Table -Vprovides specifications of a pH sensor.

TABLE V. PH SENSOR

Sensor pH SensorSensor Manufacturer Veris TechnologiesMaximum sampling speed 5 to 10 samples per acreLifetime several thousand measurementsOperating Environment −40

oC to 50oC

E. WeedSeeker sensor

A WeedSeeker sensor consists of an active light sourceand a chlorophyll identifying selective spray sensor. It detectsweeds and sprays herbicide over them without spraying overthe bare ground. Thus very less chemical can be used reducingapplication cost.

1) Basic principle to detect weeds: The sensor emits afocused beam of light on the ground and the circuitry detectsthe weeds and the valve cartilage sprays a burst of herbicideson the weeds. Figure 6 shows the weedseeker sensor.

Fig. 6. Weedseeker Sensor

2) Deployment of the Weedseeker sensor: A controller isconnected to the weedseeker and the controller can supportup to 240 such sensors. Every substance has a unique spec-tral reflectance signature. The sensor is designed to do thecomparison between soils and plants based on the reflectedlight from them. The presence of chlorophyll in plants resultsin a distinct reflectance signature at particular wavelengths ascompared to soils. The sensor compares the reflected light attwo different wavelengths. When a weed is noticed withinthe field of view of the sensor, the circuitry recognizes thechlorophyll signature and activates the extremely fast solenoidvalve in the cartridge and sprays the herbicide onto the weed.When a plant is present, the sensor circuitry activates the valvecartridge or external valve, which dispenses the chemical [13],[14]. Table -VI provides specifications of Weedseeker sensor.

TABLE VI. WEEDSEEKER SENSOR SPECIFICATIONS

Voltage 12 V D.C.Current 160 mA nominal

240mA when valve cartridge is onWeight 0.90 KgEmission wavelengths (near infrared) 770nmEmission wavelengths (Red) 656 nmSpeed range of spraying 3-10 mile per hourField Of View 12” nominal × 0.5”Mounting Height 18” × 30”

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F. Temperature sensor

Temperature determines the hotness or coldness of anybody. In agriculture the soil temperature determines the typeof crop that can be grown in an agricultural field. It alsoprovides alerts if the temperature goes above or below a certainthreshold.

1) Basic principle to calculate Temperature: The Tem-perature sensor is a p-n junction diode which is fabricatedtogether with ISFET using CMOS technology. SiOx and SiNare used to isolate the temperature sensor. The sensitivity of thetemperature sensor is of the order of 1 mV/oC. The forwardcurrent of the p-n junction diode is given by equation (7).

If = expqVf

nkT, (7)

where

If - Forward currentk - Boltzman’s constant,T - Absolute temperature,q - Magnitude of electronic charge,n - Value between 1 and 2

When a constant current source is provided then the outputvoltage varies linearly with temperature.

2) Mathematics involved: By rearranging equation (7) weget the temperature as given by equation (8).

T = q ×Vf

nk × log If(8)

In order to measure temperature a constant current sourceis needed and the temperature can be computed based on themeasured output voltage [2].

3) Deployment and specifications of the Temperature sen-sor: This sensor is normally deployed along with the soilmoisture content sensor. Table -VII provides the specificationsof a temperature sensor es-1101. It can be connected to thenode to measure the temperature.

TABLE VII. SOIL TEMPERATURE SENSOR- ES1101

Sensor model eS1101Sensor Manufacturer WatermarkRange Temperature: −40

oC to +70oC

Accuracy +/- 5%Cable Length 16ftConnector 6 pin SwitchcraftSize 1” × 1/4” (diameter)Weight 0.6 lbs

G. Wind speed Sensor

The Wind speed sensor determines the speed of the surfacewind, which is practically considered as a two dimensionalvector. Wind blowing over the earths surface is turbulent andis characterized by random fluctuations of speed and direction.Wind speed is classified into instantaneous and average types.The average wind speed is the average of the instantaneouswind speeds over a ten-minute duration. However, wind speedsfluctuate continuously, and measured values of instantaneous

wind speeds are effected by anemometer response characteris-tics. The wind speed sensor measures the value of wind speedand transmits the value in terms of an electrical parameter.The sensor given in Figure 7 is a cup anemometer. The cupassembly comprises of three cups made up of polypropylene.The stainless steel shaft is guided by precision oil bearings,lubricated by a special oil with negligible change in viscosityover a wide temperature range.

1) Basic principle to calculate Wind speed: This sensormakes use of the Hall effect. The Hall-sensor induces elec-tronic pulses while it is made to rotate by the wind. Thenumber of electronic pulses generated is directly proportionalto the wind speed. The subsequent electronic circuitry convertsthis signal to a digital output. The output frequency range of0 to 238 Hz corresponds to a measuring wind speed range of0 to 50 m/s.

2) Deployment of the Sensor: The wind speed sensor hasto be mounted at a suitable height. In any case it has to betaken care to avoid zones of turbulences.

Fig. 7. Wind Speed Sensor [15]

The sensor used is water and dust proof and is mountedalong with the cup assembly and the specifications of the Windspeed sensor are provided in Table -VIII.

TABLE VIII. WIND SPEED SENSOR

Sensors Wind Direction Wind SpeedResolution 1% 0.1 m/s to 1 km/hrRange 0

o to 360o 1 to 67 m/s

3 to 241 km/hrAccuracy 7 % 5 %

IV. NETWORK SET UP

A wireless monitoring system is created which consistsof eko view motes having ports at the bottom through whichlow power sensors are attached and are deployed randomly inthe agricultural field. There are 4 sensor ports at the bottomof the mote. The maximum allowable communication rangeof a mote is 500 feet to 1500 feet at line of sight. Varioustypes of sensors are used like soil moisture content sensor,soil temperature sensor, soil water content sensors, electricalconductivity sensor etc. The motes are connected to each otherthrough the wireless channel and are connected to the basestation through a wireless medium. The base station collectsthe data and sends the information to the gateway. A largearea can be covered with the help of these motes by placing

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them in some topology. The X-Mesh Routing protocol [1] isused to send the information from a mote hopping throughanother neighbouring motes to the base station and finally tothe gateway. The gateway has an installed software, named ekoview which records the data and shows the results in the formof graphs (point graph, line graph, weighted graph etc). Thereadings are taken every minute immediately after resettingthe mote. About after an hour the readings are sent at a fixedinterval of 15 minutes. Through the eko view software we canplot the readings within the range of hour, day, week, monthand year.

V. RESULTS

Soil Moisture ContentFigure 8 shows a plot of the soil moisture content sensorreadings over a 10- day period during rainy season. The peaksindicate the soil being dry.

Fig. 8. Soil moisture content

Soil water contentFigure 9 provides a plot of the soil water content readingsgathered by the node over a 2- day period. It can be observedthat the peaks can be noticed when sufficient water has beenapplied over the 2’ pit in which the water content sensor hasbeen installed. gradual reduction in soil water content readingswhich finally comes to a constant value.

Fig. 9. Soil water Content

Ambient temperatureFigure 10 provides a plot of the ambient temperature readingsgathered by the node which have been collected over a 2- dayperiod.

VI. CONCLUSION

There exist a wide range of sensors that are useful in thefield of agriculture and with the introduction of the Wirelesssensor network we can accumulate the data in the gatewayand can process it to give some meaningful results. If required

Fig. 10. Ambient Temperature

we can set some alerts to indicate that the measured variablehas crossed the threshold and can send the alert in the formof messages or emails. The introduction of such technologyincreases the productivity of the field and reduce the inputsrequired for agriculture.

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