DIY Soil Ph Meter using Soil Ph Sensor & Arduino

Overview

In this project, we will make our own DIY Soil Ph Meter using Soil Ph Sensor & Arduino for the measurement of Soil Ph. Soils can be naturally acidic or alkaline & can be measured by testing their pH value. Soil pH is a measure of the acidity or alkalinity of the soil.

Soil is considered a natural medium for plant growth & development. Much research is going on to determine the internal factors of farmers’ crop production failure. One of the factors is the balance of nature in unstable or unfertile soil that inhibits plant growth and plant root development. The Ph is the acidity or basicity of material measured on a scale between 0 to 14. The Ph value lesser than 7 is considered acidic and greater than 7 is considered basic. If the pH scale is 7 then the material is neutral. The most ideal soil conditions for the growth & development of plants are neutral soil. However, some types of plants are still tolerant of soils with slightly acidic Ph with a maximum pH of 5.

Apart from Soil Ph, the Soil Nitrogen, Phosphorous, Potassium, EC & Salinity content is also useful for plant growth. For the measurement of NPK Content, we can use NPK Sensor. For Soil EC & Salinity measurement, we can use Soil EC Sensor But Soil Ph is completey different factor compared to Soil NPK. We have already discussed about Water Ph Sensor in one of our previous post, but Water Ph Sensor cannot be used to measure the Soil Ph due to construction and limitations factor. This is why we need an special Soil Ph Sensor to measure the Ph of a Soil.

While browsing through the internet, I found a great Ph Sensor manufactured by some Chinese R&D Manufacturer. The sensor is manufacture by a company called HONDETEC and you can check it on Alibaba. The sensor works perfectly with Modbus RS485 and the result is highly impressive. So, in this post we will learn about Soil Ph Sensor & Arduino Interfacing and design our own Arduino Soil Ph Meter. We will display the Soil Ph value on a 0.96″ OLED Display.

Bill of Materials

The bill of materials for making your own Soil Ph Meter is given below. All the components can be easily purchased from Amazon except the Soil Ph measurement sensor. You can get it from Amazon as well but may not be available every time.

Soil Ph Measurement & its important

What is Soil Ph?

Soil pH is a measure of the acidity or alkalinity of the soil. A pH value is actually a measure of hydrogen ion concentration. Because hydrogen ion concentration varies over a wide range, a logarithmic scale (pH) is used

Most soils have pH values between 3.5 and 10. In higher rainfall areas the natural pH of soils typically ranges from 5 to 7, while in drier areas the range is 6.5 to 9. Soils can be classified according to their pH value:
1. 6.5 to 7.5—neutral
2. over 7.5—alkaline
3. less than 6.5—acidic, and soils with pH less than 5.5 are considered strongly acidic

Origins

Natural soil pH depends on the rock from which the soil was formed and the weathering processes that acted on it, i.e. climate, vegetation, topography & time. These processes tend to cause a lowering of pH (increase in acidity) over time.

Rain is also considered as one of the significant factors for the increase in acidity of the soil. Some fertilizers can change soil pH and increase or reduce the number of nutrients available to plants.

Effects

Soil pH affects the amount of nutrients and chemicals that are soluble in soil water. Some nutrients are more available under acid conditions while others are more available under alkaline conditions. However, most mineral nutrients are readily available to plants when soil pH is near neutral.

The development of strongly acidic soils can result in poor plant growth as a result of Aluminum & Manganese toxicity or calcium & magnesium deficiency. Alkaline soils may have problems with deficiencies of nutrients such as zinc, copper, boron & manganese.

Soil Ph Sensor

This is a waterproof and dustproof Soil Ph Sensor that can measure the Soil Ph value from 3 to 9 with high accuracy up to ±0.3PH. The sensor has an IP68 protective case & is sealed with High-density epoxy resin which can prevent moisture from entering the body interior part. The sensor is suitable for Suitable for agricultural cultivation, industrial production, environmental monitoring, animal husbandry, and sewage treatment.

Specifications

1.Probe type: Probe electrode
2. Measuring range: 3 ~ 9 PH
3. Measurement accuracy: ±0.3PH
4. Resolution: 0.1 PH
5. Outputsignal: RS485/0-5V / 0-10V / 4-20mA output
6. Supply voltage: 5V~30VDC
7. Working temperature range: -30 ° C ~ 70 ° C
8. Stabilization time: 5-10 Minutes after power on
9. Response Speed: ≤15S
10. Standard 2 meters Cable
11. Long-term Stability: ≤5%/y

Pinout

The Soil Ph Sensor has 4 pins as it need to be connected to RS485 or MAX485 Module. The four colored wires are Yellow, Blue, Black & Brown.

Communication Protocol & Register Address

The communication protocol that it uses is the Modbus. Thee device works as a Slave with the device address as shown below. You can send this instruction or Inquiry frame & Read soil PH at device address 0x01.

You will get the Response frame as follows. From the response frame, you can calculate the Ph Value.

From the received response, you can calculate the Ph Value. The 4th bit (starting from the 0th bit) is the value of Ph. For example, we got 0047H (hexadecimal) = 71 Decimal => pH = 7.1pH

To learn more about the Modbus RTU Communication protocol, you may follow the following tutorials:

• Modbus RTU Communication with Arduino
• Modbus RTU Communication with ESP32
• Modbus RTU Communication with Raspberry Pi Pico

Interfacing Soil Ph Sensor with Arduino & OLED Display

Now, let us interface the Soil Ph Sensor with the Arduino Nano Board using the MAX485 Modbus Module. The connection diagram is given below.

The R0 & DI pin of from the Modbus is connected to D2 & D3 of Arduino using Software Serial. Similarly, we have to enable DE & RE high which is done by connecting them to the D7 & D8 pin of Arduino. The NPK Sensor has 4 wires. The brown one is VCC which needs a 5V-30V Power Supply & can be connected to 5V of Arduino. The GND pin which is black in color needs to be connected to the GND of Arduino. The Blue wire which is the B pin is connected to the B pin of MAX485 & the Yellow Wire which is the A pin is connected to the A pin of MAX485.

The 0.96″ SSD1306 OLED Display is an I2C Module. The OLED Display VCC & GND are connected to 3.3V & GND of Arduino. Similarly, its SDA & SCL pins are connected to the A4 & A5 of Arduino. You can follow the circuit diagram & assemble the circuit on a breadboard or make a custom design PCB.

Source Code/Program

The source code for interfacing Soil Ph Sensor with Arduino & retrieving Soil Ph value from the Sensor via Modbus command is given below. You can send the command and retrieve the value in HEX Code. The HEX code needs to be converted into Decimal to get the Measured Soil Ph content data.

Since we are using OLED Display to display, you will need OLED Library. Download the following OLED Library and add it to the Arduino IDE.
1. Adafruit SSD1306 Library: Download
2. Adafruit GFX Library: Download

Here is the complete source code. Compile & upload it to the Arduino Nano Board.

Testing & Monitoring Soil Ph Data on OLED Display

Once the code is uploaded, the OLED will initialize along with the sensor. The sensor will take some time to get stabilized and the reading may be incorrect for a few seconds initially.

Once the sensor gets stabilized, you are ready to go. The sensor when exposed to air gives 7.0 as a Ph which can be observed on an OLED Display.

So this is how you interface soil ph measurement sensor with Arduino & get the Ph Reading. Similarly, put the sensor in different samples of soil. You will see a variation in the Ph Value depending upon the type of soil. The value increased or decreases like an Analog Soil Ph Sensor and can also be used as IoT Soil Ph Sensor.

Video Tutorial & Guide

Follow the following video to learn about the entire project & code information.