The soil fertility is detected using NPK sensors. A major component of soil fertilizer is nitrogen, phosphorus, and potassium. The knowledge of the soil nutrient concentration can help us to learn about nutritional deficiency or abundance in soils used to endorse plant production.

There are multiple methods of measuring the soil nutrient content using some optical sensors or using a spectrometer. But the spectral analysis method is not convenient and the drawback is the data are only 60-70% correct. While comparing the spectral analysis method with traditional wet chemistry methods, the accuracy of the products is yet to be fully resolved, given the paucity of data in that regard.

The Soil NPK sensor is a low-cost, quick responsive, high precision & portable Sensor that works with Modbus RS485. The advantage of this sensor over a traditional detection method is that it gives very fast measurement & data are highly accurate. All you need is to insert its probe into the soil and get the reading using Arduino. 

Specifications:

Basic parameters

Functions and Features

(1) The sensor compact design, easy to carry, installation, operation and maintenance.
(2) reasonable structure design, stainless steel probe life assurance.
(3) soil less affected areas are widely used.
(4) High measurement accuracy, reliable performance, ensure proper operation
(5) fast response, high data transmission efficiency.

Integration Guide with Arduino

This guide provides instructions on how to integrate the CWT Soil Sensor (NPK Type) with an Arduino microcontroller for measuring soil parameters like temperature, humidity, pH, conductivity, and NPK content.

1. Hardware Requirements

• CWT Soil Sensor (NPK Type)
• Arduino Board (e.g., Arduino UNO)
• RS485 Module for Arduino
• Connecting Wires
• Power Supply (5V-30V DC)

2. Wiring

Connect the sensor to the Arduino using the RS485 module as follows:

• Sensor Cable Color Description:

  • Brown: Power + (DC 5-30V)
  • Black: Power -
  • Yellow/Green: RS485 A+
  • Blue: RS485 B-

• Arduino to RS485 Module:

  • VCC: Connect to Arduino 5V
  • GND: Connect to Arduino GND
  • A+: Connect to Yellow/Green wire from the sensor
  • B-: Connect to Blue wire from the sensor

3. Software Requirements

• Arduino IDE
• Modbus Library for Arduino (for RS485 communication)

4. Arduino Code

Below is an example Arduino sketch that reads data from the CWT Soil Sensor using Modbus communication:

#include <ModbusMaster.h>

// instantiate ModbusMaster object
ModbusMaster node;

void setup() {
  Serial.begin(9600);       // Initialize serial communication at 9600 baud rate
  node.begin(1, Serial);    // Slave ID 1
}

void loop() {
  uint8_t result;
  uint16_t data[7];         // Array to hold the sensor data

  result = node.readHoldingRegisters(0x0000, 7);
  
  if (result == node.ku8MBSuccess) {
    data[0] = node.getResponseBuffer(0x00); // Humidity
    data[1] = node.getResponseBuffer(0x01); // Temperature
    data[2] = node.getResponseBuffer(0x02); // Conductivity
    data[3] = node.getResponseBuffer(0x03); // pH
    data[4] = node.getResponseBuffer(0x04); // Nitrogen
    data[5] = node.getResponseBuffer(0x05); // Phosphorus
    data[6] = node.getResponseBuffer(0x06); // Potassium

    Serial.print("Humidity: "); Serial.println(data[0] * 0.1);
    Serial.print("Temperature: "); Serial.println(data[1] * 0.1);
    Serial.print("Conductivity: "); Serial.println(data[2]);
    Serial.print("pH: "); Serial.println(data[3] * 0.1);
    Serial.print("Nitrogen: "); Serial.println(data[4]);
    Serial.print("Phosphorus: "); Serial.println(data[5]);
    Serial.print("Potassium: "); Serial.println(data[6]);
  } else {
    Serial.println("Failed to read from sensor!");
  }

  delay(1000); // Wait for a second before reading again
}

Explanation:

• node.begin(1, Serial): Initializes the Modbus communication with the sensor, where 1 is the default slave ID.
• readHoldingRegisters(0x0000, 7): Reads 7 registers starting from address 0x0000 corresponding to humidity, temperature, conductivity, pH, nitrogen, phosphorus, and potassium.

5. Calibration and Customization

If calibration is needed, the sensor allows writing calibration values and coefficients through specific registers:

• Temperature Calibration: Register 0x40081
• Humidity Calibration: Register 0x40082
• Conductivity Calibration: Register 0x40083
• pH Calibration: Register 0x40084
• NPK Calibration: Registers 0x41259 (Nitrogen), 0x41269 (Phosphorus), 0x41279 (Potassium)

Example: Writing a value of 32 into the Nitrogen register:

node.writeSingleRegister(0x40004, 32);

6. Testing and Validation

• Upload the code to the Arduino.
• Open the Serial Monitor to view the sensor readings.
• Adjust wiring or parameters if readings are not as expected.

7. Troubleshooting

• Ensure correct wiring, especially the RS485 connections.
• Verify that the sensor is powered correctly.
• Check the Modbus library documentation if communication issues arise.