IoT Soil Nutrient Monitor with NPK and pH Sensing

Farmers decide fertilizer doses mostly by experience or by sending soil samples to a lab and waiting weeks for results — by which time the crop stage has moved on. The gap is not precision agriculture in the abstract; it is a simple, repeatable way to read the soil's actual nutrient state in the field. This project builds that instrument: an NPK sensor probe (Modbus RS485) measures nitrogen, phosphorus and potassium in mg/kg, a pH probe reports soil acidity, and a moisture sensor adds the water context that nutrient readings need to be meaningful. An ESP32 collects the readings, displays them on an OLED for in-field use, and pushes them over Wi-Fi to a cloud dashboard where the season's trend is visible. The firmware converts raw NPK values into a simple fertility rating (low/medium/high) per nutrient so the readout is actionable, not just numbers.

1. The NPK, pH and moisture probes are inserted into the soil at the sampling point; the probes need a few minutes to stabilize in moist soil.
2. The ESP32 polls the NPK probe over Modbus RS485 for N, P and K in mg/kg, reads the pH probe through its conditioning board, and samples the capacitive moisture probe.
3. Firmware applies the stored calibration offsets, converts each nutrient to a low/medium/high fertility band, and renders all values on the OLED.
4. On each measurement cycle the ESP32 publishes the readings with a timestamp over Wi-Fi to the cloud dashboard.
5. The dashboard plots each parameter over time per probe location, so nutrient depletion or pH drift across the season is visible as a trend.
6. Before first use, the student runs the two-point pH calibration (buffer solutions) and records the offsets — the report documents the procedure and the student's own readings.

Tech stack:

• ESP32 development board (Wi-Fi)
• Arduino IDE (C/C++ firmware)
• Soil NPK sensor probe (Modbus RS485) + TTL-to-RS485 module
• Analog pH probe with conditioning board
• Capacitive soil-moisture sensor
• 0.96-inch OLED display (I2C)
• Cloud dashboard (ThingSpeak/Blynk class) over MQTT/HTTP
• 5 V USB/battery supply with weatherproof probe cabling

• [NPK sensing] A Modbus RS485 soil NPK probe reports nitrogen, phosphorus and potassium in mg/kg directly from the soil, read by the ESP32 over a TTL-to-RS485 module.
• [pH measurement] An analog pH probe with its conditioning board measures soil pH so nutrient readings are interpreted with acidity context (nutrient availability is pH-dependent).
• [Moisture context] A capacitive soil-moisture probe reports volumetric water content, since dry soil skews nutrient uptake and sensor readings.
• [Fertility rating] Firmware maps each nutrient to a low/medium/high band against standard agronomy reference ranges and shows the rating alongside raw values.
• [On-device OLED display] A 0.96-inch OLED shows NPK, pH and moisture live in the field without any phone or laptop.
• [Cloud dashboard] Readings are published over Wi-Fi (MQTT/HTTP) to a dashboard with per-probe history charts, so a season of measurements is visible in one place.
• [Calibration procedure] The kit includes a buyer-run two-point pH calibration and an NPK verification procedure with reference solutions, documented step by step.

• Working prototype: ESP32, NPK probe, pH probe, moisture sensor, OLED, RS485 module wired and tested
• Complete ESP32 firmware (Modbus polling, pH/moisture sampling, OLED UI, cloud publishing)
• Sensor calibration and verification procedure with reference values
• Cloud dashboard setup guide (channel creation, API keys, chart configuration)
• Wiring diagram and connection table
• Project report PDF (background, soil-nutrient science, methodology, working, results)
• PPT presentation for final review
• Viva Q&A preparation document (Modbus, pH sensing, NPK agronomy basics, IoT telemetry)

• The NPK probe reports indicative field values, not lab-grade analysis — the report states this honestly; lab testing remains the reference for precise dosing.
• pH accuracy of approximately ±0.2 depends on the buyer performing the two-point calibration; uncalibrated readings drift.
• Probes must be in moist soil to stabilize; readings in dry, loose soil are unreliable and the firmware flags low-moisture conditions.
• The dashboard needs Wi-Fi at the deployment point; readings are shown on the OLED regardless of connectivity.
• Nutrient bands (low/medium/high) are general agronomy reference ranges, not crop-specific recommendations — the report says so explicitly.
• Probes need periodic cleaning; salt crust on the NPK probe degrades readings over a season.