Introduction
The series of stainless steel-pin soil probes for direct field sensing, include the following basic versions:
- temperature & soil-moisture [temp-sm],
- temp, sm & electric conductivity (ec), salinity, total dissolved solids & epsilon [temp-sm-ec],
- Nitrogen (N), Phosphorus (P) & Potasium (K) [NPK], and
- pH [pH]
The four basic versions are available as separate units and as various combinations. The latter also include a 5-pin unit with the capacity to sense all the listed soil properties (except epsilon). The figure below shows the xSpectrolum+ v080 spectroemter with three of the four basic version and the 5-pin version combining all at the bottom.
RS485 MODUS-RTU
To simplify, the ANSI EIA/TIA-485, or RS485, is a hardware standard multi-point (characteristics of drivers and receivers) communications systems. MODBUS is a software protocol that uses the RS485 connection for serial communication. Some modules and sensors that operate using RS485 + Modbus are possible to use with Arduino. Among these are the SEEED technology Soil Moisture & Temperature & EC Sensor MODBUS-RTU RS485 (S-Soil MTEC-02A). I got the SEEED technology MODBUS RS485 sensor to operate using xSpectre’s version 0.78 spectrometer. Once that worked, I just needed to get the MODBUS-RTU communication parameters correct for also operate all other versions of the steel-pin soil field sensors.
temp-sm and temp-sm-ec
The most common os the soil steel-pin probes are the temperature and soil-moisture (temp-sm) and temperature, soil-moisture, electric conductivity, salinity, total dissolved solids and epsilon (temp-sm-ec) probes. These sensors were introduced as 5 volt models by SEEED studios.
Today, identical steel-pin sensors to the SEED studio version are offered on AliExpress.
NPK
Searching for 5 volt versions of the NPK steel-pin RS485 sensor revealed only a few options, all from China or India. Updating in January 2023 and then again in April 2023, a range of options for purchasing steel-pin NPK soil sensors with a watertight body and the RS485/MODBUS-RTU setup has become available.
While the wiring is straight forward and the MODBUS addresses given, I could not understand why I could not acceesing the sensor from Arduino. It turned out that buadrate for the NPK sensor must be set at 4800 (not the default 9600). Once I found this, the two RS485 NPK sensors I had ordered via AliExpress operated as expected.
I have not got the NPK sensor from the AliExpress vendor ComWinTop. But other steel-pin RS485 sensors from this vendor has worked as expected and ComWinTop maintaines a complete set of versions of the steel-pin RS495 soil probes at favourable prizes.
