Murata’s novel ground conductivity sensor now handles synthetic soils

Author: EIS Release Date: Nov 3, 2023

“In recent years, the use of synthetic culture soil such as rock wool and coconut peat is popular for crops including tomatoes and strawberries,” according to the company. “Since the water retention capability of these kinds of culture soil is extremely high compared to general culture soil, proper measurement of water content and electrical conductivity values was deemed difficult since it was difficult to have the sensor section contact with material.”

 

Murata soil sensor

To a first approximation, soil consists of mineral grains, water with dissolved salts, and air.

 

The sensor measures electrical conductivity, which is an indicator of ion concentration, using a nine-electrode array, and processing, developed by Murata to reject variations in contact quality.

 

 

Also measured are local permittivity, to determine water content (‘VWC’), and temperature – the latter to compensate the conductivity measurement as well as a soil indicator in its own right.

 

 

Electrical conductivity data is available across 0 – 5dS/m (±3% of full scale), volumetric water content from 0 to 96% for synthetic soil (0 to 60% in in normal soil ±3% full scale) and temperature from -20 to +60°C (±1°C ±1digit).

 

“A proprietary algorithm has been incorporated into the device which means that exact figures on fertiliser content can be obtained”, said Murata, which developed the sensor after farm trials including monitoring salinity in rice fields, fertiliser in fruit orchards and irrigation in peanut fields.

 

The housing is water and dust proof to IP68, and versions are available with a choice of UART, RS232E, RS485, SDI-12 (standard soil only) or Modbus-over-RS485 wired interfaces.

 

Murata soil sensor Terrace RightARM analysis softwareTerrace RightARM analysis software used with Murata soil sensors

 

Required sensor density varies with crop type, with higher value crops such as the grapes for wine-making justifying a greater sensor density, said Murata.

 

Data acquisition period can be adjusted. “For outdoor farming operations, updates every 30mins are likely to be adequate, but for indoor set-ups this might go down to as often as every five minutes,” said the company. Three AA batteries power the sensor for >6months with half-hour intervals.

 

Part Interface Maximum cable length

(probe comes with 3m) Number of sensors

on the same cable

SLT5005 RS232 10 1

SLT5006 UART 3 1

SLT5007 RS485 300 up to 32

SLT5008 SDI-12 (normal soil only) 60 up to 10

SLT5009 Modbus over RS485 300 up to 32

All run from 3 to 6V at ~30mA, except SDI-12 which needs 9.6 to 16V at ~8mA, and across -20 to +60°C.

 

Probe size is 132 x 27 x 16mm.

 

A software update is available to add rock wool and coconut peat soil electrical conductivity measurement to existing units.