For using the Soil/Leaf Interface its recommended installing the P82B715 chip on the TX board.
Its recommended to connect the Soil/Leaf Interface to the LSCL and LSDA lines on the TX board (active only when the P82B715 chip is installed), however it will also work if connected to the regular SCL and SDA lines, but in this case the cables should be very short.
If the cables between the your Soil/Leaf Interface and the TX unit don't exceed seven to eight meters you can safely left R1 and R2 empty, and just install R9 and R10 on the TX board (v3.12), 4.7K is a good value for cables up to eight meters.

Example: If you need a 20 meters cable, use UTP (STP, FTP, SFTP) Cat5E cable, and for the resistors, use the values shown in the picture bellow (documentation - Fig 9.), i.e. 4.7 kΩ for R8 and R9 on the TX board, and 470 Ω for R1 and R2 on the Soil / Leaf interface.

Despite the Soil / Leaf interface can be installed on the same box than an existing or a dedicated Transmitter unit, it makes more sense placing it in a separated small waterproof box (type junction box) near the point where the sensors will be installed than have it the TX box. Why? Simple because it's easier to use a single UTP cable from the Transmitter unit to the Soil / Leaf interface, than have all the cables from the sensors running up to the Transmitter box.

The Soil / Leaf interface and all the sensors connected to it are powered from the TX unit via the 4 wire connection.
Interface supports 3.3V and 5V analogue sensors. Power voltage for each one is selected by a jumper on the interface.

The VH400 sensor doesn't require any calibration, however for the Decagon LWS (Leaf Weatness sensor) you will need to determine the minimum and maximum output voltage at dry and wet condition (fully immersed in water). Those values should then be placed in user configuration options of the receiver software. See example bellow:

// Max and Min output voltage of the analog sensors connected to Soil / Leaf Interface
// Default value for Decagon Devices LWS Leaf Wetness Sensor (3.3V power): Max. Output Voltage - 1395mV (max wetness), Min Output Voltage - 530mV (dry)

//                                              ADC1  ADC2  ADC3  ADC4
const uint16_t AnalogSensor_MaxVoltOut[4]   = { 3000, 1395, 5000, 5000 };    // Values in miliVolts
const uint16_t AnalogSensor_MinVoltOut[4]   = {    0,  530,    0,    0 };

• Up to four analogue Soil Moisture / Leaf Wetness sensors.
  

Ex: 2 Soil Moisture and 2 Leaf Wetness, or 3 Soil Moisture and 1 Leaf Wetness. Usage definition is done in the software.

Soil Moisture: Vegetronix VH400

Leaf Wetness: Decagon Leaf Sensor (current name is METER PHYTOS 31)

• Up to four 1-Wire Dallas temperature sensors (DS18B20 or DS18S20).
  

Ex: Four sensors for Soil Temperature or three for Soil and one for Leaf Temperatures. Usage definition is done in the software.

For using the Soil/Leaf Interface its recommended installing the P82B715 chip on the TX board.
Its recommended to connect the Soil/Leaf Interface to the LSCL and LSDA lines on the TX board (active only when the P82B715 chip is installed), however it will also work if connected to the regular SCL and SDA lines, but in this case the cables should be very short.
If the cables between the your Soil/Leaf Interface and the TX unit don't exceed seven to eight meters you can safely left R1 and R2 empty, and just install R9 and R10 on the TX board (v3.12), 4.7K is a good value for cables up to eight meters.

Example: If you need a 20 meters cable, use UTP (STP, FTP, SFTP) Cat5E cable, and for the resistors, use the values shown in the picture bellow (documentation - Fig 9.), i.e. 4.7 kΩ for R8 and R9 on the TX board, and 470 Ω for R1 and R2 on the Soil / Leaf interface.

Despite the Soil / Leaf interface can be installed on the same box than an existing or a dedicated Transmitter unit, it makes more sense placing it in a separated small waterproof box (type junction box) near the point where the sensors will be installed than have it the TX box. Why? Simple because it's easier to use a single UTP cable from the Transmitter unit to the Soil / Leaf interface, than have all the cables from the sensors running up to the Transmitter box.

The Soil / Leaf interface and all the sensors connected to it are powered from the TX unit via the 4 wire connection.
Interface supports 3.3V and 5V analogue sensors. Power voltage for each one is selected by a jumper on the interface.

The VH400 sensor doesn't require any calibration, however for the Decagon LWS (Leaf Weatness sensor) you will need to determine the minimum and maximum output voltage at dry and wet condition (fully immersed in water). Those values should then be placed in user configuration options of the receiver software. See example bellow:

// Max and Min output voltage of the analog sensors connected to Soil / Leaf Interface
// Default value for Decagon Devices LWS Leaf Wetness Sensor (3.3V power): Max. Output Voltage - 1395mV (max wetness), Min Output Voltage - 530mV (dry)

//                                              ADC1  ADC2  ADC3  ADC4  
const uint16_t AnalogSensor_MaxVoltOut[4]   = { 3000, 1395, 5000, 5000 };    // Values in miliVolts
const uint16_t AnalogSensor_MinVoltOut[4]   = {    0,  530,    0,    0 };