The demand from farmers for precision farming is greater than ever. Political constraints, environmental protection and the desire to increase farm productivity mean that farm managers are always looking for new ways to apply available resources such as fertiliser as efficiently as possible. And experts use plant sensors to maximise efficiency. ISARIA supplies farmers with two reliable sensors for precision farming in the form of the ISARIA PRO Active and the ISARIA PRO Compact. But how does an ISARIA plant sensor measure crops? Read on for the answer.

Working principle of an active and passive plant sensor

Methods of measurement using ISARIA plant sensors – a comparison

The ISARIA PRO Active and ISARIA PRO Compact plant sensors use different methods of measurement.The ISARIA PRO Active is an active plant sensor, because it actively irradiates the crop with its own light source. The ISARIA PRO Compact, on the other hand, uses sunlight as a light source and does not have its own light source, making it a passive plant sensor. A passive plant sensor relies on a reference (an irradiation sensor on the roof of the tractor) to measure the intensity of the solar radiation, which is used as a reference for the level of reflection from the crop. Both plant sensors measure the light reflected from the crop in the same way while travelling over the crop.


Active plant sensor

The ISARIA PRO Active i

is an active plant sensor and can therefore be used independently of daylight. Four high-powered LEDs integrated into the sensor head provide the light source.


For this method, the artificial light source emits light waves in the near-infrared and infrared range, which are reflected back by the crop. This reflection is then measured by a detector in the sensor. The method does not rely on solar radiation, as the leaf green reflects in the near-infrared and infrared range about 7 times more than in the visible range. The two sensor heads are guided at between 80 cm and 120 cm above the top edge of the crop and measure straight downwards. The plant sensor is operated and set up via a dedicated terminal, which communicates with the sensor via Bluetooth.

Operating principle ISARIA PRO Compact

Passive plant sensor

The ISARIA PRO Compact is a new compact passive plant sensor based on ISOBUS for use during the day. The particular feature of this plant sensor is that it is easy to mount on your tractor’s wing mirrors. A light sensor is also mounted on the tractor roof to measure the current solar radiation. The two sensors on the mirrors measure the reflection of light by the crop at a 45° angle downwards. A separate light source is not used as output radiation for this method, which uses solar radiation instead. It does not matter whether the radiation is diffuse – such as on a cloudy day – or direct.
It is still possible to achieve stable measurement down to a sun angle of 10°. It is also possible to continue measuring below 10°. However, the measurement result may then be somewhat less accurate. Unlike the ISARIA PRO Active, this plant sensor is fully ISOBUS-compatible and can communicate with any ISOBUS-compatible terminal.

Operating principle ISARIA PRO Compact

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Good to know: Using the passive plant sensor, farmers do not have to work out themselves when to stop measuring. If the position of the sun is too low, the system warns the farmer that error-free measurement is no longer possible.


What do active and passive plant sensors measure?

Regardless of whether you are using an active or passive plant sensor, both plant sensors measure the light reflected by the plants as they drive above them. These reflection values are used by the ISARIA system to calculate two plant indices. The biomass index provides information about crop density. The N-supply index is a measure of the current nitrogen supply to the crop. These two indices are used to calculate the correct quantity of fertiliser for the current crop automatically, depending on the selected fertiliser strategy, so that you can always apply the optimal quantity of fertiliser to your crops.