Crop sensors never really caught on in the Netherlands with only a handful sold, because they were originally developed for wheat and oilseed rape, which are not widely grown.
In addition, Dutch farms are not large or diversified enough to make them cost effective.
See also: How a new web tool links precision farming with agronomy
However, this may be about to change with the development of new applications in fields such as in potato growing, which could see sensors become more worthwhile in the Netherlands, says Winfried Raijmakers of Yara Benelux.
Round-up of tractor-mounted crop biomass sensors
The passive Yara N-Sensor costs €20,000. So if you have a farm with 200ha of potatoes, grain and oilseed rape, and depreciate the sensor over 6 years, it works out at €25/ha, he says.
But price is just one factor in its relatively low sales. In other countries, sales temporarily peak when subsidies become available. Even then, farmers don’t always use them, and they tend to be employed in research projects and by more high-tech growers.
Evaluating crop measures
Crop sensors have been described as 'reading glasses' for farmers. They’re an extra pair of eyes that generate data on the crop’s status with each pass of the sprayer.
One limitation compared with satellite and drones is a lack of resolution, as tractor-mounted sensors only measure 60cm to 4m with each pass.
However, this is not an issue with current fertiliser spreading and spraying techniques, as their working width is much wider at 36-40m and are unable to vary the dosage within the working width.
Need to know
- Crop sensors were initially developed for wheat and oilseed rapeA crop sensor provides objective information about the conditions of the crop
- Medium costs compared with satellite and drone imagery
- Always available and instant application; no piloting licenses or aviation registration necessary or cloud cover issues
- Not just calibration curves for wheat and oil seed rape, also for potatoes
- Prices vary from €9,000 to €35,000, so compare and shop around before you buy
Support for artificial fertiliser sales
Before mobile sensors came on to the market, handheld units were used in the same way as grid-based soil sampling to determine within field variations in chlorophyll (green leaf) and biomass.
Geert-Jan Giesberts, of the manufacturer Homburg, says one of these, the Crop Circle from the American company Holland Scientific, was the basis for the Ag Leader OptRX sensors.
Over 20 years ago, the Norwegian fertiliser manufacturer Hydro Agri (now Yara) began developing its N-Sensor to vary dosages during additional fertilisation of cereals.
The firm’s goal was, and still is, to help farmers use nitrogen more sustainably. The N-Sensor was launched in 1999, and there is now an active version that can be used 24 hours a day.
Since then, the company has sold only 2,000 units worldwide, including some 600 in Germany and just 10 in the Netherlands, far fewer than originally forecast. The company says there are a variety of reasons for this limited interest, one being a reluctance to adopt new technology.
Near versus remote sensing
Crop sensors are a form of near sensing, in which the distance between the crop and the sensor is small.
The opposite of this is remote sensing, which uses satellites, light aircraft, helicopters and drones. Satellite photography has the lowest average cost on a per image and per hectare basis while professional drones are the most expensive.
The number of companies offering drone imaging is increasing fast, and prices will, therefore, probably fall. Crop sensors may be more expensive per hectare, but you can take measurements as often as you like, regardless of the weather, and do not require pilots or licences.
Perhaps their biggest benefit is they are the only technology that can instantly vary the dosage to suit the crop’s needs based on the sensor readings.
The development of new calibration curves for different crops and applications is making sensors more attractive to Dutch farmers. They are now available for additional fertilisation and growth regulation of cereals, oilseed rape, maize, potatoes and grass. There are also calibrations for the control of potato blight.
Another benefit for potato growers is better use of agchems. Tests carried out as part of the IJKakker project used sensors to tailor the dose of the desiccant diquat to the measured foliage growth, and achieved a 44% average reduction in use.
Glossary: Crop indices to measure crop condition
There are several crop indices, also known as vegetation indices.
- NDVI – Normalised difference vegetation index – is the most common index and indicates how healthy a crop is. Plants absorb visible light for their photosynthesis and reflects non visible light, near-infrared (NIR). A crop sensor calculates the difference between them and can determine the amount of chlorophyll, the mass (biomass) and the crop conditions. The difference in reflection between visible and NIR light also depends on the total amount of light that reaches the surface. This difference is corrected for it (normalised).
- WDVI– Weighted difference vegetation index – is derived from the NDVI. This index also includes soil reflection and has a higher distinctiveness when crop development increases.
- NDRE – Normalised difference red edge – measures the amount of chlorophyll and is an indication of the current health and growth potential of the crop.
- IBI– Isaria Biomass Index– is a proprietary sensor index used to estimate plant densit
- The IRMI vegetation index reflects the nitrogen supply to the crop