It's like shooting in the dark. This quickly sums up how Tony Grift, a University of Illinois (U of I) agricultural engineer, views the limitations in trying to apply granular fertilizer accurately with traditional spinner-type spreaders.

“This type of equipment is a weak link in precision agriculture since farmers don't have an effective or efficient means to know precisely how much or where granular fertilizer is being applied on fields in order to reduce under or over application,” he says. “Even calibrating such spreaders by using the bin method isn't desirable. It requires experience, it's time consuming and weather conditions need to be ideal. The producer's time is much too valuable to fool around with this task.”

For Grift, meeting that challenge to boost the accuracy of spinner-type spreaders began in the mid-1990s. That's when he began developing a special “optical sensor” for a single-disc Lowery 300 spreader. The cost was less than $500.

Grift also developed special software based on mathematical models of fertilizer spread patterns for the optical sensor as well as automatic control gates — or finger-like chutes — on the spreader.

Altogether, this unique optical sensor system allowed the operator to more precisely monitor and control the outflow and spread pattern of granular fertilizer automatically and in real time. The end result, according to Grift, is an “applicator that delivers the prescribed application rate at an optimal uniformity without any human intervention.”

The optical sensor installed on the spinner-type spreader fully automates the spreading process. It can predict a spread pattern by measuring the velocity and diameter of fertilizer particles as they pass by the sensor. A custom computer program then calculates a simulated pattern for a preset swath width.

“In precision agriculture, you need the ability to vary fertilizer application rates very accurately based on the demands of the crop and soil,” he says. “For producers trying to make do with existing granular spreaders, the situation was like shooting in the dark. Not knowing by exactly how much or where you may be over-applying or under-applying fertilizer adds up all too quickly into economic losses.”

Grift and his colleague Jan Willem Hofstee are refining the optical sensor system and the automatic control gates, which control the spread pattern, on a prototype spinner spreader at U of I.

Since the optical sensor system is fairly easy to retrofit on existing spreaders, Grift says he hopes a manufacturer will see its benefits and start making this technology commercially available.

“In Europe, virtually all spreaders used in precision agriculture applications weigh the amount of fertilizer in the hopper during spreading,” says Grift. “On either continent there is, however, no feedback mechanism that controls the uniformity. Again, this is why we began pursuing the use of the optical sensor system.”

He says the sensor could also benefit aerial application of dry fertilizers.

“The difference between aerial and ground-based application centers on the fertilizer stream density,” he says. “In aerial application, the fertilizer stream in the ducts is dense, which requires measuring mass flows instead of counting single particles as in a ground-based application. By making some modifications, this optical sensor would offer aerial applicators at least some type of feedback mechanism so they know exactly how much material is being applied, even when faced with headwinds or tailwinds.”

Grift adds, “In fact, since the optical sensor system is capable of measuring mass flows, it could be transformed into a grain yield monitor as well. In the long run, this application might prove even more beneficial to farmers than automated fertilizer spreaders.”