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Anno 2021 Numero 3-4

March - April 2021

Centrifugal fertilizer spreaders

To distribute chemical and mineral fertilizers, disk fertilizer spreaders are the most widely used machinery since they provide a high working capacity and great overall compactness. In the last 20 years, technological evolution has come a long way, and manufacturers are now offering equipment that can adapt the distribution to the most varied operating conditions

The crop fertilization technique has been known since Neolithic times. Until the middle of the last century, it was almost exclusively carried out through the use of organic matrixes, which were, however, available in relatively small quantities compared to the extent of arable soil. Since around the first half of the last century, the chemical industry's availability of large quantities of mineral and synthetic fertilizers has led to a considerable increase in yields and cultivable areas.

As a result of the need to deliver fertilizers in these new forms, the first fertilizer spreaders were immediately prototyped. Nowadays, since fertilization is mainly carried out with fertilizers of mineral origin, produced through chemical synthesis, and marketed mainly in granular form, it is common practice to distribute them mainly through dedicated machinery. Except for manure and slurry spreaders used for shovelable and non-shovelable organic matrixes, there is no doubt that mineral and chemically synthesized fertilizers are still much more frequently used than these, and therefore the related spreaders, most of which are centrifugal, are widespread throughout the territory.

In recent years, these machines have also been the object of a remarkable technical evolution, aiming at making their distribution more and more precise, also because of the reduced profit margins resulting from standard cereal cultivation, it is well known how important it is to reduce the wastage of production factors in order to keep at least a minimum profit margin.


The machinery

The operating principle of centrifugal fertilizer spreaders is based on the force imparted to the granules, which are dropped on one or more discs rotating at very high speed, on which are radially mounted some spreading blades, which, impacting with the granules, give them a solid tangential acceleration (and therefore a considerable speed) which throws them at certain distances from the point of impact with the blade so that they end up on the ground after having traveled a parabolic trajectory transversal to the direction of progress of the equipment. The machine's heart is, in fact, the rotating disc that has to ensure that the product is thrown with the least possible shock to the fertilizer granule so as to prevent it from fragmenting and creating dust. This ensures accurate distribution and optimal coverage.



With single-disc fertilizer spreaders, the fertilizer granules, after their contact with the spreading disc and its blades, are subject to an intense acceleration and are thrown towards the outside with an initial speed of 8-12 m/s. The area covered by the distribution is represented by a circular sector with a center on the axis of rotation of the disc and radius equal to the maximum distance covered by the granules. Under optimal operating conditions, the machine's distribution diagram is triangular, with a typical slight asymmetry. In order to obtain an excellent transversal distribution uniformity, it is necessary to make an overlap between two contiguous passes of the spreader between 50% and 25% of the maximum throw. The resulting working widths are usually between 8 and 15 m.


Double disc

The distribution device is made up of two counter-rotating discs. In this way, the typical distribution asymmetry of single-disk machines is reduced. The resulting distribution diagram has a semi-elliptical shape. In order to obtain a good transversal distribution uniformity, the actual working width must range between 50 and 70% of the maximum throw and, it is generally between 12 and 20 m. The dose regulation system can be by gravity or by forced extraction. In the latter case, some machines are already fitted with adjusting devices in the basic equipment to maintain an always proportional distribution to the forward speed automatically. Either mechanical or electronic control systems exist for this purpose.

There are basically two goals to pursue with fertilization: The first is to dose the supply of nutrients on the surface unit correctly. The second (and more challenging to achieve) is to make the distribution as uniform as possible over the entire treated surface. If, in the first case, the objective is relatively easy to achieve, in the second case, it is necessary to correctly overlap the different passes, which depends on the spatial conformation of the distribution diagram.

While distributing the exact amount of fertilizer per hectare is a reasonably simple target to reach, this is not the case when referring to much smaller surface units, such as the square meter or its submultiples, and this is an issue not only for the optimization of the distribution of the product (and therefore for the minimization of waste) but also for all environmental issues related to it (pollution of groundwater by nitrates in the first place). It is also essential to know that, even though a recent study promoted by the European Commission has quantified in about 1 million tons/year the amount of nitrogen lost because of inappropriate application methods, other recent studies have shown that in Italy, about 60% of cultivated areas are still fertilized using single-disc fertilizer spreaders. Some of these can be more demanding to set up in order to obtain a perfect distribution on the ground, primarily because of the shape of their distribution diagram.

In any case, obtaining perfect distribution uniformity on small areas is almost always a dream, even if it is much easier to approach perfection with double-disc models than with single-disc models. However, it is well-known that in both cases, when setting the machine, the best possible compromise has to be found, partly because some of the problems involved in obtaining (and maintaining over time) a perfect distribution diagram are caused not by the machine but by the product to be distributed, which sometimes does not have a uniform grain size.

The latest manufacturer innovations have significantly improved the centrifugal fertilizer spreader's operating capacity, thanks to the use of devices that have improved the flow of fertilizer from the hopper to the distributor (by controlling it in real-time). Furthermore, efforts have been concentrated on optimizing the fall of fertilizer onto the rotating finned disc. In this way, using special sensors and the ever-present electronic control unit, it is now possible to adjust from the driver's seat both the working width and the actual amount of product distributed per unit of area. The next step was to develop machines capable of operating in DPA mode (Distribution Proportional to Advancement), that is, able to maintain a constant dose (in kg/ha) even when the advancement speed varies.


The latest innovations

Additional goals included the reduction of the off-target product (e.g., by taking more outstanding care in the distribution carried out in the perimeter areas of the plots), but above all the adoption of variable rate distribution techniques, i.e., with the distribution of differentiated doses according to the productivity found in the various areas of the plots. Initially, the dose to be distributed was determined to rely almost exclusively on the field's productivity inferred during harvesting by combines equipped with yield sensors, which were geo-referenced via GPS.

Working in this way, it was possible to obtain graphical maps with differentiated productivity, enabling site-specific action to be taken with the subsequent fertilization plan, i.e., increasing or decreasing the fertilizer dose based on general agronomic indications. For variable rate fertilizer distribution, it is vital that the yard has a satellite navigation system, and that the fertilizer spreader is also correctly prepared. From this point of view, ISOBUS-compatible tractors and equipment are well suited. They make it possible to simplify the control and management of the fertilizer spreader with a single terminal onboard the tractor without being forced to install additional control units.

With the progressive reduction in the cost of remote sensing data (via satellite or with planes, drones, or helicopters), the "precision" distribution of fertilizers has now become a consolidated reality. However, for some time now, several sensors have been available on the market that can detect the "health status" of the crop in real-time, through the analysis of solar radiation reflected by the crop also using special optical sensors placed on the front of the tractor. In this case, it is a significant operational simplification since it disregards the prior preparation of prescription maps.

by Davide Facchinetti

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