Talking about the quality of the spread image, primarily the lateral distribution of the fertilizer over the working width (distance of the machine track) is meant. The spread image of the conventional 2-disc spreader results from the overlap of the spread fertilizer amount, i.e. the granule is being thrown considerably further in distance than the desired working width. If the fertilizer quantities do not complement in the overlapping area in the correct extent, a deficient lateral distribution is the consequence and therefore spread failures. Following conditions have an impact on the lateral distribution of the fertilizer:
According to the type of fertilizer the product groups differ in shape, size and weight. It has to be distinguished between prilled fertilizer (urea), granulated fertilizer (calcium ammonium salpetre, NP- and NPK-fertilizer), compact fertilizer (e.g. kalichlorid) or crystalline fertilizer (e.g. ammonsulfate). Prilled urea with 670 kg/m³ has a considerably lower specific weight than e.g. NPK fertilizer (15/15/15) with 1100 kg/m³. Heavy granules are thrown considerably further which leads to a bigger oberapping aea and herewith to a better lateral distribution. The single granule shapes (smooth and round, rough and edged) behave differently already with the abandoning of the thrower blade. As a consequence each fertilizer spreader has to be adjusted to the granule features. The well-known manufacturers of fertilizer spreaders provide therefore the spreader settings for their machines. If several fertilizer components are mixed to a certain nutrition formula, a higher failure quota of the nutrition distribution can be expected. Settings for mixed fertilizers do not exist.
Due to the usage of a short thrower blade and a long blade per spreader disc the distribution accuracy is enhanced since this leads to an increased overlap (figure 5). With long thrower blades the granule stays longer on the spreader disc and is abandoned later and therefore in a different angle. It is accelerated more and thus is thrown further. Thrower blades are wear and tear parts which wear off after some time. If they are not substituted in due time spread failures of 30% or more occur.
Due to different specific weights of granules they are thrown differently far with the same abandoning speed. Or the other way around: If the same abandoning distance is targeted with the single spread types, different abandoning speeds are necessary. According to a mockup trial of University of Applied Sciences in Mannheim the NPK fertilizer with a packed weight of approximately 1.1 to/m³ with an abandoning speed of 25 m/s is thrown approximately 12 meters (in both directions). In order to reach the same distance with a prilled urea, an abandoning speed of 100 m/s is required. More lightweight fertilizers are therefore considerably more suscepible to wind in comparison to heavier granules. While the spread image of prilled products shifts heavily with side wind, the image stays relatively constant with the application of fertilizers with a specific weight of more than 1 to/m³ also with a wind speed of 3.5 m/s.
Modern spreaders can be set today with adjustable thrower blades or by change of the contact point of the fertilizer onto the spread disc to the different fertilizer shapes and working widths. The bigger the required working widths the more important is the choice of fertilizer. Only heavy big granules can be distributed evenly also with working widths of 12 meters and possible wind impact. Attention: the limits for the exact nutrition distribution are reached if round, smooth granules as well as rough, edged granules are distributed in one working process.