Effective bout widths for uniform spread of lime from ground spreaders

Abstract

There is a strong correlation between crop yields and evenness of application of lime. Lime is the most widely used fertiliser/soil conditioner product in New Zealand, but it is not very mobile in the soil, so it is important to achieve uniform application of lime. Lime is a very challenging material to spread, due to its fine particle size and its non-uniform size distribution, resulting in poor ballistic properties. Further, lime also has adhesive properties that cause it to come off the conveyer belt in discrete chunks or cakes rather than as a smooth continuous granular flow. Field testing was conducted to measure the uniformity of lime spreading over a range of application rates relevant to variable-rate application. Five different trucks were tested over application rates from 500 to 5000 kg/ha, and also at different driving speeds. Spread patterns were measured across three tray lines for each test condition, and coefficients of variation (CV) calculated as a function of bout width for each line. Measurements were also made of the particle size distribution for the lime used, and videos were taken of the lime motion in the vicinity of the spreader disks. The results of the field trials show that on average a bout width of 10 m was obtained at a coefficient of variation (CV) of 25%, and a bout width of 5 m at a CV of 10%. There was variation of lime in the longitudinal as well as the transverse direction, with a CV of 22% averaged across all trucks in the direction of truck travel. This variation is caused by the caking of the lime as it comes off the belt, which can be observed both directly and indirectly in the videos taken by the truck-mounted camera.