The uniformity of ground spreading common New Zealand blended fertilisers

Abstract

Farmers increasingly ask spreading contractors to apply fertiliser blends. Most fertiliser in NZ is spread using twin-disc fertiliser spreaders mounted on trucks or tractors. These spreaders aim to achieve a uniform spread by overlapping passes in the field. Fertiliser spread testing in NZ uses the Spreadmark® scheme as a fertiliser placement quality assurance programme. This study assessed the Field Coefficient of Variation (Field CV) achieved when spreading commonly blended compound fertilisers in NZ, including measuring the Field CV of the individual components of the fertiliser blend. Fertiliser blends were tested using commercially available, Spreadmark® accredited, twin-disc spreaders on flat fields in the Waikato and South Canterbury with a standard pan test method. Across the four blended fertilisers tested in the Waikato, there were significant differences in the Field CV achieved when spreading the fertiliser component alone, versus its spread as a component of the fertiliser blend, in 80% of comparisons. For the two blends tested in South Canterbury, there were significant differences in the Field CV achieved when spreading the fertiliser component alone, versus its spread as a component of the fertiliser blend, in only 40% of the six comparisons. The average deviance between spreading a fertiliser component alone, versus its spread as a component of a fertiliser blend between the two spreaders in Waikato, and the two spreaders in South Canterbury, was 11.8%. The average deviance between spreading a fertiliser component alone, versus its spread as a component of a fertiliser blend between the Waikato, and South Canterbury, was 16.8%. The differences between the spreading variability of components lead to differences in the maximal Bout (working) Width of spreading such fertiliser mixes. The tested non-nitrogen fertilisers can only be spread at a 22 m working width to stay within maximal variability (25%), whereas the tested N-fertiliser can be spread at a 27 m working width to stay within maximal variability (15%).