Precise aerial fertiliser application on hill country

Abstract

The fertiliser requirements of hill country vary with soil type, slope and aspect-related factors which govern pasture production potential and species composition. In most situations, the topographic complexity is such that only very broad differentiation in land units can be made when aerially applying fertiliser. The traditional method of aerial topdressing is for superphosphate to be flown on at a common rate over large blocks of complex topography by fixed-wing aircraft. Advances in geographical positioning system (GPS) and aircraft technology now allow aircraft to fly accurately defined track spacing and so achieve optimum uniformity of fertiliser spread. The same technology could be used to vary fertiliser application rate along a flight path according to predetermined recommendations and through links to a farm geographic information system (GIS) map. This approach could also be used to apply different fertiliser types. In a desktop study the effects of differential, compared with uniform, fertiliser application policies, on animal productivity and economic returns were examined for three contrasting hill farm situations using a combination of trial results and the AgResearch PKS Lime Programme. Results showed that for a farm with a low soil P status (Olsen P =9), that stocking rate could be increased by 0.5 su/ha, and the economic return by 7.5%, by differential, rather than uniform fertiliser application. In a similar but higher soil P status farm (Olsen P = 15), the increase was 0.9 su/ ha and 10.1% respectively. In a summer-dry situation where nitrogen fertiliser could be substituted for some P fertiliser, a differential policy designed to optimise production gave a 2.1 su/ha and 43% net margin increase, compared with the uniform application of a typical rate of maintenance P fertiliser only. The results from the desktop study are discussed in relation to the practical aspects of developing differential fertiliser application methods. This will relate to extra fertiliser application cost, and the definition of practical sized land units and fertiliser forms, which will all have some effect on the net economics of a differential application policy. Despite these unknowns, the technology would appear to offer real gains to the hill country farmer. Keywords: economic return, fertiliser application, GIS, GPS, hill country, phosphate