Application of nitrogen and sulphur to sunny and shady aspects on South Island dry hill country

Abstract

There are large differences in pasture growth and composition between aspects in dry hill country (annual rainfall 300-600 mm) in South Island dry hill country that could result in different fertiliser nutrient requirements for the grasses and legumes present. To determine these requirements, small plot mowing trials were carried out on sunny and shady aspects on Glenfoyle Station near Tarras and on Omarama Station over 3 years. At the Glenfoyle sites, nitrogen (N) was applied as urea at an average rate of 0, 15, 30 and 60 kg/ ha/yr and sulphur (S) was applied as gypsum at 0, 10, 20 and 40 kg /ha/yr. At the Omarama sites, N and S were applied at 0, 18, 36 and 71 kg/ha/yr. Soil total N (0.18- 0.33%), sulphate-S (3-5 ìg/g) and organic-S (1.5-3.5 ìg/g) levels were low at all sites. Pasture species on the sunny north-west aspect at Glenfoyle were mainly ryegrass (35% of total pasture content), flatweeds (20%) and subterranean clover (10%) with a predominance of browntop (31%) and white clover (20%) on the shady south-east aspect. There was a total pasture production response from 15-60 kg N/ha/yr (12-20 kg DM/kg N) associated with an increase in ryegrass content in Year 2 on the sunny aspect. On the shady aspect there was a total pasture production response from 30-60 kg N/ha/yr (16-21 kg DM/kg N) associated with an increase in browntop content. Increasing rate of N reduced subterranean clover on the sunny and white clover content on the shady aspect. On the sunny aspect, total pasture production and ryegrass content was increased from the application of 40 kg S/ ha/yr. Pasture composition at Omarama consisted mainly of Danthonia species (41%) and Bromus tectorum (30%) on the sunny north aspect and cocksfoot (39%) and browntop (11%) on the shady south aspect with less than 3% legume content on either aspect. On both aspects, there was a response in pasture production from rates of 18-71 kg N/ha (19-36 kg DM/ kg N) but this was not associated with a consistent response from individual pasture species. There was no response in total pasture production to S. The overall results indicate that high responses to N can be achieved from a range of grass species on both sunny and shady aspects and that soil sulphate-S levels of 3 ìg/g and greater are required for optimal growth of higher fertility responsive pasture species. Keywords: aspect, differential application, dry hill country, grasses, legumes, nitrogen, sulphur