Increasing the yield and quality of broadleaf planting stock through higher N fertilisation in the nursery.

The planting of broadleaf species has increased in Ireland in recent years with a consequent upsurge in the demand for planting stock of common alder (Alnus glutinosa L.), downy birch (Betula pubescens Ehrh.), and pedunculate oak (Quercus robur L.). However, the yield of usable plants and the quality of planting stock produced in Irish nurseries could be improved. The better use of N fertiliser and cloches might help address this issue. To this end, the effect of 40, 80 (standard amount), 120 and 160 Kg N ha-1 (applied as calcium ammonium nitrate) on seedling growth and yield in these species was assessed. Seedbeds were sown with seeds of each species and were covered with cloches before seedling emergence had commenced. The cloches were removed in mid June, after which the fertiliser treatments commenced. Fertiliser was applied in five equal quantities at two-week intervals until mid August. Higher N fertiliser than the standard (and the 40 kg N ha-1) amount improved growth and yield in all species. For example, 50, 48 and 67% of the seedlings had a mean height >40 cm in plots that received the highest N level, compared with 30, 21 and 24% in the plots that received the standard amount in alder, birch and oak, respectively. Differences in treatment responses to the two highest N levels were small in alder, but were less clear in birch and oak. Furthermore, seedlings grown at the highest N level also had higher root growth potential (a measure of plant quality) than those grown at the standard level, suggesting that the former plants would probably perform better in the field than the latter ones. The N cost per usable plant was lower for seedlings grown at the higher than the lower N rates in alder and birch, but not in oak, but further testing is needed to confirm this.