Response of Caucasian clover to phosphorus and sulphur fertilisers on a high country soil: A dissertation submitted in partial fulfilment of the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Two trials were carried out to investigate the response of Caucasian clover to phosphorus and sulphur fertilisers on a high country soil. A trial to measure the effects of phosphorus and sulphur fertiliser application on Caucasian clover (Trifolium ambiguum) was carried out between September 1998 and April 1999. The site consisted of a seven-year old Caucasian clover sward that had been established on a low-fertility, depleted, short tussock grassland site at Mesopotamia South Canterbury, New Zealand. The trial contained 30 main plots, each divided into 4 subplots. Phosphorus was applied to each of the main plots at rates of 0, 20, 40, 60 or 80 kg P/ha, while sulphur was applied to subplots at either 0, 20, 40 or 80 kg S/ha. Dry matter harvests were taken in December 1998 and April 1999. Fertiliser application increased over all dry matter production (kg/ha), due predominantly to the effect on Caucasian clover. Caucasian clover dry matter production increased from 141 kg DM/ha at nil fertiliser to 3894 kg DM/ha at combinations of 80 kg S/ha and 80 kg P/ha. Application of fertiliser had virtually no effect on dry matter production (kg/ha) of other components of the sward. The effect of phosphorus fertiliser was minimal without sulphur applications giving less than 776 kg DM/ha of Caucasian clover averaged over all levels of phosphorus, however, as long as sulphur was present dry matter production increased with increasing levels of phosphorus. Application of sulphur fertiliser alone increased dry matter yields of Caucasian clover to 1527 kg/ha at 20 kg S/ha, with no further increase in dry matter at increased levels of sulphur. This indicates that sulphur is more limiting for Caucasian clover growth in this environment than phosphorus, but that once sulphur levels are adequate, increased yield depends more on the level of phosphorus. In February 1999, a glass house pot trial was established at Lincoln University to further investigate the effect of phosphorus and sulphur fertiliser on Caucasian clover and white clover (Trifolium repens), and also to look at the effect of sowing depth on Caucasian clover rhizome development. Soil was taken from the Mesopotamia site used in the field trial and sieved, before phosphorus at either 40 or 80 kg P/ha, and sulphur at 20, 40, 60, 80, 120 or 200 kg S/ha was applied. Caucasian clover seeds were either surface sown or sown at a depth of 10 mm to encourage rhizome production, while white clover seed was only surface sown. Increasing phosphorus increased dry matter production from 2.27 g DM/pot at 40 kg P/ha to 2.99 g DM/pot at 80 kg P/ha, five months after sowing. Caucasian clover treatments, with 2.73 g DM/pot and 3.01 g/pot for shallow and deep sowing respectively, yielded more than white clover treatments with 2.16 g DM/pot. White clover yielded more above ground herbage (2.66 g DM/pot), than Caucasian clover (2.09 g DM/pot for shallow sowing, 2.38 g DM/pot for deep sowing). However, Caucasian clover had over twice the root mass (1.51 g DM/pot for shallow sowing, 1.66 g DM/pot for deep sowing) in compari~on with white clover (0.659 gDM/pot). Sulphur had no effect, however it was thought that sieving of the soil may have caused mineralisation of both S and N in the soil, preventing a nutrient deficiency occurring. Soil tests showed that there was adequate sulphur in the organic pool, and that the C:N ratio was satisfactory for nitrogen mineralisation to have occurred. Sowing depth also had no effect and production of rhizomes was low. However, the majority of rhizome production was found on those plants that were in the deep sown treatments, possibly indicating that sowing at depth helps initiate rhizome production