The interactive effects of phosphorus, sulfur and cultivar on the early growth and condensed tannin content of greater lotus (Lotus uliginosus) and birdsfoot trefoil (L. corniculatus)

Abstract

Although soil phosphorus and sulfur influence the establishment and condensed tannin (CT) content of perennial Lotus species, previous field responses to superphosphate fertiliser applications have been confounded by moisture stress and weed competition, and the association of CT content with soil fertility has not been consistent between sites and species. In a glasshouse experiment, 2 cultivars of L. uliginosus (Grasslands Maku and Sharnae) and 1 cultivar of L. corniculatus (Grasslands Goldie) were grown in sand culture under 3 levels of phosphorus (0.1, 0.2 and 0.7 mmol/L) and 3 levels of sulfur (2.6, 6.4 and 92.4 mmol/L), applied as nutrient solutions, to examine the interaction of cultivar, phosphorus and sulfur on dry weight, CT content and mineral nutrient concentrations. Dry weight of the Lotus cultivars 68 days after sowing was significantly increased by increasing levels of phosphorus and sulfur, and there was a significant phosphorus × sulfur interaction, characterised by a much smaller response to increasing phosphorus at low levels of sulfur compared with the response at intermediate and high levels. Results indicated that positive responses to superphosphate applications at establishment can be expected if weed competition is controlled and that both soil phosphorus and sulfur should be monitored in order to predict responses to superphosphate applications at sowing. In L. corniculatus cv. Grasslands Goldie, low levels of CT were maintained at increasing phosphorus and sulfur treatment levels, whereas the CT content of the L. uliginosus cultivars increased with increasing levels of phosphorus and sulfur and was positively correlated with dry weight accumulation. This result, allied to measurements of CT in grazed swards, suggests that a management strategy of frequent cutting or grazing during the active growth phase will maintain optimal CT levels for grazing in L. uliginosus cultivars.