Factors contributing to reduced productivity of subterranean clover (Trifolium subterraneum L.) pastures on acidic soils
In a field study on four sites, soil acidity, root rot (Phytophthora clandestina), and soil phosphorus were identified as causes of 'subterranean clover decline'. Liming increased herbage and seed production at four sites, with a tendency for lime to increase herbage yields in autumn (22%) and winter (15%) but not in spring. The presence of ryegrass with clover increased total herbage yields, and reduced clover seed production, but there was no interaction with liming. Losses caused by root rot associated with P. clandestina were quantified for the first time in New South Wales. Root rot reduced survival of seedlings as well as herbage production in autumn and/or winter at three of the four sites. In the presence of the disease, lime did not improve root health or seedling survival. On two sites with high aluminium saturation of exchangeable cations (> 17%) and high phosphorus sorption index values, subterranean clover growth responded to high levels of P fertilizer. On one site, where lime increased the soil pH to above 5.5, the P sorption index was temporarily increased, and this was associated with a temporary adverse effect on herbage yields. Some possible mechanisms underlying the seasonality of lime responses are proposed and the practical implications of our findings are discussed.