Field benchmarking of the critical external phosphorus requirements of pasture legumes for southern Australia

Abstract

In recent decades several pasture legumes have been available in southern Australia as potential alternatives to the most widely used annual pasture legume Trifolium subterraneum. Little is known about their soil phosphorus (P) requirements, but controlled environment experiments indicate that at least some may differ in their P fertiliser requirements. In this study, pasture legume varieties, including T. subterraneum as the reference species, were grown at up to four sites in any one year over a 3-year period (in total, seven site × year experiments) to measure herbage growth responses in spring to increased soil P availability. A critical soil test P concentration (corresponding to 95% maximum yield) was estimated for 15 legumes and two pasture grasses. The critical soil P requirements of most of the legumes did not differ consistently from that of T. subterraneum, indicating their soil fertility management should follow the current soil test P guidelines for temperate Australian pastures. However, the critical P requirement of Medicago sativa was higher than that of T. subterraneum, but remains ill-defined because extractable soil P concentrations in these experiments were often not high enough to permit a critical P estimate. Three forage crop legumes (Trifolium incarnatum, Trifolium purpureum, Trifolium vesiculosum) and two pasture legumes (Ornithopus compressus, Ornithopus sativus) had lower critical soil test P concentrations. It may be feasible to manage pastures based on these species to a lower soil test P benchmark without compromising yield.

The development and application of functions describing pasture yield responses to phosphorus, potassium and sulfur in Australia using meta-data analysis and derived soil-test calibration relationships

“…2), the determined 95% critical value of 15 mg P/kg was the same value previously determined for dryland and irrigated Victorian pastures (Montgomery and Rubenis 1978;Gourley and James 1997) and also consistent with a previously proposed critical value of 20 mg P/kg for 7.5 cm sampling depth for subterranean clover pasture in NSW (Spencer et al 1969). More recent experiments have also found that there was no pasture production response to soil Olsen P values above currently recommended optimum concentrations of 13-16 mg/kg (Cotching and Burkitt 2011;Aarons et al 2015a;Simpson et al 2015;Sandral et al 2019).…”

“…Many shorter-term and smaller-scale field studies also support the derived soil test critical values, notably addressing P and K (e.g. Cotching and Burkitt 2011;Aarons et al 2015a;Simpson et al 2015;Sandral et al 2019). Moreover, higher soil test P concentrations are associated with greater and unnecessary rates of P accumulation in the soil (Simpson et al 2014(Simpson et al , 2015 and incur larger risks of P loss to waterways (Melland et al 2008;Gourley and Weaver 2012).…”

“…The critical values are estimates derived by measuring the growth response to a limiting nutrient generally from a grass-legume pasture dependent on the fixation of atmospheric nitrogen by its clover component. In many cases, it is the critical soil nutrient requirement of the clover that essentially determines pasture nutrient requirements because they typically have higher P (Ozanne et al 1969(Ozanne et al , 1976Helyar and Anderson 1971;Jackman and Mouat 1972;Hill et al 2010;Sandral et al 2019), K (Hunt and Wagner 1963;Bolton and Penny 1968;Brockman et al 1970;Simpson et al 1988;Bolland et al 2002) and S (Gilbert and Robson 1984;Warman and Sampson 1994;Tallec et al 2008) requirements than grasses in the sward. For the determination of a particular critical nutrient requirement, other nutrients, soil physical and/or soil chemical conditions must be non-limiting for pasture growth.…”

Dry matter and nutritive value responses of native, naturalised and sown pasture species to soil Olsen P

“…Rankings of critical external P requirements were consistent with Hill et al (2005), where P. aquatica, M. stipoides and T. subterraneum were in a group with high external P requirements and Rytidosperma richardsonii (Cashmore) Connor & Edgar in a low group. A similar study by Sandral et al (2019) also found that rankings in the field matched those of controlled conditions. There is no evidence to date that our field studies produced a different or more valid ranking.…”

Australian Grassland Association – Soil Constraints on Pasture Productivity 2019

“…Soil fertility has long been an important limitation for pasture production in Australia, and Gourley et al (2019) presents new information on pasture yield responses to addition of phosphorus, potassium and sulfur for regions around Australia. There is also an increasing emphasis on the development of more phosphorus efficient pastures which is highlighted by Sandral et al (2019) and McCaskill et al (2019), while the phosphorus requirements of native pastures in south-eastern Australia was reviewed by Mitchell et al (2019). Nitrogen fertiliser application has become increasingly important in the Australia dairy industry and the balance between production and potential for nitrogen losses was examined by Rawnsley et al (2019).…”