TFS - Potential for phosphorus efficent legumes

Summary

​Research was conducted by the Tablelands Farming Systems Group (TFS)  to establish if legume species with a critical phosphorus requirement lower than subclover, such as Serradella, would survive and be productive in the high altitudes of the Southern Tablelands of NSW. Two trials were conducted with the first trial comparing the performance a range of legume species at two levels of soil phosphorous and the second testing the adaptation of these legumes in different soil types and locations.

The cost of phosphorus fertilisers is high (SSP = $350 per tonne spread in 2016) and increasing scarcity could see an increase in the cost in the future (Simpson et al 2011).  Given there is also mounting evidence that soil acidity also poses a limitation for traditional use of sub clover and their rhizobium, (Hackney et al 2017)  the use of an acid tolerant legume that has a low P requirement such as Serradella could see an improvement in carrying capacity of the low pH soils typical of  the region.

Farms in the Southern Tablelands predominantly graze beef cattle and sheep for meat and wool production.  The findings of this trial will assist farmers in making decisions about plant selection and fertiliser use to improve the productivity of their properties.

The Phosphorus Buffer Index (PBI) of the soils represented in the trial were very low (30 to 100) and the results show that plant available soil  P can change markedly with lower than expected rates of fertiliser. The responsivness of these soils to applied P meant in trial 1 the target Colwell P level in the high and low Phosphorous treatments were exceeded.  On the low treatment, Colwell P exceed the critical P level for sub clover growth (35 mg /kg ) rather than reaching the expected level of 20 mg/kg.

Unfortunately this meant there was no difference  observed in the growth of legumes between plots with high and low fertiliser applications. At most sites in both 2015 and 2016 Pink Serradella (Ornithopus sativus cv Margarita) produced comparable biomass to Leura Sub clover and the naturalised clovers although due to the higher than expected soil P response in trial 1 it cannot be determined whether this level of production would have been maintained at lower P levels.  Caucasian Clover (Trifolium ambiguum) cv Aberlast survived at high elevations in the trial area but was slow to establish so its potential productivity was probably not realised within the short time frame of the research.

Crimson clover (Trifolium incarnatum) cv Blaza and pink serradella had the highest production of the novel legumes tested.

Trial results show that comparable performance can be expected from the naturalised clover base with improved management of soil fertility, but in areas where application of fertiliser is limited by topography or if phosphorous price increases, pink serradella would appear the best of the alternative legumes tested.

This project did not evaluate conclusively the role of pink serradella in the region as more time will be needed to determine if it can successfully regenerate from seed in the long term.