Richard Simpson, Research Project Leader – Grassland Agronomy, CSIRO Agriculture & Food
The serradellas (Ornithopus compressus [yellow serradella] and O. sativus [French serradella]) have many attributes which make them useful alternatives to subterranean clover. Among these attributes is a considerably lower requirement for soil P (Fig. 1). This is not news to many farmers because serradellas have long been prized for their ability to grow on infertile soils. Recently, we have been able to estimate the “critical” soil test P (STP) concentration for yellow and French serradella and have confirmed that it is significantly lower than that of subterranean clover (Fig. 2). The “critical” STP concentration is the soil test P concentration of the topsoil layer (0-10 cm) that supports 95% of maximum yield when measured in spring (i.e. when pasture growth rates are at their highest).
Figure 1: Early research from WA clearly
showed that serradella can yield as well as sub. clover but with a much lower level of applied P.
Figure 2: The critical soil test P concentration (corresponding to 95% of maximum yield) of various pasture legumes and two perennial grasses grown at up to 4 sites in southeast NSW (PBI range: 40-80) over a 3-year period (in total, 7 site-year experiments). There were no significant differences in the critical P requirements among cultivars from a single species. Bars indicate 1x standard deviation. They provide a measure of the repeatability of the critical P estimate. The graphs are drawn using data from Sandral et al. (2019).
This allows us to revise the STP benchmark for managing serradella-based pastures. The critical P requirement for serradella is still greater than that of some companion grasses (e.g. ryegrass, silver grass, etc.) and it is roughly equivalent to that of other grasses (e.g. phalaris, cocksfoot, etc.). Because of this, it is now feasible to use the lower critical STP requirement of the serradellas as the benchmark STP target for fertilising serradella based pasture. In a soil where sub clover pasture needs to be fertilised to 30-35 mg Colwell P/kg soil, serradella will yield near its maximum at about 20 mg Colwell P/kg (Fig. 2).
Figure 3. Photographs of typical roots of serradella and sub. clover showing how serradellas have thinner roots with very long root hairs in contrast to the thicker roots and very short root hairs that are
characteristic of the sub. clovers. These differences in root morphology explain the difference in the ability of these species to take up P from low P soils.
How is the lower critical P requirement achieved?
The serradellas have long, thin roots and produce much more root length per gram of root mass (e.g. ~250-300 metres of root per gram root dry mass, compared to sub clover ~100-150 metres/gram). They also have long root hairs (~0.7 millimetres compared to ~0.25 millimetres for sub clover) (Fig. 3). The
result is that serradellas develop a much larger root surface area for P uptake than sub clover and this allows them to extract more P from soil at a lower STP concentration; hence their lower critical STP concentration.
How can we apply this information in deep sandy soils?
Most of the soils in the experiments across the “P-efficient pastures” project have a low to moderate Phosphorus Buffering Index (PBI) of at least 30. These soils retain most of the P that is applied as fertiliser in the top 20 cm of the soil profile and, of that, about 70% is held in the top 10 cm. This makes it easy to soil test for P and to apply the critical STP benchmarks for soil P management.
There is a real dilemma in the deep sandy soils being managed by ASHEEP and Southern Dirt which have PBI values of 10-11 or less. These soils do not retain P in the topsoil for long and it soon moves to depth in the soil profile (Fig. 4). We believe that
serradellas are well equipped to capture this P because of their deep rooting habit. However, soil sampling for P in the top 10 cm of the soil alone is unlikely to provide a useful picture of the amount of P that is potentially available to the pasture.
Figure 4: The Colwell P profiles of soil at Neridup after two years of relatively high P applications showing how P has
moved below 20 cm depth relatively quickly. This data was generated by Dr Brad Nutt (Murdoch University) in his
experiment that is adjacent to the ASHEEP plots at Neridup. The soil P profile at Grass Patch is similar.
During the P-efficient pasture project, we have changed tack to address this issue and are now sampling soil at 0-10 cm and 0-60 cm depth with the aim of relating the amounts of Colwell P found
in each soil zone to pasture growth. Our hypothesis is that deeper soil samples may better reflect P availability in these “leaky” soils. Unfortunately, droughts have been against us during the last two growing seasons so it seems unlikely, at this stage, that we will have tested the hypothesis adequately by the end of the project.
References: Bolland and Paynter (1992) Comparative responses of pasture species to super… Fertilizer Res. 31, 21-33 Sandral et al. (2019) Field benchmarking of critical phosphorus requirements… Crop Pasture Sci. 70, 1080-1096
