Report by Inaya Stone, South East Agronomy Research on behalf of ASHEEP
The P Efficient Pastures project is in its last year this year and has brought some interesting results despite the unfavourable year. This project aims to reduce the
Phosphorus (P)-dependence of Australian temperate pastures by expanding the use of high yielding pasture legumes that have lower fertiliser-P requirements. ASHEEP has two trial sites, one located in Grass Patch and one at Neridup. The sites were chosen
for their sandy, low nutrient soil profiles. The sites were sown to serradella in 2017 and treated with low, medium and high rates of P fertilizer to determine the effect of these treatments on serradella and subterranean clover production.
Whilst 2019 had a very dry start to the season with no summer rain to speak of, there was still a measurable germination at the Neridup site. With the nature of the sandy soils on which the trials were sown, germination rates in both years were relatively low but consistent. Graph 1 demonstrates the yield comparison between the 2018 and 2019 seasons as a percentage of 5 x 5 cm squares occupied in a 50 x 50cm quadrant. The counts clearly show that whilst overall numbers (percentages) were low, it’s clear that the serradella numbers were significantly higher this year compared to last year, with the percentage of cover only varying slightly between P treatments. The lack of significant frequency gain as P increases is promising for reducing the need for P in serradella, but we must bear in mind we have had 2 very dry starts in a row.
Figure 1: Grass Patch site.
Sub clover germination frequency across the two years has been quite poor compared to the serradella, but despite this, Sub Clover showed no significant gains as P increases, which at this early stage also demonstrated a lack of dependence on Phosphorous.
Due to the lack of summer and opening rains, germination at the Grass Patch site was very poor, with some plots having nothing but the occasional cape weed plant. Considering this, it was decided to not conduct the plant analysis measurements, but
instead conduct soil sampling to gain an understanding of P levels down the profile. Cores were taken down to 90cm and separated into 10cm increments in early October.
Results from soil samples taken in spring 2019 are not yet available, however results from the previous two years testing may help explain these results. The two soil core graphs taken at the Neridup site help understand the P levels throughout the soil
horizon. Both graphs demonstrate that P levels drop quickly below 15cm. P levels tend to decline through the sandy profile until a depth of 35cm where there is clay and the levels spike slightly. Soil at depths greater than 35cm are lower in P and continue to decline at depth. It was interesting to see that the P levels in the A horizon were
measured to be far higher in 2018 compared to 2017. One reason why both the Sub Clover and Serradella aren’t performing significantly better following the application of higher P rates is potentially due to the fact that the critical P (Colwell) levels for s Sub clover is 25-30 but lower in serradella due to Serradellas ability to forage deeper in the soil profile and access greater amounts of P. This ability for Serradella to forage to
greater depths is potentially why it is performing better than the sub clover, especially in the low P plots.
The Grass Patch sites appears to have a shorter/ poorer fertiliser (P) history, with low Colwell P in the top 15cm. As a direct result of the lower P history and low buffering (sandy soils) soil analysis shows what P that is in the profile has leached down to 35cm, where there appears to a slight increase below that level where there may be a slightly
higher clay content at depth . Overall, P levels are classified as very low inadequate in this soil.
As a result of the combination of low rainfall autumns, dry, sandy (potentially non – wetting) soils, the pasture species struggled to germinate at the Grass Patch site. The fact that there is also a very low P bank would have also prohibited the germination and growth of legume-based pastures.
To find out more on this project contact eo@asheep.org.au.
