Does BSES P testing have a fit?

Phosphorus (P) is an essential nutrient for plant growth. It plays a significant role in photosynthesis, respiration, energy storage and several other processes. A deficiency in P often leads to delayed maturity and fewer tillers in cereal crops.

As P deficiencies are difficult to correct after sowing, P inputs are most effective when applied at seeding which involves a significant up-front cost for growers. To ensure P is applied at the appropriate rate, soil testing is imperative.

There are many tests available to analyse soil, all of which assess different pools of P. Colwell P measures soil solution P concentration as well as some of the P sorbed onto soil particles. It is the most common P soil test used, and has good reliability in predicting responses to added P fertiliser. The DGT-P test measures the rate of supply of P from the soil and has been shown to give more reliable P response predictions than Colwell P on calcarosol soils.

The BSES-P was developed by the former Bureau of Sugar Experimental Stations (BSES) for acidic subtropical soils and is a test of the level of acid extractable P. This test extracts solution P and sorbed P, as well as slow release P reserves from minerals and phosphorus fertiliser products (Moody et al. 2013). The difference between BSES-P and Colwell-P is that the former indicates the amount of slow release P reserves in the soil.

Research published in 2013 compared multiple soil tests. The crop P response relationship for different soil tests was measured in a wheat crop. This comparison, which used 164 soil samples from field P experiments, showed that on calcarosols DGT-P had the most reliable response prediction and also performed slightly better than Colwell P on other soils (Speirs et al. 2013). The soil data used for this comparison was from the Better Fertiliser Decisions for Cropping (BFDC) database. BSES-P performed similarly to Colwell-P on all soils and slightly better on calcarosols.

This research investigated the ability of DGT-P, Colwell-P and BSES-P soil tests to predict the response of wheat to the up-front application of P. The trial was designed specifically to discover whether BSES-P testing could form part of the soil testing regime in the Victorian Mallee. A high P status from BSES-P test (relative to Colwell-P) could indicate that slow release P may be sufficient to compensate for deficiencies in the more readily available P reserves. If deemed useful, such tests might allow growers to more accurately predict rates of P requirements for crops.

TAKE HOME MESSAGES

• Used in conjunction with Colwell-P, BSES P soil analysis gives an indication of low availability P pools in soils.
• Poor seasonal conditions which produced low trial yields (<0.6 t/ha) meant that no conclusions could be drawn on the reliability of the use of these soil tests as the additional P did not result in further gains.

AIM

To assess the viability of BSES-P testing as a guide to phosphorus application in wheat in the Victorian Mallee.

TRIAL DETAILS

Location: Jil Jil
Soil type: Clay loam
Annual rainfall: 253mm
GSR (Apr-Oct): 160mm
Crop type/s: Mace wheat
Sowing date: 28 May
Seeding equipment: Knife points, press wheels, 30cm row spacing
Target plant density: 150 plants/m²
Harvest date: 13 November
Trial average yield: 0.48t/ha

Trial inputs

Fertiliser: refer to Table 1
Herbicides: Pre-sowing: TriflurX® @ 1.5L/ha + RoundUp® PowerMAX @ 2L/ha + Sakura® @ 0.18L/ha
In-crop: Lontrel™ Advanced @ 1.5L/ha + Velocity® @ 670ml/ha + ENHANCE® @ 0.5%

METHOD

A field trial was sown using a randomised complete block design. Assessments included establishment counts, tiller counts, crop biomass and grain yield and quality parameters.

Soil sampling was conducted on transects of three paddocks identified as being low in P and samples assessed for their suitability for the trial. The site was chosen based on responsive Colwell and DGT P test results, combined with a higher BSES test result. The selected paddock was then sampled within the trial location to ensure reliability of the starting P reserves (see Table 2). Soil sampling depths were 0-15cm and 15-30cm which are deeper than standard depths of 0-10cm. Consequently, soil test results (Table 2) need to be interpreted in terms of this deeper sampling which will show lower soil test values than shallower tests. This is because P is not very mobile in the soil and is at its highest concentration in surface layers.

Fertiliser treatments (Table 1) were applied as urea and MAP. The urea was broadcast prior to seeding to balance the N supplied with the MAP.

Weeds were managed to best commercial practice to reduce effects on yield results.

Table 1. Trial treatments (fertiliser inputs).

RESULTS AND INTERPRETATION

Even when adjusted for the deeper sampling depths, soil tests results (Table 2) suggest that for Colwell
P, PBI and DGT-P the site had a very low P status, but that quite large amounts of slow release P, as indicated by the difference between the BSES-P and Colwell-P tests, were present.

Table 2. Soil test results.

VL = very low status, L = low status, M = medium status.

There were no differences at emergence due to P application (data not shown), but tiller counts were lower for the Nil treatment than where P was applied (Table 3). This was to be expected as poorly tillered plants are a known consequence of low P levels (Glendinning 2003). There was no difference between the varying P rate treatments.

At anthesis (GS65), no significant differences in crop biomass were observed between the treatments. Similarly, although visual differences between treatments were noted, NDVI assessments carried out at GS30 and GS65 recorded no significant differences.

Similarly, grain yield and quality parameters revealed no significant differences among treatments (Table 3). Given the obvious visual differences throughout the growing season, it is apparent that the seasonal conditions, with very poor spring rainfall and hot weather at flowering and grain filling, prevented any P deficiency from being expressed in biomass and grain yield. Al ternatively, the P available from the slow release P reserves countered the deficiency highlighted in the Colwell, PBI and DGT P test results.

Table 3. Treatment grain yield, biomass and tiller counts.

COMMERCIAL PRACTICE

The difference between the Colwell P and BSES-P test results for this site suggested that there were significantly low P availability reserves, principally in the topsoil. The other assessments (PBI, Colwell P and DGT-P) suggested that the crop was likely to respond to added P, but additional P did not have an effect on yield.

These results should be treated with caution, as the season was limited by an unfavourable spring.

Further research should be carried out for this particular soil type, to gauge the benefit of BSES-P testing.

ON-FARM PROFITABILITY

Using Colwell P and BSES-P tests can indicate the amount of P reserves in different pools for Wimmera and Mallee soil environments. BSES-P costs $8.80 per sample. A small cost to more accurately gauge P levels could have significant economic benefits in the form of more precise fertiliser applications.

REFERENCES

Glendinning JS (2003) Australian Soil Fertility Manual, 33.

Mason S (date unknown) Soil phosphorus by availability, https://soilquality.org.au/factsheets/dgtphosphorus, date accessed: 1 December 2015

Speirs DS, Scott BJ, Moody PW, and Mason SD (2013) Soil Phosphorus tests II: A comparison of soil test crop response relationships for different soil tests and wheat, Crop and Pasture Science 64, 469-479.

Moody PW, Speirs SW, Scott BJ, Mason SD (2013) Soil Phosphorus tests I: What soil phosphorus pools and processes do they measure? Crop and Pasture Science, 64, 461-468.

ACKNOWLEDGEMENTS

This trial was funded by BCG members. BCG acknowledges the GRDC More Profit from Crop Nutrition initiative through related research including the Better Fertiliser Decisions for Cropping database.

A. Elliott, T.McClelland, R.Norton., 2015, BCG Season Research Results ‘Does BSES P testing have a fit?’. pp no. 93