Excess phosphorus in low pH soils: Should you invest in phosphorus or lime?

A field day at the trial site in 2015

Currently in Western Australia, about twice as much phosphorus is applied to grain crops as is removed in harvested grains. The proportion of unused phosphorus that accumulates versus that lost will vary with soil type, landform and climate.

Nearly 90% of agricultural soils in south-west WA have soil test phosphorus levels above the critical concentrations for crop growth, while about 70% have soil pH values that are lower than recommended levels.

On phosphorus-adequate soils, there is a need to shift phosphorus fertiliser application to maintenance rates, while increasing the application of lime if the soil is also acidic.

A replicated trial was conducted 21 km south-West of Kojonup by Southern Dirt and Professor Richard Bell from Murdoch University between 2014 and 2018.

The trial aimed to examine the concept of a balanced approach to managing inputs that decrease phosphorus application to a maintenance rate, and increase liming to avoid sub-soil acidity.

The soil was a grey sandy duplex that had been under pasture for seven years.

The soil had a Colwell phosphorus level (0-10 cm) of 42 mg/kg, which is well above the critical phosphorus concentration.

Soil pH (CaCl2) was five in the 0-10 cm zone, and 4.5 at 20-30 cm. This compares to minimum recommended soil pH of 5.5 (0-10 cm), and 4.8 in the sub-soil.

Replicated treatments included five phosphorus rates between 0 and 21 kg/ha/yr, with half of the plots limed at two tonnes/ha, and half un-limed, resulting in a total of 40 plots.

Results and Discussion

Over time, soil pH in un-limed plots has declined. Colwell phosphorus levels have remained fairly steady between Feb 2014 and May 2016. Soil test results from January 2018 sampling are not yet available.

Canola yields in 2014 were high (3.4 t/ha) but were unaffected by either phosphorus rates or lime application. There was no significant effect of the phosphorus rates and lime levels on Colwell P after harvest in 2014. Lime increased pH at 0-10 cm depth from 4.9 to 5.5. The pH  of un-limed plots below 10 cm was 4.5 – 4.2. Removal of phosphorus in canola seed was equivalent to 20.5 – 23.8 kg P/ha.

In 2015,  there was again no effect of phosphorus treatments on barley yield. However, lime increased mean barley yield from 4.1 to 4.4 t/ha.

In 2016, lime again increased canola yield by 20 % (0.81 to 0.97 t/ha) while phosphorus treatments had no effect on yield.

In 2017, barley yields increased with liming (4.5-4.9 t/ha) but phosphorus treatments had no effect.

The results are in line with the hypothesis for the experiment.  Given initial Colwell phosphorus levels of 42 mg/kg, which was well above the critical concentration for a sandy duplex, we predicted that crop phosphorus supply would be more than adequate for canola growth. The Colwell phosphorus levels, even with no phosphorus fertiliser added, have apparently continued over four years to be sufficient for yields of canola and barley.

A lime response was not expected in year one, given that pH in the 0-10 cm layer was five.  In 2015, the  two tonne of lime/ha increased barley grain yield, which may be related to either on-going acidification in the subsoil of control plants, the sensitivity of barley to acidity, or to the lower than average rainfall in July and October, which may have accentuated the effects of subsoil acidity on root growth in un-limed plots.  Lime continued to boost canola yield in 2016 and barley yield in 2017.

These results, albeit from a single site, suggest that on acid soils with more than adequate Colwell phosphorus, there is scope for shifting some expenditure from phosphorus fertiliser to lime application.

Acknowledgements

This article was written by Professor Richard Bell and Q. Ma (Murdoch University), and Kayla Ringrose and Emma Russell (Southern Dirt), and edited by South West Catchments Council (SWCC). We gratefully acknowledge the support of Andrew and Rachel Plowman who hosted the experiment on their farm and the funding support from SWCC, through the Australian Government’s National Landcare Program, and CSBP. This project was managed by Southern Dirt with support from SWCC.

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