Grower group road-tests subsoil phosphorus nutrition

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Image of grower John Cameron

Southern Queensland grower John Cameron is testing the long-term profitability of subsoil phosphorus nutrition in EGA GregoryA wheat on the box clay soils across his property at Bongeen, west of Toowoomba, as part of GRDC-funded trials through the Central Darling Downs Grower Group.

PHOTO: Clarisa Collis

On-farm trials through the Central Darling Downs Grower Group are testing the long-term profitability of deep-banding phosphorus over three winter grain seasons on a less typical, but nevertheless widespread, soil type in southern Queensland.

Research meets adoption in the box clay soils across John ‘Cowboy’ Cameron’s 1400-hectare property at Bongeen, west of Toowoomba in southern Queensland, where on-farm trials are helping to measure the cost–benefits of subsoil nutrition.

Prompting these on-farm trials are the findings of earlier long-term soils research that showed a persistent decline in subsoil fertility on 25 farms from Gunnedah, New South Wales, to Capella, Queensland.

This earlier research led by Professor Mike Bell from the Queensland Alliance for Agriculture and Food Innovation (QAAFI) looked at a range of representative soil types in the northern grains region, then covering northern NSW and Queensland (the GRDC’s northern region now encompasses the whole of NSW).

Of particular interest to many northern growers was the finding that subsoil phosphorus deficiency was limiting grain yields by up to 20 per cent.

Now, John and other members of the Central Darling Downs Grower Group are testing this finding, and the profitability of subsoil phosphorus nutrition more generally, on their own local soil type: box clay, which is known for its low phosphorus content.

Although less typical of the northern grain-growing region generally, this soil type is widespread in some northern farming systems. However, it is only now being rigorously studied through GRDC-funded subsoil nutrition research.

To date, 18 trial plots of EGA GregoryA wheat established on the Camerons’ property last year have demonstrated that deep-banding phosphorus on box clay soil takes time to deliver cost–benefits with the practice actually resulting in a loss in the first season following application. This was due to the high initial cost of application including expenditure on high rates of phosphorus fertiliser, plus the outlay on labour and fuel as part of deep-ripping, which is a separate on-farm operation before seeding.

The first year of trial data showed that the $315/ha cost of applying 40 kilograms of phosphorus per hectare as part of about 185kg/ha of starter fertiliser at depth exceeded the estimated $245/ha net return from EGA Gregory wheat – meaning a $70/ha loss.

In contrast, the traditional practice of applying 8.7kg of phosphorus as part of 40kg/ha of starter fertiliser near the soil surface was shown to be more cost-effective. Using this approach, investing about $32/ha in a starter fertiliser containing phosphorus provided a net return of about $358/ha.

So the conventional practice of applying starter fertiliser phosphorus with the seed was about $113/ha more profitable than deep-banding phosphorus in the first year where all deep-banding costs were attributed to that first crop.

However, phosphorus fertiliser placed deep in the soil profile is known to provide benefits for up to four years and the trials on the Camerons’ property are examining the cost–benefits over three winter seasons from 2014 to 2016.

The expectation is that deep-banding phosphorus will prove, ultimately, to be profitable in the longer term.

John says the preliminary trial findings have been encouraging as they showed a significant yield response to subsoil phosphorus applications “in tough seasonal conditions”.

Although rainfall was well below average in the Bongeen district in 2014, applying 40kg/ha of phosphorus about 22 centimetres into the soil profile on 50cm row-spacings provided a 79 per cent yield increase in EGA Gregory wheat.

The conventional practice of applying 8.7kg/ha of phosphorus in 40kg/ha of starter fertiliser with the seed, about 5cm into the soil, provided a 47 per cent yield increase – almost half the yield benefit of deep phosphorus applications.

Although the return from investment in deep-banding phosphorus may be slow to emerge, the significant yield lift demonstrated in the 2014 trials, which is expected to continue over three to fours years, should offset the initial application costs in the long term.

Preliminary data from the 2014 trials also suggests that subsoil applications of phosphorus at higher rates may provide almost twice the yield benefit of phosphorus applied at low rates near the surface as starter fertiliser.

However, these yield gains were found to be linked to higher rates of phosphorus removal by the crop. Analysing the trials is another member of the Central Darling Downs Grower Group, Bede O’Mara, who is also a Toowoomba-based agronomist from Incitec Pivot. Bede says the trials showed increased concentrations of phosphorus in grain, which suggests that the applied phosphorus had made its way into both the plant and grain, and that plants drew more phosphorus from the soil where higher rates were applied deep in the soil.

He says this finding highlights the need for growers to monitor phosphorus removal rates using grain analysis to help determine the optimum frequency for deep-banding phosphorus.

For example, growers might need to reapply phosphorus at depth in the soil after three high-yielding seasons, whereas the nutrient supply might persist for four years in the event of low-yielding seasons.

Bede says another lesson from the 2014 trials was that applying phosphorus fertiliser in bands that follow the same direction as the crop row was more profitable than applying it in bands at 90 degrees to the crop row.

The yield response where phosphorus was deep-banded directly beneath the crop row demonstrates the importance of placing immobile nutrients, such as phosphorus, in the root zone.

“Placing phosphorus in bands close to the crop row is important for crop access, and helped provide the maximum yield benefit in the first crop following application,” Bede says.

A cost-saving measure that helped reduce John’s initial application costs was a homemade linkage chisel plough, converted into a seeder, which he used to deep band phosphorus beneath his crops rows as part of the trials on his property. This dual-purpose set-up cost about $15,000, and John says it takes just 10 minutes to switch from seeding to deep fertiliser applications.

“It basically involves adjusting the height of the coulter gauge wheels so that the planter tynes penetrate 22cm into the soil, instead of the 5cm when seeding crops,” John says.

To help avoid breaking tynes when applying phosphorus at depth in the soil, he slowed the average speed of his operations to about five kilometres per hour, compared with an operating speed of 9km/hour at planting. However, this meant the machine used twice as much diesel for deep-banding than seeding: “The machine uses about 3.5 litres of diesel/ha at seeding, and about 7L/ha was needed to pull the same machine when deep applying phosphorus,” John says.

More information:

John Cameron
0427 037 212;

Bede O’Mara
0417 896 377

GRDC Project Code CLI00001

Region North, South