THE placement of added phosphorous to top up depleted subsoils has delivered immediate yield returns in trials on John Cameron’s Darling Downs property, Kintyre, Bongeen.
Native fertility in some of the black and grey cracking clays of the northern grains region has been depleted by years of continuous cropping, and traditional methods of applying immobile nutrients like phosphorus (P) to the topsoil in minimum tillage and dryland systems have failed to replenish stores below the top 10 centimetres.
However, preliminary trial results are offering farmers new hope, with high levels of P placed at depths of 20cm using conventional farm machinery returning yield increases in the first year that match the outlays for the one-off investment.
“The bottom line is that the preliminary trial results show that the cost of applying deep phosphorus and the additional yield achieved was about break even in the first year, despite the dry seasonal conditions,” Mr Cameron said.
“What we’re interested in now is whether there are any on-going benefits to be realised in years two and three and four and beyond thanks to a one-off investment of P at depth.”
The Cameron family dryland farms about 1000 hectares of owned, leased and contract farming country.
They follow a strict rotation of long fallow into summer crops of dryland cotton or sorghum, then double crop back to wheat before a return to a long fallow period.
Mr Cameron has been soil testing all his paddocks to a depth of one metre for more than 20 years, providing a valuable historical benchmark of background nutrient levels across his operation.
However, when he took on a new block in recent years the testing regime revealed a paddock with severe P deficiencies. In the top 10cm the site recorded a Colwell P score of less than 5 and a BSES-P of around 10, and for both test phosphorus was hardly detectable in the 10-30cm bracket.
It prompted him to offer the site for a deep fertiliser placement trial coordinated by the Central Downs Grower Group with support from Incitec Pivot and the Grains Research and Development Corporation (GRDC).
A winter wheat crop was planted across the site on June 3, 2014, with a full moisture profile.
The control plot received no starter fertiliser, while the balance of the area received the ‘normal’ application of 40 kilograms per hectare of Starter Z (approximately 9kg/ha of P) with the seed at sowing.
The high-P areas then received a further 180kg/ha of Starter Z at a depth of 20cm for a total P application of 49kg/ha. The fertiliser was placed using a conventional winter crop planter with Flexi-coil chisel plough tynes and narrow point opener.
Despite only receiving one useful fall in crop – 22mm in the first week of August – the results were clear.
The control plots with zero fertiliser yielded an average of 0.94 tonnes a hectare; those with the standard application of 40kg/ha of Starter Z averaged 1.46t/ha; and those with the standard starter fertiliser plus the P supplement at depth averaged 1.78t/ha – 80 per cent better than the control plot.
“I suspect the real difference may come in the next crop in the rotation,” Mr Cameron said.
“Our intention now is to monitor yields and quality over time in the rotation.
“It wouldn’t be something you’d want to do every year due to the costs – we need now to monitor P run down over time to understand what the payback period is and how often we need to repeat the treatment.
“I suspect that a lot of our industry has adequate P levels but there are also a number of soil types that don’t have adequate P levels, and unless we can work out what the economic responses are versus the soil test levels, we really won’t know where we’re going.”
Mr Cameron said the trials results reinforced the importance of soil testing to depth across all paddocks of the farm. He said many Downs growers had become frustrated that they were not seeing yield responses to P, despite long-term small applications at surface level – a sign that their background nutrition levels may already be adequate.
Close analysis of soil test results would also allow farmers to better manage nutrient applications, he said, by matching the application rates of P to other essential nutrients, particularly nitrogen (N), in line with the adjusted yield potential.
“The suspicion is that we’ve been mining it at depth and depositing it through trash back onto the surface,” Mr Cameron said.
“During the growth phase of our crops when we actually need it, the surface is actually dry and our crops are not able to access that nutrient if it is stranded up on top.
“The ambition now is to put it further down the soil profile so that it may be in a position where there’s moisture the crops can access for their growth.”
Mr Cameron’s soil test data has also prompted a rethink of his fertiliser investment strategy on the remainder of his country, which is not P deficient.
“That’s the beauty of having a 20-year history of soil testing – we can see that our background levels are not dropping on the majority of our soil types,” he said.
“We’re now searching our soil tests a lot closer and where appropriate we’re dropping P out of some of our program in selected crops, while keeping an eye on our base levels all the way through.”