Conventional wisdom says that a higher yielding crop needs more of the major nutrients than one that's less productive. But what if that's not the case?
Researchers are attempting to answer that very question with a three-year project to assess just how much nitrogen and phosphorus is needed by modern cotton cultivars.
"Over time cotton varieties have become more productive, and cotton growers are wanting to know whether the nitrogen and phosphorus requirements have also increased as yields have increased over time," said University of Queensland soil scientist Dr Tim McLaren.
University of Queensland agronomist Professor Mike Bell said much of the yield increase has been attributed to modern cotton varieties more efficiently converting their efforts into bolls and lint, rather than vegetation.
"The plant physiologists tell us that we're not really growing a lot bigger plant, but we're yielding a lot more," he said.
"The critical question is, if you're not growing any more biomass, do you really need all this extra fertiliser that people think they're having to put on?
"We're testing whether there is a clear link between the yield you get out of the crop, and the amount of fertiliser that it takes to deliver that yield."
They said being able to reduce inputs of major nutrients such as nitrogen and phosphorus would have benefits for the grower's bottom line, as well as reducing how much was lost through denitrification, leaching and other processes that can negatively affect the environment.
The pair are analysing data from the first year of field experiments at Gatton, in the Lockyer Valley, and Hopeland, near Chinchilla, which has given some interesting albeit preliminary results.
Dr McLaren said the main "take home messages" from the nitrogen trial at Gatton were: extra nitrogen produced more biomass, but there appears to be little difference in biomass production between plots with different yield potential, and split application of nitrogen generated a significant increase in cotton yield compared to that applied upfront.
They had originally hoped to test conventional cotton, an old transgenic variety and a modern variety with naturally different yield potentials, but weren't able to obtain seed for all three.
"We were also told the difficulty of managing a conventional cotton would have been horrendous," Dr McLaren said.
Instead, they used one modern cultivar and tested two ways of manipulating yield potential at both sites: using short periods of shading to reduce boll load, and spraying with the plant growth regulator Pix to limit light interception.
"We weren't too sure how effectively we could manipulate yield potential for the same variety growing in the same seasonal conditions, without physically getting in there and having to pull bolls off, which is almost impossible," Prof Bell said.
"Ultimately the Pix treatment proved ineffective. However, by putting shade cloths over the crop for 10 days and then taking them off, we simulated cloudy conditions which growers tell us often causes the crop to throw squares and bolls and reduce yield potential.
"We were able to drop yield potential from say 14 or 15 bales, down to about 11 to 12 bales."
In the nitrogen trial, plots with different yield potentials received one of eight different rates (from zero to 350 kilograms of nitrogen per hectare in 50kg increments). The urea was applied all upfront or split, with half applied pre-planting and half in-crop.
The phosphorus trial was based on increasing concentrations of Colwell-P in the soil 0-10cm and 10-30cm layers, which were established in a previous project.
Dr McLaren said they expected to indirectly gain some insights into the fate of nitrogen that wasn't taken up by the plants, but direct tracking would require the use of stable isotopes which was beyond the scope and budget of the project.
"Without isotopes you can tell that the crops got different amounts of nitrogen or phosphorus, but you can't tell whether that nitrogen has come from the background soil fertility or from the fresh fertiliser that you've just applied," Prof Bell said.
"Typically, in the year of application we would see ... fertiliser nitrogen recovery at anywhere between 20pc and 40pc of the nitrogen that's applied in cotton.
"Where's the rest of it going? Some of it goes into that soil mineral nitrogen pool, some into the soil organic matter pool, and some of it is lost to the environment, but you can't tell the proportions unless you can specifically track the fertiliser itself."
Prof Bell said the prime focus of the project, which is funded by the Cotton Research and Development Corporation, was to identify what fertiliser rates were needed for different yield potentials.
"That's where we'll have another crack in year two," he said.
"And the phosphorus work will be focused no longer on residual soil phosphorus, but will be looking at fresh rates of fertiliser phosphorus, just like nitrogen.
"At this early stage we have no real evidence that you need to have higher rates to achieve higher yield potentials, but it's very preliminary."
Statistical analyses from the first growing season are still underway, although turnout figures have yet to be received due to queues for ginning, which have been exacerbated by late harvesting of the large national crop.
Planning is underway for the second year of field experiments.
