Over-application of nitrogen (N) fertiliser to Australia’s cotton crops has proved a headache - and a wallet ache - for growers in recent years as they try to maximise yield while minimising risk in Bollgard II cotton production systems.
But a new user-friendly test kit is set to make it easier for growers or consultants to estimate how much fertiliser to apply - saving fertiliser, money and CO2 emissions.
Many growers “err on the side of caution” when fertilising cotton crops because of the risk of lint loss from too little N, says Agronomic Business Solutions (ABS) agronomist Chris McCormack.
But much added N is wasted.
“Recent industry data from 147 irrigated sites from Hay in Southern NSW to Emerald in Queensland found that 74 per cent of irrigated cotton sites were below optimum range for nitrogen fertiliser use efficiency, showing that an average of 40kg/ha too much N is applied,” says CRDC CottonInfo technical specialist, Jon Welsh.
The quest for the optimal N application rate can be complex and exasperating.
Many variables influence plant-available N such as mineralisation, cropping history, soil type, slope variability, and waterlogging.
Yield target, underlying soil health, and a grower’s experience should also be factored in to N decision-making, says Agronomic Business Solutions agronomist Chris McCormack.
To achieve the optimal rate growers must take note of such variables for a ‘tailored fit.’ It’s certainly not a ‘one-size fits-all’ solution.
Pre-plant soil testing and in-crop plant tissue testing (petiole or sap testing) are currently the key measurements used to determine how much fertiliser is needed.
Now a user-friendly online model and test kit aims to take out a lot more of the guesswork.
With CRDC funding, UQ researcher Dr Francois Visser and his team are close to trialling the N Optimisation Management Tool, which aims to help growers improve the efficiency of N applications on cotton farms.
Their research looks specifically at the role of N mineralisation.
“Mineralisation is a significant source of N for the crop but mineralisation is the one area that we haven’t really looked at in enough detail,” Dr Visser says.
“Farmers have also recently indicated it is the one area where they’d like to have some more information and understand how they can maximise the contribution of mineralised N.”
N mineralisation occurs naturally when microbial activity breaks down organic matter and supplies N to the soil. Healthy soils supply more mineralised N naturally, thus reducing the need for added N fertiliser.
Dr Visser says more than half of a cotton crop’s N can come from the soil and mineralised N. This is based on research carried by the late Dr Ian Rochester where they found that at an optimum fertiliser rate of 175 N at the trial site, the average crop uptake N was 247 N. Therefore at an average 50 per cent fertiliser loss rate it appears that fertiliser N may only have contributed 35 per cent of the N taken up by the crop.
“If mineralised, N can provide 50 per cent of what a crop needs under normal growing conditions, therefore coming from the soil itself … it can be a major source of N for your crop,” Dr Visser says.
“And it’s obviously a cheap source of N and also a more stable source of N because applied fertiliser N may be susceptible to losses and can create harmful greenhouse gases in the form of nitrous oxide.”
Every year up to half the average 100,000 tonnes of N fertiliser applied to Australia’s cotton crops can be lost through a number of pathways including leaching, tailwater run-off and gaseous losses to the atmosphere as N2O emissions, according to research undertaken by Dr Rochester at the Australian Cotton Research Institute.
Dr Visser and his team are busy developing a methodology for the online N optimisation model.
As part of the methodology they are validating a test kit that can estimate the N mineralisation potential in the soil under normal growing conditions, to assist growers when planning their fertiliser applications.
Growers can then make a more accurate prediction about how much N can come from the soil, and therefore the balance of fertiliser that needs to be applied.
Importantly, the online model will include financial implications as well. Updated regional data sets from the 2015 Cotton Gross Margin procured by ACRI Research Economist Janine Powell and CottonInfo’s Jon Welsh add a fresh dimension to the model features.
“The online model allows the farmer to look at various practice changes in terms of what the effect of N efficiency will be,” Dr Visser says.
“But it will also track the cost and profit implications of these different practice changes.”
This summer, Dr Visser is undertaking sampling in all of Queensland’s cotton growing areas with the aim of benchmarking their research and developing the methodology, so growers have more confidence in environmental and financial outcomes of their nitrogen budgeting.
He expects both the online model and test kit to be available this year.
Previously, the UQ team developed a Carbon Calculator for the cotton industry, also with CRDC funding.
“That is now being upgraded into two tools: the Whole-Farm Carbon Management Tool and the N Optimisation Management Tool,” Dr Visser says.
“This new tool is very much complimentary to the N budgeting software such as Nutri-logic. The more confidence growers and advisors have in these complex decisions, the greater the accuracy of product rate and the less impact on crop gross margins and the environment” says Jon Welsh.
Grower Peter Watson, who grows 300–500 hectares of irrigated cotton between Boggabri and Narrabri NSW, is doing his own research with the help of agronomists.
He is in the second year of N efficiency trials on his property and says the impact of N wastage is “significant”.
“Along with all the (greenhouse gas) emissions and things like that, there are lot of downsides to putting on extra N, especially if you’re not getting any benefit,” Mr Watson says. “It also has an impact on the finances.”
While results of the N trials so far are mixed, Mr Watson is keen to continue trials because of the downsides he sees to N over application.
He has already changed his practices as a result.
“Last year our average rate of N across the farm was 330kg. This year was 260kg,” Mr Watson says.
“Last year was enough to show me we were putting too much nitrogen on.”
