With a varied start to the growing season across the southern region, questions are being raised about chasing yield with in-season applications of fertiliser nitrogen.
In most cases, the amount of nitrogen a grower applies is estimated from the expected yield and crop demand for nitrogen, and the anticipated soil nitrogen supply.
Fertiliser nitrogen makes up any difference between demand and supply and nitrogen applications should be dictated by a realistic estimate of the yield, according to International Plant Nutrition Institute regional director Australia and New Zealand Dr Rob Norton.
He says every nitrogen decision involves risk — particularly seasonal risk with either losses in wet seasons or lack of demand by the crop in dry seasons.
In making nitrogen decisions, Dr Norton said it was important for growers to revisit the 4R nutrient stewardship principles to help them achieve effective and efficient nitrogen use: right time, right rate, right place and right source.
It is common for growers to apply urea ahead of rainfall as a way of reducing volatilisation losses.
Previous research in the Wimmera and Mallee regions of Victoria has shown losses of up to 23 per cent from top-dressed urea, which can be halved when there is rainfall within a day of application.
‘‘Soil texture, wind speed, crop cover, stubble load, soil organic matter and temperature all affect the rate of volatilisation,’’ Dr Norton said.
‘‘Where crop cover is high, temperatures cool and windspeeds low, the amount of nitrogen volatilised can be relatively small.
‘‘In addition to volatilisation, poor responses to top-dressed urea often occur in the absence of rain because it becomes stranded in the dry top-soil where roots cannot access it.’’
Dr Norton says the main difference between making nitrogen decisions for wheat and canola is that canola can still show yield responses to late nitrogen applications, while late nitrogen applications on wheat will most likely end up as protein.
Nitrogen budgets reviewed with yield estimates, such as from Yield Prophet, are important in estimating nitrogen demand.
The nitrogen budget should include estimates or measurements of the supply of nitrogen from in the soil at seeding plus mineralisation during crop growth, while accounting for any potential losses due to volatilisation and/or leaching.
‘‘The rate can be determined based on having adequate nitrogen in the crop by anthesis to match the yield and protein target,’’ Dr Norton said.
‘‘A 3.5tonne/ha grain yield will probably come from a biomass at anthesis of 7tonne/ha and to meet an 11 per cent protein target, the crop will need around 140kg of nitrogen per hectare.
‘‘If the post-anthesis conditions and yields are better than predicted, then nitrogen will be diluted by the extra growth and grain protein declines.
‘‘If conditions and yield are worse, then grain protein increases,’’ Dr Norton said.
Dr Norton said there were a number of different methods growers could use to get nitrogen into the crop.
Among these methods are putting it all up-front (either top-dressed before sowing or banded at sowing), inter-row banding pre-crop or in-crop or top-dressing in-season either as dry fertiliser or as liquid.
‘‘In most in-crop situations, the nitrogen is top-dressed over the top of the crop,’’ he said.
‘‘But there is some exciting new work from Agriculture Victoria looking at mid-row banding of nitrogen in-crop, with good results.’’
Previous experiments on an alkaline vertosol soil have shown losses to ammonification of 23 per cent for urea, 12 per cent for liquid UAN, and 12 per cent for sulphate of ammonia during a nine-day period between application and a light rainfall event.
Other experiments showed similar nitrogen uptake 10 days after applications at growth stage 31, or first node stage, with UAN applied with streaming nozzles, urea solution applied with flat fan nozzles and dry urea top-dressed.
‘‘There are also many other factors at play rather than just selecting a nitrogen source to get more nitrogen into the crop and then achieving a profitable yield response,’’ Dr Norton said.
‘‘Fluid fertilisers offer the opportunity to combine fertiliser and chemical, thereby reducing paddock traffic as well as giving options for additional nutrients such as sulphur or micronutrients.’’
The GRDC recently invested in a review of the nitrogen decision support tools available to growers and advisers, to help determine which were most useful in the southern region.
Many of these tools were developed prior to the advent of continuous cropping, when legume-based pastures were a significant contributor to soil fertility.
Such tools included simple rules of thumb, Excel spreadsheet models, stand-alone mineralisation models for computers, paper-based models incorporating wheels or slide rules, and applications or apps for smartphones and tablets.