Trials looking at placing nitrogen fertiliser into the soil have shown some different outcomes. In the southern region adding mid-row banding (MRB) to the crop nutrition toolkit looks interesting. A trial showed modest yield gains and better fertiliser uptake. In the northern region, rate looks more important than how nitrogen (N) is applied.
But, the actual trials were quite different. They used different methods for placing nitrogen into the soil. Those techniques could produce different results even used under controlled conditions at the same site.
The southern trial placed urea in widely spaced bands. This alters the chemistry of the surrounding soil, slowing the release of nitrate-N. So, we can’t directly compare the different outcomes north vs south. Comparing mid-row banded with broadcast and incorporated urea in the north would be interesting.
We do know the risks of nitrogen losses are different in the north and south. Both trial outcomes are consistent with that.
Mid-row banding nitrogen in the south
In the South, Victorian trials found applying in-crop nitrogen by MRB increased yield. But only very slightly. MRB had more impact on fertiliser-N uptake by the crop. Crops took up 20% more fertiliser-N with MRB compared to surface spray or streaming.
Incorporating nitrogen in the north
In the North, surface spreading urea is less risky than we thought. Research in the Northern Region tested different urea application rates, methods and timing. Only rate had a significant effect on yield. Spread and incorporated urea produced similar yields and grain protein contents.
Climate makes a difference
Risk of nitrogen losses
There’s more chance of big N losses in the south. Some N losses happen through volatilisation of ammonia from the soil surface. So safeguarding fertiliser-N by placing it in the soil is more important there. The recent northern region trials have confirmed earlier results from NSW DPI that N losses are a lot lower there. Volatilisation when spreading urea was only about 5 – 19%.
Getting nitrogen into the root zone
Placing N into the soil reduces the need for rain to wash fertiliser into the soil. Surface spread fertiliser needs rainfall to move the nutrient into the root zone. In the North, half the sites had no rain two weeks after spreading, but all sites received rain within 25 days. This seemed to be sufficient. In the South, placing N into the soil worked better than surface spreading. Even when heavy rain followed. In-crop surface spreading did do better when followed by rain, but not as well as placement in the soil.
In the south, crops seem to take up more N from fertiliser. In the southern MRB trial 53–71% of grain-N came from applied fertiliser. In the north only 20–25% of grain-N was sourced from recent fertiliser.
In the north, fertiliser-N isn’t moving down through the soil as fast as expected. Sixty to eighty percent of N applied to a dry soil profile in November-December 2016 was still in the top 15cm at planting. Eighty to ninety percent was in the top 30cm. At harvest, 30-60% of the applied N was still in the soil.
What does this mean for growers?
In the North, N rate is more important than timing or method. Fertiliser-N is more stable in the soil there unless waterlogging conditions exist when large N losses can result. In most seasons, N losses are lower and leaching seems to be less a problem than previously thought. This means there’s less need to split N applications. In the North, there are also fewer opportunities for split applications to be followed up by good soaking rain.
In the south N losses from surface spreading are important. Growers need to reduce their risks by monitoring soil nitrogen and matching fertiliser applications to crop demand. Placing N fertiliser into the root zone for in-crop applications does seem to have benefits.