As the time for top-dressing approaches, the threat of an El Nino developing this season is challenging growers as they make their in-crop nitrogen (N) management plans.
While it is important to consider the seasonal forecast and the impact it may have on harvest expectations, growers should not forget the yield potential that currently exists. Stored soil moisture varies dramatically across all districts and soil types, so blanket recommendations may not be the best approach.
The northern half of Victoria has received above average autumn rainfall this year, which will motivate more optimism in the Mallee. However, regardless of where you farm, before topdressing, it is worth considering the following:
What is your yield potential?
Before deciding on how much N you need to apply to your paddock, you first need to have a sensible expectation of your potential yield (t/ha). This can be determined by: gut feel and experience; ; or crop modelling systems such as Yield Prophet®. Whatever your view is on the season – conservative or optimistic – in some way how much N you eventually decide to top-dress will be ultimately based on your personal attitude to risk.
Identifying your target yield (e.g. 2.5t/ha) will make determining the amount of N required easier. In general, without any further N applications, the water and nutrients currently stored in the soil should deliver at least 0.5-1t/ha.
Calculating your N requirement
A back of the envelope calculation that takes into consideration the paddock’s N balance from the previous year and then estimates potential balance at the end of the current season (N balance = N applied – N removed) is the simplest way to determine . To do this, you will need to know what the yield was of the previous crop and the amount of N applied during the season. Remember, it takes about 40kg of N to grow one tonne of wheat but it only removes of 20kg N, so for a three tonne crop we can assume 60kg of N was removed.
How do soil tests help?
Soil test results are very useful when making N decisions. Sampling that occurs closer to sowing will give a more accurate indication of the crop available N as it takes into account any mineralisation that occurred during autumn. In-crop sampling can also provide some useful information but you will need to factor in what the crop has already taken up.
What about mineralisation?
Usually viewed as a bonus, can provide a ‘buffer’, allowing you to take less risk to achieve the yield potential. Depending on your soil’s organic carbon (OC%) and spring rainfall, anywhere between five to 60kg N/ha could be potentially mineralised. Mineralisation can be estimated using the following calculation: Mineralisation = Organic Carbon (%) x Growing Season Rainfall (mm) x 0.15
What Nitrogen product do I use?
When selecting your top-dressing product it is worth considering its composition and how much nitrogen you are getting for the money spent. Considering the importance of timeliness when topdressing, availability can also influence product selection decisions.
More Profit from Crop Nutrition
BCG, through the GRDC’s More Profit from Crop Nutrition (MPCNII) initiative, is working on delivering tools and training to growers and advisers that will help them to make better fertiliser decisions with the aim to increase the efficiency and profitability of their businesses. More information about this project is available at: www.bcg.org.au/cb_pages/grdc_current_funded_projects.php
Another great tool for supporting your in-season nitrogen management decisions is Yield Prophet®. For more details phone BCG on (03) 5492 2787.
