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. Table 1 details the N requirement of various crops. Note that to work out the amount of urea (kg/ha) to apply, divide the nitrogen requirement (kgN/ha) by 0.46.
Table 1. Nitrogen (N) needed to meet target yield
| Crop | Grain yield target | |||
| 1t/ha | 2t/ha | 3t/ha | 4t/ha | |
| Wheat | 40kgN/ha | 80kgN/ha | 120kgN/ha | 160kgN/ha |
| Barley | 35kgN/ha | 70kgN/ha | 105kgN/ha | 140kgN/ha |
| Canola | 80kgN/ha | 160kgN/ha | 220kgN/ha | Good luck to you! |
Calculating your N requirement
A back of the envelope calculation that takes into consideration the paddock’s N balance from the previous year (see example 1 below) and then estimates potential balance at the end of the current season 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.
Example 1: calculating the N balance
N balance = (N applied) – (N removed)
= 100kg/ha urea (46kgN) – 3t/ha wheat yield (60kgN*)
= 46 – 60 = –14kg N (-ve)
* It takes 40kg N to grow 1t wheat, but only removes 20kg N/t.
From this example we can see that it is likely that the 14kg N was supplied by the soil, however this suggests that the total soil N has been mined.
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. Working out how much soil N is available using soil test data can be determined with the following calculation:
Plant available soil N = Nitrate (mg/kg) x Bulk density (g/cm3) x (Sample Depth (cm)/10).
The same calculation can be used to determine the amount of N contributed from ammonium. This BCG 2015 SSR – Guidelines for interpreting soil test results provides further details on how to interpret and use soil test data.
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 OC% and spring rainfall, anywhere between 5 to 60kg N/ha could be potentially mineralised. Again, working from soil test results, 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. Table 2 is a guide only. As growers well know fertiliser prices can vary considerably between resellers and also from season to season. Considering the importance of timeliness when topdressing, availability can also influence product selection decisions.
Table 2. Fertiliser comparison
| Product | Composition | Price ($) | Price ($/kg N) |
| Urea | 46% N | $485/t | $1.04 |
| UAN | 42% N | $0.69/L | $1.64 |
| SOA | 20% N, 24% S | $360/t | $1.80 |
| Ranger | 24% N | $0.42/L | $1.75 |
| Sulsa | 26.7% N, 6.8% S | $0.48/L | $1.80 |
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 here.
A great tool for supporting your in-season nitrogen management decisions is Yield Prophet®. For more details phone BCG on (03) 5492 2787.
