Applying fertiliser is considered a ‘risky’ decision because of uncertainties associated with a lack of return on investment in the year of application, negative yield responses to applied N or losses of N. This often leads growers to take a conservative approach to their N inputs. The largest difficulty with N fertiliser management is the capacity to match N supply to crop demand and being able to do this when variable climate conditions significantly change the yield potential, and hence likely response to additional N inputs.

Tactical approaches to seasonal N supply

In the past, efforts have often focussed on trying to match seasonal N fertiliser inputs to seasonal yield potential using crop N budgeting approaches. To confidently set the yield target for each year relies on accurate information on soil water status of the crop (at sowing or during the season), seasonal forecasts (which often have limited skill when decisions are being made), detailed soil information (e.g. soil plant-available water-holding capacity), as well as agronomic information such as crop cultivars, sowing dates and starting soil mineral N status. To inform these decisions a range of tools have been produced and used, such as YieldProphet, Whopper Cropper, CropARM, but these are often data hungry and take time to use effectively. Even then, decision makers are often faced with a range of potential outcomes depending on how the season will unfold.

Strategic longer-term approaches to system N management

An alternative to taking a highly responsive approach to the seasonal outlook is to apply a longer-term approach to N management. This could reduce some of the complexity with the decision making and if applied appropriately could reduce situations where grain yield is limited by inadequate N inputs or when N is not located in the soil profile where crops can access it. This approache also often has the advantage of shifting the cost of fertiliser into the year following a high production season when income is likely to be high. However, these more strategic approaches will work best in situations when most of any surplus N is retained in the soil system and there is low risk of N losses (e.g. leaching, denitrification and volatilisation).

Replacement strategy

Replacement strategies are where the crop N removed in grain is applied in the following season to maintain the system in balance. This level of replacement may then be geared upwards to account for losses or downwards if other inputs of N (e.g. legumes) are included in the system. This has the advantage, that once you know your yield and protein you can calculate and budget for the N to be applied to each field or management zone, and potentially spread your risk of application over various times the following year.

Financial strategy

A financial strategy can be used by setting the amount of N to be applied based on a percentage of your net return from the previous year. For example, if you aimed to allocate 10% of the previous income to N fertiliser, following a year where your crop income was $900/ha, you would apply $90 worth of urea in the following season. This approach takes out the influence of fertiliser price, so that in seasons when the cost is high you would apply less and when the cost is low you would apply more. Such an approach could also be applied spatially to target extra inputs into the higher producing parts of the farm.

Nitrogen banking

Nitrogen banking draws on the idea that you want to maintain reserves of N (in the soil bank) available to your crop should it need to draw upon them. ‘N banks’ require growers to set a locally relevant target for crop N supply (soil mineral N plus fertiliser N) that is enough to maximise yield in most seasons. Soil mineral N is then measured (or estimated) prior to, or early in the growing season, and if less than the target N bank, is topped up to the target value with fertiliser N. This approach does not attempt to match seasonal demand each year, but simply requires fertiliser to be applied to meet the N bank target to supply the crop sufficient N to meet its water-limited yield potential in most years. The challenge with this system is working out the N bank target relevant to your production environment, soil and farming system.  

More information and guidance on how to budget N fertiliser and the relative advantages of different approaches can be found here.