Soils characteristics can be inherently variable across a paddock, therefore soil analysis results can be substantially different. As part of a National Landcare Project, BCG is aiming to use paddock scale data to inform crop nutrition inputs and the application of nitrogen bank strategies.
At the heart of the project, BCG looked at grid sampling as a way to use variable rate technology to match nitrogen inputs to crop needs and production estimates, however the work has also allowed comparison of some potential strategies growers may currently use, or look to use as alternatives, including whole paddock nutrient assessment, and soil based zone sampling on two paddocks in the southern mallee.
Alleviating nitrogen deficiency in Australian broadacre cropping systems has the potential to increase national wheat yields by 40 per cent (Hochman and Horan 2018). With nitrogen fertiliser a costly input and a lack of accurate seasonal forecasts for rainfall at the decision-making time, it can be difficult to match nitrogen supply with crop demand.
The findings to date have shown that managing a long term N bank strategy during two highly productive years is difficult, as growers within the project look to capitalise on the above average season conditions and stored subsoil water. However BCG project manager, Kate Finger, has highlighted while the intended outcomes have been challenging, there have been many other learnings along the way that can help growers better match nutrient inputs to crop requirements.
During BCGs 2024 Trials Review Day, Kate discussed how nitrogen fertiliser application strategies need to be effective in maximising yields without causing environmental losses through leaching or denitrification. Soil sampling is an important tool to assist in developing the nitrogen application strategy, and getting it right can have big implications on farm bottom lines.
The core approaches of the strategies in question were:
Paddock scale sampling
Paddock variability for this approach is often disregarded, and one representative sample taken. This may sometimes be based on the predominant soil type. Often this is the cheapest approach for sampling/testing costs, but may have higher costs in over/under applying nutrition.
Zone sampling
Multiple cores collected per zone and bulked into one sample for each area and allows some account of variability based on soil type. This approach can be utilised in variable rate application strategies to manage yield potentials in line with soil type. Management zones can be created by utilising suitable information such as visual surface soil differences, satellite imagery, elevation maps or other technology such as EM maps (Electromagnetic induction).
Grid Sampling
Grid sampling is considered a robust and accurate method for measuring spatial variability across a paddock, however is also most costly. Both paddocks during 2022 and 2023 were divided into a 2ha grid with soil sampling undertaken prior to sowing to 60cm depth. Electromagnetic induction (EM) mapping was also undertaken to develop a baseline soil map of the paddocks and help to explain results in line with sub-paddock level variability
Which was the most effective method?
The 2023 research suggested that all strategies have a fit subject to the outcome the grower is looking to achieve, but generally more information = better placement of nutrients. The work carried out was on relatively small paddocks, making grid sampling competitive for nitrogen applications. This would likely change for larger paddocks. It was noticed that the biggest error in nutrient placement occurs when a single paddock sample is used for making whole paddock input decisions. “It comes down to how variable your paddocks are, how expensive the input is that you are using to correct nutrient deficiencies or soil conditions, and whether you have the technology to manage where the inputs are placed, such as using variable rate spreading” Kate concluded.
The recording from the session at Trials Review Day can be found here, and if you need further information on crop nutrition needs and input strategies some great resources featuring crop nutrition expert Rob Norton are below.
An additional recording from Dr. Rob Nortons presentation on crop nutrition at the 2023 BCG Trials Review Day can be found below.
