Sustainable Vegetable Systems progress update - Monitoring wins over leaching model

The Sustainable Vegetable Systems (SVS) programme continues to progress on all fronts with an ever-growing data set to support growers’ nitrogen decisions. Analysing, presenting and incorporating this into the nitrogen model and tool is currently a central focus of the programme.

Trials

A final crop of ryegrass is to be planted in three of the four intensive trial rotations. The other rotation is entering cauliflower before it too is sown with a final ryegrass crop. Analysis of the crop nitrogen uptake and concentrations from the previous crop harvests is underway for incorporation into the plant-nitrogen model. Work is also underway on understanding the drivers of leaching events observed so far in the trials, and on the impact that nitrogen application rates and crop residue breakdown has on soil mineral N.

Regional monitoring  

Regional sampling continues on a monthly basis, with most sites having had their summer crops harvested, and at some sites new crops have been planted. In addition, a winter potato crop will be sampled alongside one of the existing sites to fill a crop gap. Additional pre-harvest biomass sampling is continuing to increase the data available on crop yields and nitrogen content. Site reports on mineral N have been distributed to growers and monitors, and crop nitrogen budgets have been drafted using the sampled biomass data.

Model development

A working prototype (N-Sight) has been developed. This will continue to evolve into the farmer-facing tool.

Dissemination 

Preparation for a series of three videos is underway. These videos include a project overview, a look at the monitoring sites and the nitrogen cycle, and a science story – following the process from trial site to laboratory to data modeller.

N-Sight

Perhaps the most exciting development for the project is the work being done on the prototype farmer facing tool – N-Sight. Examples of the types of outputs generated by the tool are shown in Figure 1, though it should be noted that this tool is very much a work in progress and will go through several iterations before being released to end users. There is currently a lot of discussion about the different ways the tool could be used, including stand-alone and open source so it can be integrated into other providers’ tools.

The tool currently takes inputs for the current crop, the previous crop, and the subsequent crop. These inputs include planting and harvesting dates, harvest stage, yield, initial fertiliser applied, and quantity and treatment of crop residues. Users can also input soil nitrogen test values, as well as the number of post-planting fertiliser applications they plan to make. The tool will then calculate the quantity of nitrogen to apply and the approximate dates to apply it on, in order to match the applied nitrogen as closely as possible with crop demand.

It is recognised that building a tool with lots of inputs will not result in the most user-friendly experience, so the programme is aiming to strike a balance as far as possible between accuracy and useability. The tool needs to function with minimal inputs yet allow those growers that want to tune the tool more accurately to their conditions the ability to do so.

The SVS tool will not model N leaching. This is extremely complex, with an enormous number of inputs and assumptions which build upon each other increasing the uncertainty. The leaching number also becomes the focus when it is more productive to direct time and energy towards better understanding the nitrogen flows that we do have more control over, which in turn will optimise nitrogen fertiliser applications. Nevertheless, nitrogen leaching is a reality that growers need to account for. Simply saying that it is too hard to model doesn’t help anyone. Therefore, the alternative requires soil nitrogen testing.

While a model could estimate the impact of a 100 mm rainfall event on soil mineral nitrogen levels, it is a massive leap of faith to have confidence in what that model is telling you. A representative soil N test (and of course what is ‘representative’ is up for debate) can be used instead as the starting point, or as a reset during the season, to then guide and justify the nitrogen fertiliser programme based on what is there.

SVS will provide growers with the evidence they need in their Farm Environment Plan, in the form of a nitrogen budget, to justify their fertiliser applications.

The programme will be taking onboard the feedback of growers, agronomists, and other industry experts to help guide the tool’s development over the next 18 months.

SVS – Looking ahead

There is much to be excited about within SVS over the next six months. As the intensive trials come to an end a wealth of data will become available for analysis by statisticians, scientists, agronomists, growers and model users. The data will shed further light on the complex relationships between plants and soil, giving us a better understanding of the different forms of nitrogen and how these change over time in the soil. The impact of crop residue breakdown is of enormous interest.

Going into winter it will be interesting to see what changes the wet and cold conditions promote in soil mineral nitrogen stocks across the regional monitoring sites. As the highest risk period for leaching, this is always of special interest.

All this data will continue being fed into the model underpinning N-Sight. With a new community of practice using the prototype, and comparing regional observations and industry experience, the tool will see significant development over the next six months.

First published in the July 2022 issue of the NZ Grower magazine.