Soil and plant testing efficacy in practice
Grower surveys conducted by GRDC in 2016 indicate low adoption of soil and plant testing data to inform nutrient decisions, highlighting an opportunity to assist growers to improve their nutrient decisions and create better crop outcomes.
The wider industry often promotes the benefits of using testing to guide nutrient decisions both pre and in-season, however soil testing rates have remained quite low.
Agronomy Solutions is leading efforts to increase soil testing rates in a GRDC-funded project running until 2021. We are working with AgCommunicators, Eyre Peninsula Agricultural Research Foundation, Hart Field Site Group, Mallee Sustainable Farming and Southern Farming Systems, Australian Precision Ag Laboratory (APAL), the CSIRO and Landmark to reach growers and advisers across South Australia, Victoria and Tasmania.
Eyre Peninsula Agricultural Research Foundation,
Hart Field Site Group,
Mallee Sustainable Farming,
Southern Farming Systems,
Australia Precision Ag Laboratory (APAL), CSIRO and Landmark.
South Australia, Victoria and Tasmania
The project incorporates a range of extension and communication activities to gauge the barriers to adoption and provide education opportunities. An intensive, high-impact soil and plant testing demonstration program is being conducted with growers interested in improving fertiliser returns. The economic value of nutrient management practices in terms of increased nutrient use efficiency, including savings where soil nutrient levels are high, will be widely promoted throughout the duration of the project and beyond its completion.
Ultimately, the investment aims to improve nutrient management best practice through the increased use of soil testing and provide grain growers in GRDC’s southern cropping region with the confidence, knowledge and ability to make more effective and profitable nutrient management decisions.
The project is in its early stages with grower surveys and workshops conducted in early 2019. Soil testing of trial sites to measure pre-sowing soil N and P was completed in February and March 2019.
Soil testing was conducted pre-sowing in 2019 to get baseline information for our trial sites. Landmark’s Riley (left) and Charles (right) are pictured soil sampling Elmore and Bannockburn, Victoria.
Now that most winter crops in the southern region have been sown, the nutrient assessment focus moves to plant testing, especially for trace elements.
Soil analysis is not particularly accurate for trace elements, but plant tissue testing is a much more reliable indicator of deficiency for these nutrients.
Plant sampling can also provide a useful check of the uptake of other nutrients and the effectiveness of fertilisers applied at or near sowing time.
“For phosphorous, plant tissue testing is a gauge of whether the fertiliser application at sowing delivered enough nutrient and whether the plant has been able to access the fertiliser,” Dr Mason says.
“Unfortunately, rectifying any phosphorous deficiency cannot be done in-season, but it is useful information for following seasons.”
When plant tissue testing, Dr Mason recommends taking samples at growth stage 30, or the end of tillering.
“For nitrogen, GS30 is just before a crop’s peak nitrogen demand so if a laboratory can offer a suitable turnaround time, growers can make a quick decision on rates and whether they need to apply more or less nitrogen just before the crop really needs it.”
Similarly, trace element deficiencies identified early in the season can be rectified with foliar sprays.
When taking plant tissue samples, it is recommended to take samples from good and poor zones of the paddock to help diagnose causes of any variable crop performance.
Plant Sample Collection
“Taking whole plants is recommended for accurate nitrogen and phosphorous measurements because these are highly mobile within the plant and deficiencies will show up in the older leaves,” Dr Mason says.
In contrast, trace elements tend to be immobile in plants and symptoms show up in the youngest leaf. Previously, it was recommended to collect the youngest leaves to accurately diagnose trace element deficiencies, but whole plant samples also provide a useful measure of trace element status.
“Nowadays we recommend taking whole plants for a snapshot of all nutrients,” Dr Mason says.
Check with your laboratory or agronomist for their preferred sampling method.
Some recommend collecting all the plants on rows either side of a 30 centimetre ruler and repeating this three times in each of the pre-determined monitoring zones. Cut the plants off at ground level with clean scissors or secateurs and include all above ground shoot material in the sample.
Others recommend pulling up a predetermined number of plants (e.g. 15-25) randomly over the sampling zone and then cutting off the roots.
Record the growth stage of the crop, time between emergence and sampling, the row spacing and seeding rates, so that biomass production and plant nutrient status can be calculated.
Samples should be placed in clearly labelled bags and sent by express post to the chosen laboratory. Only use paper bags, often provided by the laboratory, as samples tend to sweat and decompose if plastic bags are used. As with soil samples, plant samples can be stored in the refrigerator for a day or two if they can’t be mailed immediately after sampling.
Plant testing should not be conducted within 36 hours of a frost or heat shock event, or herbicide application.
Key Nutrient Levels
Dr Mason says critical soil and plant nutrient levels are determined by interpreting databases that contain data from replicated field trials.
He says the bulk of soil and plant testing trial data is for wheat, though recent research efforts have aimed at improving the database for barley, canola and other crop types that have become popular options in current rotations.
“The Better Fertiliser Decisions for Crops database is a powerful tool which allows advisers to view thousands of field trials to identify critical soil nitrogen, phosphorous, potassium and sulphur levels,” Dr Mason says.
“It is important to note that critical plant nutrient levels will change according to crop stage, due to nutrient dilution as the crop grows, and potentially between years at the same growth stage given seasonal conditions and variations in dry matter production.”
GRDC’s soil testing for crop nutrition fact sheet states that when considered in combination with information about target yield, available soil moisture, the previous year’s nutrient removal and soil type, soil tests are essential to making accurate fertiliser decisions ahead of each season. Soil tests are best conducted over summer or early autumn.
More information on soil testing is contained in an expanded version of this Paddock Practices on the GRDC website.
‘Using soil and plant testing data to better inform nutrient management and optimise fertiliser investments for grain growers in the southern region’ is being led by Dr Mason and Harm van Rees in conjunction with Australian Precision Ag Laboratory (APAL), CSIRO, Landmark, Hart Field-Site Group and AgCommunicators, with support from Mallee Sustainable Farming, Southern Farming Systems and Eyre Peninsula Agricultural Research Foundation.
GRDC research code: 9176604
Please summarise the protocol for the 2019 sampling strategy?
Each grower/agronomist will select six paddocks to be sown into wheat in 2019, with two sampling zones selected in each paddock. Within the two zones, a one-hectare (100mx100m) area should be sampled.
What is the sampling frequency and sampling depth?
Sampling frequency is the numbers of cores taken from within each zone. When deep sampling, 6 cores will be taken within each zone. When shallow sampling, 6 small cores (0-10cm) should be taken around each deep core.
The sampling depth is the depth under the paddock surface at which soil samples are collected. The sampling depths in this project are 0-10cm, 10-30cm, 30-60cm and 60-90cm. A step-by-step guide to soil sampling is available on pages 6-7 of the Protocol document.
What will be measured?
For the soil testing component, the following will be analysed: pH, EC, OC, Mineral N, DGT P, PBI, Colwell P, Al if pH is less than 5.5 (surface soils – 0-10cm). The following will be assessed when plant cuts are taken but not during soil testing: K, Ca, Mg, Na, S, Cu, Mn, Zn, B, Fe and Al.
Other information that will be recorded at each site includes:
- Row spacing
- Stubble type, stubble management and the approximate amount of stubble present when soil sampling
- Sowing date or emergence date if dry sown, cultivar, sowing rate, fertiliser type and rate used at sowing, emergence date if no rainfall at seeding
- Sowing equipment
- In-crop fertiliser type, rate and location
- Daily rainfall for four weeks post N spreading
- Frost and heat shock events during flowering to grain fill
What if farmers don’t have 6 paddocks of wheat? Is there flexibility to allow for another variety being sown?
Wheat is the preferred variety, though barley is another option but needs to be reported so recommendations can be altered slightly.
Another option is, non-cereal paddocks can be sampled for soil test data only, but these paddocks cannot be used for fertiliser strips. In this case, try to pick paddocks going into wheat next year where we can re-sample.
Grain protein measures – will this be done?
Grain protein measures are not included in the initial budget but can be measured on request with potential savings from soil analysis (e.g. 60-90cm). For those with technology on headers, that can measure protein, the data is encouraged.
Depth sampling by horizons – what is the standard?
Some agronomists are keen to perform sampling by horizons to better capture soil constraints. This is fine as long as the N bucket is captured. The sample set from this is expected to be small.
Do photos of deep cores need to be taken?
This is a suggestion which will help with interpreting soil test results at depth. Take photos of deep cores if possible.
Liquid vs granular set ups?
The type of fertiliser application setup is something that will be taken into account when making P recommendations. This should be noted in the data entry.
Do all paddocks need to be going into wheat?
Wheat is the preference, though barley is another option but it needs to be reported so recommendations can be altered slightly.
Another option is, non-cereal paddocks can be sampled for soil test data only, but these paddocks cannot be used for fertiliser strips. In this case, try to pick paddocks going into wheat next year where they can be re-sampled.
Further analysis of soils – some agronomists and growers were potentially keen to get some further analysis performed at their own cost after seeing the trial results.
For soil testing promotion purposes, all project samples will be archived in the case of further analysis being requested by agronomists or growers.
With regards to soil sampling, should samples be taken on the inter-row, on the row, or a combination?
It is suggested that you concentrate on the inter-row when sampling because that is where the crop is going. Soil testing protocols suggest a 1:10 ratio of row to inter-row sampling.
Plant tissue sampling – when?
Two plant tissue timings for P and N is not practical. Given they are both only contributing data to the project and not affecting recommendations for that season, we have agreed that GS30 is most sensible.
Are there any sensitivities with when I do the plant tissue sampling?
Yes, it is advised that plant tissue sampling not be conducted until at least 36 hours after a stress event such as frost or extreme heat.
The project involves a number of outputs to ultimately give growers and advisers a good understanding of nutrient management decision making processes and the benefits to their operation.
Economic framework detailing likely returns from investing in improved nutrient management practices.
An economic framework analysing the value of soil and plant testing will involve survey and workshop analysis, farm-scale assessment via fertiliser test strips and existing literature.
The framework is designed to be used together with large scale on-farm paddock trials to demonstrate how fertiliser decisions informed by test results can lead to increased profitability. Results from this trial program will be included in the ongoing development of the economic framework.
Intensive and high-impact soil and plant testing demonstration program including 100 growers from South Australia and Victoria.
A precision agriculture and farm management platform will be used for data management, trial paddock setup and detail procedures for soil and tissue testing. This will ensure consistent results are gathered from the 100 growers with six paddocks each to be sown to wheat in 2019. Of these trial paddocks, 50 per cent will have a fertiliser strip trial included. Throughout the growing season there will be N and P nutrient strip trials and tissue sampling and grain yields will be collected to assess nutrient responses.
Promotion of nutrient management best practices using key influencers of practice change in varying communication and extension activities.
Initial focus group workshops for growers and advisers participating in the trials have sought feedback around soil testing. This series of workshops outlined the CSIRO-developed economic framework established and protocols for soil sampling, demonstration trials and data collection.
A survey of more than 200 growers will also provide insight into current attitudes of growers and advisers towards soil and plant testing.
Some of the other projects we’re working on