Fertiliser applicator manufacturers do provide guidelines and procedures to calibrate equipment. While newer technologies aim to achieve an acceptable uniformity of application, many determine only the bulk application for each hectare. This is a critical omission because poor distribution can significantly affect crop performance and yield as well as increase the risk of leaching losses.
The LandWISE-led project promotes calibration checks that include assessment and correcting of both application rate (kg/ha) and uniformity (coefficient of variation).
Farmers said determining the rate is reasonably easy and commonly done. Some make checks after the first run or first load. Others do the sums only after the job is completed – a bit late to take corrective action. And there can be questions around tonnes applied to the true area of the paddock.
However, few farmers report completing any form of objective uniformity assessment. Some walk across the area to make a visual check although this method’s accuracy is questionable. Acceptable uniformity becomes increasingly difficult as spreader bout widths increase. Massey University’s Miles Grafton has shown that the effects of fertiliser ballistics become critical as throw distance increases.
Trying to spread too wide is a big issue if trying to evenly spread fertiliser blends because different products with different ballistics will spread differently. Indeed, throwing fertiliser a long way is probably the best way to separate out the different products.
Fertiliser application uniformity is described by the coefficient of variation (CV), a statistic focused on variation from the average application rate. CV is considered acceptable if it does not exceed a specified level that is related to agronomic effects. The accepted maximum allowable CV is 15% for nitrogenous fertilisers and 25% for other fertilisers.
Uniformity requires the collection of samples from a spreading event and the calculation of a uniformity value. To account for overlapping it will involve either physical or theoretical overlapping of adjacent swaths.
Internationally, there are many protocols for testing the spreading of fertiliser products. Massey researcher Hayden Lawrence compared six test methods.
The protocols used 0.5-by-0.5m trays organised in a line across the travel path to capture the spread pattern of the spreader. Use of standard test trays is strongly advised given the need for baffling to stop fertiliser bouncing out.
- Dan Bloomer is manager of LandWISE, an organisation that promotes sustainable crop production through leadership, support and research.
Equipment set-up includes making sure the spreader is level and all door and vane settings are correct.
On top of crops
Onfarm testing of the efficacy of fertiliser spreading should use a sufficient number of trays to adequately cover the full application swath. As a guide, trays should be no more than 2m apart and preferably closer to ensure little chance of missing banding effects. About 20 trays will give a good set of data.
It is possible to apply overlapped swaths in the field and measure the overlapped results. An alternative is to measure just one pass then rely on software to do the overlapping. The advantage of this is that a range of different bout widths can be assessed.
Weighing samples is complicated by the small quantities involved – often a single prill in the outer containers. Scales weighing 0.01g are required but satisfactory options are readily available at a reasonable price.
If a larger sample is wanted, two or more runs at the chosen application rate should be made rather than applying at a higher rate. This is because the performance is very dependent on the rate fertiliser is discharged.
There is also some doubt about the percentage of fertiliser caught by some types of tray so a separate calculation of the average application rate is still important.
Online software has been developed to process data and generate performance reports. Key outputs include the measured application rate, the CV at the specified bout width and the bout widths at which CV is within accepted limits. Graphical outputs make it easy to see the spread pattern and the effect of changing bout width.
Spread-pattern checks performed as part of the SFF project show there is a need for wider testing by farmers. Many new machines are just achieving acceptable CVs. Indications are that older equipment cannot achieve high uniformities and there is some indication that incorrect application rates are the norm.
Fertiliser application calibration procedures suitable for farmers applying nutrients with their own equipment allow onfarm checks to ensure and demonstrate application equipment is performing to expectations. They offer an important assurance for nutrient management and compliance and may also aid the self-audit component of Spreadmark.
The On-Farm Fertiliser Applicator Calibration project is co-funded by the Foundation for Arable Research and the Fertiliser Association of New Zealand.
