From now on, the fields will be busy with the application of chemical fertiliser to replenish used nutrients in the soil. Nearly all fertiliser spreaders in use in Ireland work off the principle of broadcasting fertiliser from either the spinning disc with veins, or the timeless pendulum wagtail spreader. The latter-mentioned wagtail spreader is really only suitable for 12m working widths due to mechanical limitations.

Some designs have inward turning discs while others have outward spreading rotations. But, if set correctly, every type of machine is technically capable of achieving a good spread pattern. The big variable is both the weather and the quality of fertiliser. Higher working widths place more demand on the broadcast ability, especially when trying to get fertiliser to travel over 24m. Many manufacturers when promoting their machines will use data from test halls on spread patterns. This practice is perfect for them to demonstrate that the machine design is effectively working and is capable of achieving the target spread pattern. In reality, many fields in Ireland and the variable weather are a million miles off the conditions in a test hall. The only way for any contractor or farmers to check their machine for accuracy is to perform a tray test in the field.

The tray test

To get the best overall picture of the spread pattern, a series of trays should be placed across the working width. Ideally, the more trays the more accurate reading can be obtained. The trays should be of uniform size and with the same type of baffles placed inside to collect the broadcasted product. Collection of the fertiliser into the funnels needs to be carefully done, as only a small few grains dropped will have an influence on the overall result.

The net result of this test is to get a coefficient of variation (CV) value. In the fields, results of 10% and under are good and results of 5% are excellent. The other side of the spectrum is that results of 20% upwards can have a financial loss with stripping or lodging in some cases in the cereal crop.

Spreading in windy conditions

In what started out to be a test to examine the ideal spread pattern, the gusty conditions didn’t inspire any confidence that we would get any fair result.

The day of the test was typical of a showery and windy day, where a lot of contractors or farmers could decide to apply fertiliser.

In the end, the decision was made to carry on and see what happens when the weather conditions are less than ideal. To be fair to the spreader manufacturer and fertiliser company, they will not be mentioned in the report.

The fertiliser used all had a particle size between 2mm and 5mm and required a 4kg breaking force to shatter the granule. This meets the desired standards, and the fertiliser was deemed of good quality. The machine likewise, was fitted with new veins on the disc, and calibrated.

The first pass was made over 24m, with 13 trays laid down to collect the fertiliser. It was obvious that the trays behind and to the side of the tractor had more fertiliser, when compared with the trays in the centre of the pass. The result calculated for the first pass was a 35% CV. This result would show clear signs of stripping in the field later on. For the second run, the working width was reduced to 21m, using the exact same fertiliser and machine. The CV dropped to 21%, showing that trying to get coverage at 24m in windy conditions was beyond the limit of both the machine and the fertiliser product. Unfortunately, the weather deteriorated further, forcing us to cancel the test at 18m. It could be expected that at 18m the CV could have dropped to 15% or under in the same conditions.

Overall conclusion

Borderline weather days with moderate wind will affect the spread pattern on wide widths over 24m. If conditions are less than suitable, reducing the working width – particularly in grassland – is useful to maintain spreading quality. In these conditions, 18m appears to be the threshold for the fertiliser product. Top-dressing of cereal at 24m and upwards should be avoided in these conditions.

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Special focus: fertiliser