Archive Article: 1997/11/08

8 November 1997

GREEN stripes may look good on a lawn but down a cereal field they represent a financial loss and a potential source of nitrate leaching.

Fortunately, improvement in machine design and a better understanding of fertiliser quality have reduced the potential difficulties in recent years.

"There is no room for complacency," warns Ted Crooks, of fertiliser calibration specialists SCS. With three partners around the country, the group tests about 1,500, mostly spinning disc fertiliser spreaders a year, representing about 10% of the UK total.

"Growers are well aware that uneven fertiliser application can result in reduced yield and lodging but visible signs in the growing crop are only apparent when the coefficient of variation (CV), a measure of accuracy, exceeds 20%," explains Mr Crooks.

In other words, at a CV of 20%, actual fertiliser rates can vary by as much as plus or minus 40% from the mean or desired rate. Put another way, fertiliser rates can vary from 60 to 140% of the desired rate.

Enormous variation in fertiliser rate and yield could occur without growers even noticing. "I dont wish to stand in the way of progress, but growers should aim to get the best out of existing equipment and fertilisers before considering precision farming techniques and variable application rates," recommends Mr Crooks.

When spreader testing using a series of trays across the width of the machine, CVs up to 10% are considered good, 10 to 15% may be acceptable according to circumstances, and 15 to 20% not acceptable by either fertiliser or spreader manufacturers. "On-farm CVs of 10 to 20% are relatively common," claims Mr Crooks.

And switching to pneumatic spreaders may not be the answer. "Many growers have been sold pneumatic spreaders on the basis they will spread poor quality fertilisers accurately, and even in the wind, but in practice that is often not the case," says Mr Crooks.

However, a good quality fertiliser, applied by a well set-up pneumatic spreader may be able to work in windier conditions than the equivalent disc machine, and especially at wide bout spacing.

On the other hand, the spread pattern from a well-set disc machine spreading good quality fertiliser can be just as good as a pneumatic and at considerably higher work rates. It should also be remembered that pneumatic spreaders have many more moving and wearing parts than spinning disc machines.

"Accurate fertiliser spreading starts with the right choice of product," maintains Mr Crooks. Spreader manufacturers test their machines with the most commonly available fertiliser types.

Problems arise with imports, especially ammonium nitrates and ureas, the quality of which can vary from year-to-year and batch-to-batch. And with no compensatory setting information from the manufacturer.

Prills are often too small, or irregular in size, or soft, breaking up on contact with the vanes, or already having turned to dust.


"Blends too are not without their problems," he warns. "Theres nothing wrong with a good quality blend consisting of equal-sized particles but spreading problems arise when there is a big size variation.

Poor size matching in a blended fertiliser can also lead to an uneven distribution of nutrients. When opening a big bag, for example, growers should make as wide a gash as possible. If the hole is insufficient, smaller particles will congregate in the middle of the hopper while the larger particles roll to the outside. The ratio of nutrients applied could then differ significantly from one end of the field to another.

"Operators may not appreciate the physical properties of differing fertiliser types," continues Mr Crooks.

For example, Hydro Extran is a granular fertiliser and not a prill so machines should be set accordingly. Blends and compounds may behave completely differently although containing the same nutrients. In a blend containing nitrogen, the grower needs to know in what form is the N source.

Assuming fertiliser quality and loading to be satisfactory, on-farm spreader testing starts with the tractor, not the spreader.

A hand-held digital meter is used to test PTO speed. The majority of spinning disc and pneumatic spreaders are designed to run at 540 rpm.

Occasionally, tractors need to be run at 300 to 400 rpm more than the manufacturers recommended setting to obtain 540 rpm at the PTO shaft. Its hardly surprising spreading accuracy falls with engine speed when going uphill.

Moving on to the machine, Mr Crooks warns it is worth checking the spreader has been correctly assembled and inspected, even from new. He occasionally finds the wrong vanes or discs or combinations fitted.

Shutter openings

Shutter openings on double-disc spreaders must be equal to ensure an even spread pattern. On older machines check the apertures for wear or partial blockage.

Not surprisingly the vanes on spinning disc spreaders are frequently the cause of poor distribution and hidden striping. Abrasive fertilisers wear the surface of the vane.

Even slight wear to the vane can have a profound effect on the spread pattern over 20m or more. Dusty material sticking to the vanes in damp conditions again result in high CVs.

Rather more expensive to rectify, worn gearbox bearings allowing the discs to rock must be replaced.

A well-maintained machine goes a long way to ensuring an even spread pattern but operator adjustments in the field may be just as important. Some machines are designed to run at an angle, with adjustment to the top link to vary spreading width.

Others should be level in all directions and at the right height above the crop so its important to readjust the height setting as the crop grows.

Understandably, Mr Crooks recommends that spreaders be independently checked, ideally every season, using at least one tray per metre of spread. Basic cost per spreader is £125 for tray testing two fertiliser types.

Growers should receive a written report, including computer-generated spread patterns, CVs and a diagnosis of any problems.

Dick Palmer outlines the benefits of proper calibration.

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