In the third and final part of his 'Back to Basics' article on fertilisers, Scotts International Technical Manager, Stuart Staples, advises on selecting and calibrating a spreader and tackles common problems with fertiliser application
There are basically two main types of granular fertiliser spreaders; drop and rotary.
Drop spreaders, as the name suggests, drops the fertiliser granules through holes along the bottom of the hopper. Altering the spreader setting will in turn alter the size of these holes and the flow rate of fertiliser and, therefore, the application rate. These types of spreader are best for precision application over smaller areas and, for this reason, they are perhaps not the best choice for larger areas such as sports fields.
Great care needs to be taken with the walking pattern and getting the correct amount of crossover on each return run, since fertiliser is either applied at 100% rate or 0%.
This means that if the walking pattern is not correct, you will either have small stripes where no fertiliser has been applied resulting in yellow turf or, where fertiliser has been applied at twice the desired rate, stripes of very dark turf and possibly scorching.
Rotary spreaders are more suitable for applying fertiliser to larger areas and are more commonly used in the maintenance of sports fields. Instead of the fertiliser dropping straight down from the hopper onto the turf, the granules will first hit a spinning disc(s) that propels the granules over a wide distance and so speeds up the application process.
Accurate swath widths are also very important with rotary spreaders but are less critical than with drop spreaders. This is because the amount of product applied gradually decreases at the edges of the swath width, so there is no on/off line as with drop spreaders making misapplication errors less visible.
Rotary spreaders may be tractor mounted or pedestrian propelled. The choice depends on the desired accuracy and the total area to be covered, combined with your fitness and enthusiasm!
Selecting a spreader
There are many factors that contribute to a well-designed and constructed fertiliser spreader but, for turfcare professionals, there's only one aim; the accurate application of your selected fertiliser. So, what is important?
The most important thing is the compatibility of your spreader with your selected fertiliser.
From a practical perspective, they should be comfortable to operate and well constructed for longevity. Wheels should have flotation tyres to minimise the impact to turf, especially when using it with a full hopper.
The best rotary spreaders have some kind of mechanism that can alter the spread pattern according to the different physical characteristics (particle size and bulk density) of the available granular fertilisers. If your spreader does not have this feature, it can only accurately apply one type of fertiliser.
Within the turf industry, the most popular type of rotary spreader that can be adapted to accommodate a wide variety of fertilisers with different physical properties uses a helical cone to adjust the spreading pattern.
The Scotts AccuPro 2000 and SR-2000 models are popular examples of this patented technology. By adjusting the helical cone, the location of the fertiliser coming from the hopper to the spinning disc can be adjusted and, therefore, fine-tuned to give a consistent spread pattern and left/right swath width.
How fertiliser selection can affect spreader performance
The characteristics of your selected fertiliser can have a big impact on the performance and reliability of your spreader.
Bulk density will affect the weight of product flowing through the aperture. Denser material will require less granules to flow through versus a product with the same particle size that is less dense.
Granule size is very important. Larger, heavier granules will travel further once propelled by the spinning disc of the rotary spreader and therefore altering swath width. Conversely, smaller, lighter granules will not travel so far and the swath width will be reduced. Different settings would be required for fertilisers with different particle sizes.
If the fertiliser you have selected has a wide particle size range, some granules will travel a long way from the centre line while others will not. If these different sized granules are also sources of different nutrients (i.e. some nitrogen and potassium) then it is possible that striping could occur following application, even if the overall rates are correct, so a narrow particle size range is essential for the uniform application of nutrient across an area.
Dusty fertilisers affect application rates and also affect how the operator uses the spreader and when, for example in windy conditions or when other people are in the vicinity.
Calibrating your spreader
Spreaders should be calibrated prior to use. Even if there are recommendations on the bag, or in brochures and on websites, it is important that you calibrate your spreader with the products that you are using. The advice in literature or on the bags are guideline recommendations only, and you need to calibrate your spreader and setting to your individual equipment and conditions.
The calibration process is relatively simple:
• First, check your spreader over visually to ensure all parts are present and correct and in good working order.
• Work out your walking speed by timing yourself over a set distance, do this three times to get an average.
• Set out calibration trays to catch the full width of fertiliser as it is spreads. Walk through the trays at the correct speed with the spreader applying fertiliser.
• Weigh the fertiliser from the trays and assess the spread pattern, this will tell you how wide apart you should walk.
• Put a known quantity of fertiliser into the hopper and spread it in a straight line. You can then measure the distance you have covered and work out your application rate.
• Adjust the spreader apertures to the required setting for your required application rate.
If you prefer, you can get a specialist company to calibrate your spreader for you. Companies such as Spreader and Sprayer Testing Ltd offer an MOT-style check and calibration of your application equipment.
Fertiliser application still needs some human intervention, and with this another variable is added.
Walking speed will have a big impact on application rates, so it is important to note what speed is advised per spreader. Guideline settings are normally based on 5km/h (3mph) which is considered normal walking speed.
Increasing walking speed will mean that for a given time a greater distance will be covered, but since the flow rate does not change according to speed, this also means that the application rate per m2 decreases.
This is further compounded by the fact that, with increasing walking speeds, the spinning disc will also increase in speed and, with this, the distance that the granule is thrown from the spreader increases.
The result is that the same quantity of material is being spread over a wider distance (and also forward distance) meaning a reduction in application rate. The reverse is true if you decrease your walking speed so, if you're doing a lot of spreading in one day, remember to recalibrate yourself regularly!
It's important to try and keep the spreader parallel to the ground so that an even spread pattern is achieved. Tilting the spreader too far forwards results in the application of a lot of material in front of the machine, creating a band of over-application within the centre of the spread pattern.
To increase the uniformity of application, it is best to apply the fertiliser in two passes at half rate, as this minimises the risk of over/under applying due to incorrect spreader spacings. These two passes should run at half spacings run parallel to each other to ensure the best spreader pattern over-lap.
Double passes with the second pass conducted at right angles is also practiced, however, incorrect overlap can still occur on both passes resulting in a 'checkerboard' effect.
Always empty the remaining fertiliser out of the spreader and return it back into its original bag, sealing it tightly to prevent moisture entering.
Clean the spreader in an appropriate area using either compressed air or water to ensure that the aperture, helical cone and spinning disc are all clean with no remaining debris. If the cone or spinning disc is not cleaned following application, it can alter the spreading characteristics for the next application. Ensure the spreader is dry before using it again to avoid the product forming clumps.
Some common fertiliser problems and causes
Problem: Dark stripes
Cause: Swath widths to narrow/close
Problem: Light stripes
Cause: Swath widths too far apart
Problem: 'Checkerboard' effect
Cause: Double pass at right angles used with incorrect spacings (too near/too far apart)
Problem: Tiger stripes in turf
Cause: Walking too slowly with rotary spreader causing a curved band of fertiliser to be applied
Problem: Spotting on turf
Cause: Application rate too low (various reasons for this)
If you'd like advice on your choice of fertiliser or guidance on applying it, contact your distributor rep who will be pleased to advise you.
A list of Scotts distributors and the contact details of the Scotts Turf & Amenity technical team can be found on the website at www.scottsprofessional.co.uk.