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Top tips to calibrate your fertiliser spreader

As fertiliser can vary in shape and density, it is important a few simple steps are taken to get your spreader calibrated accurately. We find out more with some expert top tips.

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Spreader calibration is essential to get the most out of your fertilisers.
Spreader calibration is essential to get the most out of your fertilisers.

Growing a good grass crop requires attention to nutrition. And just like any other crop it is important to ensure it receives the correct amount. Calibrating your fertiliser spreader is essential before you spread.

 

Richard German, customer service engineer at Sulky UK has been advising customers on calibrating and setting up their fertiliser spreaders for 28 years. Here, he outlines the steps you need to take, before you go spreading this spring.

 

He says; “Each year, the prill size and density of a given fertiliser changes, even from the same manufacturer. Calibrating your spreader each time you change fertiliser type, manufacturer, and ideally, each time you use it, will ensure that you are applying the correct rates at the time of spreading.”

 

The weather also influences how fertiliser behaves. Mr German says; “On a dry, sunny day, fertiliser will flow through the machine and spread better than on a dull, damp day. Where you have vastly different weather on days that you are spreading, it is worth re-calibrating the machine to ensure you are still applying the correct rate.”

 

To do this you need to calibrate the spreader for both the rate in kg/ha (the amount coming out of the spreader) and the spreading width in metres (ensuring the correct overlap and even distribution).


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Step 1: Obtain your settings

Step 1: Obtain your settings

You will need to obtain the spreader settings for your chosen fertiliser from the machine manufacturer, if available. This could be a calibration app, like Sulky’s Fertitest, or another programme or application.

 

You will need to know your planned forward speed, the bulk density of the fertiliser and it’s prill size and type. If you cannot find the settings for the fertiliser listed, you can calculate this manually using a granulometer.

Step 2: Set up machine for rate calibration

Step 2: Set up machine for rate calibration

Fill the hoper with sufficient fertiliser for the spreading you will be doing, or at least 100kg for calibration. Remove one of the discs from the spreader and place the manufacturer-supplied calibration chute onto the machine. Hang a bucket onto the back on the calibration chute.

 

Using a slide rule or similar calculator from the machine manufacturer, calculate the test distance you need to drive for the working width you will be spreading. This is roughly one fortieth of a hectare in area.

 

If you do not have a slide rule or calculator, divide 500 by the working width, to give you the distance in metres.

Step 3: Carry out rate calibration test

Step 3: Carry out rate calibration test

Mark your required distance out in the field, giving yourself space to ‘run-up’ to the first mark, and sufficient space to turn around after the second mark.

 

Set the test opening on the rate slide on the machine, according to manufacturer guidelines. Start tractor, run the PTO at 540rpm, select the gear you will be using and drive towards your first mark in the field. Allow yourself sufficient space to run-up to the first mark to ensure you are at your planned forward speed.

 

When you hit the first mark, open the shutter your bucket is under, and when you hit the second mark, close the shutter.

Turn the tractor off, remove the bucket and weigh the quantity of fertiliser collected, minus the weight of the bucket.

Step 4: Calculating application rate

Step 4: Calculating application rate

If you have a manufacturer’s slide rule, use it with the weight of fertiliser collected to achieve the required rate scale settings for the spreader.

 

If you do not have a slide rule, simply multiply the net weight of fertiliser collected by 40, this will give you the amount of fertiliser being applied in kg/ha.

 

To check this rate setting is correct, repeat the test using the new setting. If the result is a little different from the rate you set, tweak the settings on the spreader up or down accordingly.

 

Remove calibration chute and replace the disc.

Step 5: Perform an overlap check test

Step 5: Perform an overlap check test

Check that the spreader settings are giving you the correct amount of overlap. Use the spread width settings determined in Step 1, set the height of the spreader to at least the minimum required distance from the ground and ensure it is level.

 

Using four trays, place them equally across half the working width on the ground from the centre of the first tramline. For example, for a 24m working width you need the trays to cover 12m, by placing the trays at 3, 6, 9, and 12m from the tramline.

 

With the trays on your right hand side, drive up the tramline with the machine spreading at your chosen settings. Turn the tractor right handed at the top of the test run, and come down the next tramline, still spreading over the trays, which will now be to your left.

Step 6: Collect and assess

Step 6: Collect and assess

Collect the fertiliser from each tray in individual test tube. If you have collected an equal amount in each tray the spread pattern is correct.

 

If you have collected less in the trays nearest the tractor and more in the middle two trays, the machine is spreading the fertiliser too widely, creating an excess of fertiliser in the overlaps. In this case, reduce the working width setting.

 

If you have more fertiliser in the trays nearest the tractors, and less in the middle two trays, you are not spreading wide enough. Increase the spreading width setting to allow it to throw the fertiliser out further.


Impact of prill size and density

Impact of prill size and density

The trajectory of a fertiliser is influenced by its size, type and density. Urea is light for its size and does not travel as far as a denser fertiliser for the same centrifugal force applied to it.

 

Prilled N is denser, with a shape like a smooth ball bearing, which allows it to flow quickly and smoothly through the machine and fly much faster through the air. Compound fertilisers will vary in their size, density and shape.

 

This is why it is important to re-calibrate your spreader each time you change fertiliser type, manufacturer and at the beginning of each season.

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