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Spread accurately and evenly

25 February 2020
Type Media Article

By Dermot Forristal, Teagasc Oak Park

Today’s fertiliser spreaders are increasingly complex with advanced control systems often coupled with on-board weighing and GPS control.  But whether a spreader has complex electronic or simple mechanical controls, its setting and adjustment to ensure accurate and even spreading is absolutely essential.

Accurate and even fertiliser spreading is important to optimise yield and crop/grass quality and to protect the environment.  While there is lots of research on the rates and timing of fertiliser application, we rely on the spreader to apply that fertiliser accurately.  The challenge to spread very variable fertiliser products at bout widths of between 12 and 30m in variable fields is immense.   

Spreading evenly
Spreading evenly across the width is the first priority.  There are a number of steps that will ensure this is achieved.

  • Correct choice of machine for the fertiliser being used at the chosen bout width
  • Use of fertiliser with good physical quality characteristics
  • Correct setting of the machine for fertiliser and bout width using setting resources based on comprehensive tests.
  • Correct setting and use of the machine on the field headland and narrow bout widths

Spreader setting resources – comprehensive testing essential
For wider spreading machines, the importance of the availability of good setting resources, cannot be over-emphasised.  These are generated by the spreader manufacturer carrying out multiple spreading tests at different rates with a huge variety of fertiliser products.  Access to a suitable test hall is essential for this.  Depending on the manufacturer and the machines characteristics, once a basic set of tests are complete, the correct settings for particular fertiliser product can be achieved by size and physical quality matching the fertiliser with some of those already in the database.  There are two ways that this can be achieved; the simpler form involves characterising the fertiliser using information such as: 

  • Particle size distribution by sieving (hand held testers available)
  • Density by weighing a volume
  • Particle strength by crush testing
  • Particle shape by matching to images

Most of the spreader manufacturers use a system like this today.  It’s accuracy depends on how representative the sample is; how sensitive the spreading mechanism on the spreader is to variation in quality; and how comprehensive the manufacturers full spread-test database is.   An alternative approach is to subject a sample of the fertiliser to a standardised ‘throw’ test where the spreading characteristics are assessed and again compared to database information to get a proper setting.

The right machine
Having a spreader capable of spreading the particular fertiliser evenly over the desired bout width is essential and can only be verified by having full spread test information which should show the shape of the spread pattern and verification of even spreading by having a low CV (coefficient of variation).  Test hall CV values should be less than 10% and preferably in the range 2-7%.  Independent test results would provide more complete verification, but these are difficult to secure, forcing us to rely on manufacturers’ information.   It’s often wise to stay well within the claimed limits of the spreader; if the manufacturer claims 27m bout widths are possible, it might be wise to stick with bout widths of 24m or less!

Settings to spread evenly
The adjustments to be made to ensure even spreading with a particular product depend on the design of the spreading mechanism which differs significantly between makes and models.  Note too that the setting appropriate for one model in a range may not be correct for a different model.  The spreader components that may be adjusted to ensure even spreading include:

  • Disc type
  • Disc speed
  • Vane type and position on disc.
  • Position of fertiliser drop point onto disc.
  • Spreader angle to crop and height over crop
  • Disc height over crop/soil, or spreader / disc angle.

Source of setting data
The manufacturers setting database, can be accessed by a number of means.  The operator’s manual shows how the adjustments can be made.  A settings manual is usually still available to give settings for fertiliser types and even specific products.  In recent years there has been a move to either ‘on-line’ or smart phone ‘app’ based systems.  These offer three main advantages:

  • They allow the type of fertiliser being spread to be matched to something in the database by inputting physical characteristics like size distribution, density etc.
  • The database that underpins the setting information can be continuously updated unlike setting ‘books’
  • You cannot mislay the app or on-line website unlike the setting book!

It’s possible to have this information directly available on the in-cab monitor for the spreader also.

Setting the machine
Using these resources is not difficult, but seek help from smart phone-savvy (younger!) people if necessary. There are often two ways in which these resources can be used.  In many cases, the manufacturer may have tested the actual product (including Irish fertiliser) that you are using and the settings are provided on this basis.  However fertiliser can change from season to season and with blended products, even from batch to batch. This makes it very difficult to rely on the database information without verifying that the physical quality matches that in the database.  The second method allows the physical quality characteristics to be inputted.  In an ideal world, this information should be provided for each batch of fertiliser supplied by the fertiliser industry, but typically it will require simple sieve box testing of representative samples in addition to appearance and maybe strength testing.
When the fertiliser data, and the desired bout width are entered, the app or website will then give the setting to use to ensure the fertiliser is spread evenly across the width.  Most of these setting resources will also give an approximate rate setting too if the desired application rate in kg/ha and forward speed are inputted. However the accuracy of these will depend on how well the database sample matches the product being used, and weather conditions, as high humidity will reduce flow rate.

Rate setting / calibration
Getting the correct rate of fertiliser out (kg/ha or bags/acre) is also important and while manufacturers setting guides are a starting point, some level of field calibration is often needed.
Many high-spec spreaders now have on-board weighing systems which allow automatic calibration; just input the desired application rate and the controller will take over from there.   A full calibration requires the fertiliser to be run-through the spreader and flow rate to be calculated, typically with a disc removed.  Forward speed needs to be accurately measured too.  Some manufacturers have very useful setting aids such as calibrated flow bags that guide setting without full calibration.  Field calibration (without on-board weighing) where the area on which a known quantity of fertiliser is spread is measured is possible. But with 500kg bags, a lot of area may be spread before the correct rate is achieved.

Headland spreading
Current Oak Park research suggests that fertiliser distribution in the headland areas of fields is quite uneven compared to the in-field area, contributing to yield loss.  There are two challenges: 1) spreading to the boundary and 2) merging the in-field runs with the headland runs.
To spread evenly to the field margin, manufacturers offer different adjustments such as: deflectors; altered disc speed and fertiliser drop point etc to alter the headland pattern, but these must be set very carefully.
Merging the in-field runs with the headland runs requires the spreader to be turned on and off at a precise distance from the headland.  This can be very difficult with spreaders which throw fertiliser considerable distances.  Accurate GPS systems can automatically control the on/off point making this more precise than can typically be achieved manually, thus avoiding fertiliser waste and crop lodging at the headland.
Finally - The impact of fertiliser spreader use on crop/grass performance and profitability can be enormous.  Fertiliser must be spread evenly and accurately.  Careful setting is essential.

Videos on how to spread accurately and evenly can be found here