On many farms around the country, the same settings are used year after year to spread certain rates of fertiliser. This often leads to inaccuracy as the density and flow rate of the fertiliser can change between two bags from the same pallet.

There are many differing systems to help set the rate correctly. The removal of a disc and the weighing of the fertiliser that falls into a bucket in 60 seconds and some simple calculations can give large improvements in rate control for a small investment of time. This is often more difficult as the spreader ages due to corrosion of the disc-removal mechanism. And more harm may be done to the pattern and rate during the removal of the disc than would be gained by doing the test.

As an example, if the flow rate for the machine is measured at a particular regulator setting the rate per hectare can be using the formula below.

Fertiliser calibration formula

Application rate kg/ha = 600 x flow rate (kg/min)/forward speed (km/h) x bout width (m)

In the case of a twin-disc spreader, the flow rate should be doubled.

Bogballe has the S-indicator, which enables the flow rate of the fertiliser to be assessed using a hanging funnel separate from the machine. Vicon has an app where you input the density of the fertiliser and it calculates the setting required. Most of the manufacturers have online resources where you can enter the compound being used and it will suggest what setting will give the correct rate from the machine. These online and app-based tools are useful as a guide but do not take into account differing qualities of fertiliser and also the weather. Dampness in the air can greatly affect the flow of fertiliser so, if possible, any test should be carried as close as possible to the time of spreading.

At the higher end of the market, many of the spreaders are now self-calibrating. This is possible as there are weight cells on the machine and once the rate is set on the in-cab monitor, the programme constantly uses the manual formula to constantly update the regulator for the correct rate to be spread. This is done by using the weight cells to constantly update the flow rate, a forward speed sensor and an input bout width. This type of machine is becoming the norm across the country.

Spreaders are now starting to move away from the mechanical drive systems we are used to, where the PTO shaft turns a gear assembly and the discs turn at a constant and equal speed.

This is the next step in hydraulically driven discs and will allow for not only much greater control of disc speed but also the ability to run each disc at a different speed to modify the spread pattern, similar to section control on a sprayer.

The spread pattern can also be modified by differing the drop-point of the fertiliser on to the disc. If the fertiliser is dropped closer to the centre of the disc it will leave at a greater speed giving you a wider spread width.

Both the variable speed disc drives and the differing drop area combined can give the high accuracy required to match up to the data that can be gained these days. Yield mapping from combines, and sensor data and drone sensor data can be overlaid on to field maps and then with a combination of the latest technology available on spreaders the fertiliser can be targeted where it is needed.

All of this extra data brings more complexity to the spreading process and the “brains” behind these machines need to be constantly updating the rate that the fertiliser is falling on to each disc separately. Whether calibrating a single-disc manual spreader using a bucket and a piece of paper or inputting figures into a screen, calibration of your spreader is time well spent.