Trailed fertiliser and lime spreaders, many of which are used by contractors, with a capacity of 5 tonnes or more have a conveyor belt in the bottom of the hopper which carries the material to one or two spinning discs at the rear.
Some models have a hydraulic motor drive for the floor conveyor, while others are chain driven from a land wheel or a cage wheel running on one of the spreader wheel tyres. The latter arrangement maintains a constant relationship between forward speed and conveyor speed to ensure a consistent application rate is achieved, although ground speed and field conditions may vary. The spinning disc or discs are driven by a hydraulic motor. Application rate is altered with an adjustable shutter which releases fertiliser onto the discs and by the speed of the floor conveyor. The adjustable shutter also shuts off the flow of fertiliser when turning on the headland. Spreading widths vary from 10 m for lime up to 15 m for granular fertilisers.
Oscillating spout or pendulum broadcasters.Mounted and trailed broadcasters with a swinging or oscillating spout spreading mechanism are an alternative to the spinning disc broadcaster. The spout, which oscillates through an arc, is power take-off driven through an eccentric unit.
The spreading width varies between 6 and 18 m depending on spout length, the height of the spout above the crop and litre weight of the fertiliser. As the fertiliser is thrown from the machine with less force than from a spinning disc the litre weight of the fertiliser has a significant effect on the spread pattern.
Some models of oscillating spout broadcaster distribute the fertiliser over a narrower width than a comparable spinning disc machine and this limited width of spread makes these models unsuitable for top dressing cereal crops with widelv spaced tramlines.
The hopper capacity of mounted swinging spout broadcasters ranges from 250 to 1,500 kg. Trailed models have a capacity of 2 or 3 tonnes. Application rate is varied with an adjustable shutter in the hopper and tractor forward speed. A calibration kit is provided with the machine.
Special spouts can be used with this type of broadcaster suitable, for example, for spreading narrow bands of fertiliser in orchards and border spreading to keep fertiliser away from ditches.
Using fertiliser broadcasters.Mounted broadcasters must be held rigid on the three-point linkage with external check chains or stabiliser bars. It is important to set the machine so that the hopper is level and the spinning disc or the oscillating spout is the correct height above the ground. These settings should be checked in the instruction book. Failure to follow these instructions will affect the width of spread, which in turn may cause inaccurate matching between bouts.
Tramlines make it easy to achieve the correct overlap on adjoining bouts. When there are no tramlines, and when working on bare soil, marker stakes placed across the field at the required intervals are the only certain way to achieve accurate matching between bouts. High lift attachments are made for most broadcasters. They are used to increase the height of the spinning disc(s) or spout by about 300 mm when top dressing cereals and other tall crops. The hopper can be lowered to its normal level for filling.
Checking application rate.Environmental considerations and the high cost of fertilisers call for its accurate application. Instruction books give the settings for a wide range of application rates, which can be checked, or calibrated, in the farmyard.
Most spinning disc broadcasters are supplied with a calibration kit consisting of a bag or container that is attached to the machine while it is run for a period of time specified in the instruction book. The fertiliser collected in the container is weighed and a simple calculation converts this weight into the rate per hectare.
Some broadcasters can be electronically calibrated in the field without catching and weighing the fertiliser in a bucket. One design has weigh cells linked to a computer which monitors the flow of fertiliser from the hopper. After the required application rate and initial shutter setting are entered, the computer constantly checks and maintains the correct flow of fertiliser to achieve the required application rate.