Factors affecting spreading performance

There are many factors that affect the rate and uniformity of fertiliser distribution during an application spreading event. Some are readily controlled, others not.  Correct machine maintenance is critical. Calibration of equipment helps ensure an acceptable application rate and distribution are achieved.

Machine settings

Machine settings that control application rate and spread pattern include:

• Machine travel speed
• Machine alignment – horizontal axes
• Discharge door opening
• Discharge height
• Disc rotation speed
• Disc vane settings
• Where fertiliser drops on to the disk
• Oscillating spout size and shape
• Spout oscillation rate

Application conditions

Application conditions also impact spread pattern. Important factors include:

• Humidity and temperature
• Wind speed
• Wind direction
• Slope/Ground contour

Product characteristics

Fertiliser is a highly variable product. Spreading rates and patterns are affected by many factors that affect the particle ballistics, including:

• Fertiliser density
• Particle size
• Particle shape
• Moisture content
• “Flowability”

Factors affecting placement performance

Machine settings

Machine settings that control application rate and spread pattern include:

• Machine travel speed
• Machine alignment – horizontal axes
• Discharge door opening
• Roller condition
• Duct design and settings
• Fan speed
• Drop tube design and condition

Application conditions

Application conditions also impact spread pattern. Important factors include:

• Humidity
• Ground contour
• Soil condition especially wetness

Product characteristics

Different products have different physical properties that affect how they flow through application equipment. Even, rounded particles may flow freely. Uneven, irregular products may flow less easily, especially if inherently sticky. Some fertiliser materials are hygroscopic, i.e. they absorb water from the atmosphere. Changing temperatures and humidity during the day can affect their flow rate through machinery. Drag (wind resistance) restricts the distance a particle can travel horizontally. The effect changes with particle shape, size and density. If particles cannot travel far enough for satisfactory overlap, there will not be even coverage. When differences can be seen as striping in a crop, the yield impact is large.

The Massey University Centre for Precision Agriculture has completed a number of fertiliser distribution studies. These have helped explain why spreading issues are being experienced.

One Massey study (Grafton, 2014) showed that the higher the fertiliser particle velocity, the more particle ballistics affect the distance they travel. Some common blends such as potassium chloride and ammonium sulphate have particles with very different ballistic properties.  This can lead to the products separating if spread together. Urea and potassium chloride appear to have similar ballistic properties, although quite different shapes and densities, so they may be suitable to blend (see Table below).

A 30m swath spacing (bout width) requires a total spread pattern of about 45m for overlap to achieve the uniform application required. This means fertiliser particles must spread 22.5m in each direction. To propel particles 22.5m from a height of 1.5m above ground, they must be ejected at around 60ms^-1 (216km/h). At these speeds blended fertilisers separate because their rate of deceleration through drag force is based on (in order of importance) particle density, size and shape.

Spreaders are pattern type tested by manufacturers to ensure a pattern overlap which delivers the minimum spread variation possible, given the fertiliser particle parameters. Even using manufacturer recommended settings, problems arise when fertiliser particles are not uniform. This is particularly the case with blended products. Their particle density, size and shape all differ, so the products have different ballistic properties.

Fertiliser ballistic differences start to become significant at between 30m/s and 40m/s, for common materials with different physical properties. Therefore, unless mixes have similar properties then blends should not be sown at bout widths much greater than 20m. Applicators contemplating spreading at bout widths greater than this should test their spreader for the material being spread and use homogenous products, as proprietary mixes are likely to separate.

Impact of striping

Crop striping is a significant issue when spreading blended fertilisers at wide swath spacing. The latest generation spreaders increase recommended swath spacing from about 20 – 24m to 30m or more. This reduces the number of tram lines and crop damage and makes application much faster. However, striping problems are seen when very wide throw equipment is used, even if set-up correctly.

Striping is only visible at in-field coefficient of variation (CV) of around 40%.  This gives a yield reduction of at least 20%, (Mersmann et al, 2013) and (Yule and Grafton, 2013) so has large economic impact. The economic impact of striping increases exponentially as in-field CV increases. That means that if CV doubles, the economic loss increases by four times. If CV increased three times, the economic loss would increase nine times.

Conclusion

Farmers and farm advisors must be aware of the ballistic properties of the products they use and recommend. Blending products with differing ballistic properties will increase the in-field CV and this will have an adverse impact on yield.

In many situations the cost saving of increasing bout widths will be less than the reduction in income by reducing yield by increasing the in-field CV of the spread. Reducing the number of applications by blending fertilisers may also be a false saving.

* Grafton, M.C.E., Yule, I.J. and Robertson, B. 2014. The Ballistics of Separation of Fertiliser Blends at Wide Bout Widths.