Interactions between the form of fertiliser and the type of application equipment can have serious effects on the evenness and accuracy of application. Terrain and the task to be done often dictate the type of application system used – e.g. aerial spreading on steep hill country.

Weather conditions can significantly affect both the containment of fertiliser on the application site and the evenness of application within this site. The importance of weather conditions depends on the form of fertiliser, the application method and equipment used.

Recommendations and nutrient management plans from fertiliser and agricultural consultants assume the fertiliser material will be spread evenly and accurately over the target area at the target application rate. Poor spreading can negate the best management plans and result in significant production losses and pollution of waterways.

Ground based application

Ground based application includes a wide range of application methods to apply a vast array of fertiliser products, requiring careful matching of equipment and technique to the fertiliser and production system.

Spreading operators must understand the spreading characteristics of all products they spread, and how their equipment and equipment settings affect spreading performance. For example, products may be solid (free flowing particles or mass material) or fluid (solutions, suspensions, slurries). Particle sizes in free flowing solid fertilisers typically range from less than 1mm to over 5mm in diameter. When ejected laterally from spreading equipment, particles of different sizes have different ballistic trajectories and therefore variable spreading patterns. Particle shape also varies but is usually near spherical in manufactured products. Particle shape, density and surface roughness all affect the flowability of the product.

There are two broad types of ground based spreading equipment:

• ground based equipment that spreads fertiliser beyond the width of the machine – e.g. bulk spinners
• ground based equipment where the swath width is equal to or less than the width of the machine – e.g. boom sprayers, combine drills, pneumatic top dressers

Factors that may affect ground based fertiliser spreading performance:

• Calibration – application equipment must be calibrated for the fertiliser product to be applied. Different products have different bulk densities and even different lines or batches of the same product can vary in bulk density.
• Slope – the performance of all ground based application equipment is likely to be affected by sloping ground. It is generally preferable to operate up and down rather than across slopes. Unless the spreader is computer controlled, variations in surface roughness may lead to uneven spread as vehicle speed varies.
• Weather, atmospheric conditions – 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.
• Soil conditions – slippery ground conditions can interfere with accurate fertiliser placement. Avoid operating machinery on soft soils where there is a risk of compaction. On slopes, slippery conditions can create a safety issue for the operator.
• Speed of spreader.
• Broadcast (spinners, reciprocating spouts, muck spreaders) – because the material is thrown beyond the width of the machine there is a risk that driver error and wind will make it difficult to keep the fertiliser within the target area and achieve a low CV%.
• Other equipment (e.g. drills, pneumatic booms, boom sprayers) – these are capable of achieving lower CV% results, especially where tramlining and bout markers are used, but only if they are accurately calibrated.
• Irrigators and sprinklers – the volumes applied must be controlled so that nutrients are not washed off the surface or subject to deep percolation through the soil. Application evenness and distribution pattern should be calibrated as for other application equipment.

The application of fertiliser from ground based machinery should comply with the Code of Practice for the Placement of Fertiliser in New Zealand (Spreadmark).

Aerial application

In many situations, aerial application is the only practical means of applying fertiliser. Where fertiliser is applied by air, the minimum acceptable standards for evenness of spreading should be the same as for other application methods used on similar classes of land. Where the risk of environmental contamination is low, higher CV values for evenness of distribution may be acceptable.

Keys to quality aerial topdressing include:

• good communication and direction from land manager
• the calibration efficiency of the equipment being used
• the accuracy of spread
• the skill level of the pilot
• high environmental standards
• use of GPS to achieve higher accuracy of fertiliser placement

The aerial application of fertiliser should comply with the Code of Practice for the Placement of Fertiliser in New Zealand (Spreadmark), Part The Aerial Spreadmark Code.