Fertiliser is applied to maximise the production of pasture, crops or trees. It is always assumed that it will be spread evenly over the entire area, at the required rate.
A completely uniform spread is hard to achieve over a whole paddock. This is because the application rate (from both groundspreaders and aircraft) is usually highest directly behind the machine, and diminishes outwards on either side. This is called the swathe pattern, which is typically triangular. It is produced by the combined effects of the size of the fertiliser particles and the way they are discharged. It may also be affected by cross-winds.
When fertiliser is applied, the operator, farmer or pilot tries to partly overlap the previous swathe pattern with the next pass. This helps to compensate for the low rate at the edge of each swathe. Variation in the distance between successive passes can improve the uniformity of spread. However, as the distance reduces, the time taken to apply all the fertiliser will increase, and so will the cost of spreading.
When even spread is important
- When a fast-acting fertiliser like nitrogen is used to get rapid grass growth over a short period. This is usually expensive and the farmer wants all the pasture to receive the right rate.
- In horticulture, where there may be frequent applications – three or four times on some crops. Uneven application can cause crop variability, which leads directly to lower returns.
- In large-scale forestry, particularly for high-value trees such as veneer or clear-wood logs. Fertiliser is typically applied at 10-year intervals, so there is no opportunity to counter or remedy uneven spread the next year.
When it is not so important
- In routine annual application of phosphate fertiliser to pasture. This is because non-uniform application in one year tends to be partly compensated for in the following year.
- When the nutrient being applied is only a small proportion of the level already in the soil.
The New Zealand fertiliser industry has set standards for uniformity of application. They represent an acceptable compromise between the cost of application (which will rise to achieve a more even spread), and of lost production from uneven application.