Fertiliser use is often blamed for causing surface water quality problems, such as nuisance algal growth. While nitrogen (N) and phosphate (P) are required to assist in algae growing in rivers, streams, and lakes; summer conditions of low water flows, warm temperatures and sunlight are also required. Unless you (or your fertiliser spreader) actually chucks the fertiliser into the water, very little fertiliser N or P finds its way directly into waterways.

Most N and P that does reach waterways are indirect losses from the farming system itself  - N leached from urine patches of grazing animals, or P run-off. Of the small amount of run-off that does occur, 90% (on average) in most New Zealand regions is P attached to fine clay particles as a result of soil movement. There are a number of mitigation practices to assist with reducing N leaching and P run-off. Best management of fertiliser N and P will assist to minimise any potential direct losses.

N fertiliser:

• Apply to match plant demand and to plants that are actively growing.
• Apply in smaller dressings more often.
• Do not apply to saturated, dry or compacted soils.

P fertiliser:

• Apply to match plant demand.
• Apply to reach and maintain the economically optimal soil P level.
• Do not apply to saturated, dry or compacted soils.

Use soluble P fertilisers e.g., superphosphate, DAP where:

• A rapid plant response is required
• Soil P levels need raising rapidly.
• There is a low risk of run-off.

Use slow release P fertilisers e.g., RPR where:

• A rapid plant response is not required.
• Soil P levels are adequate.
• There is a high risk of run-off.
• Soil pH is less than 6.0, and rainfall greater than 800mm.