Plants need phosphorus for healthy roots. They need much less phosphorus than nitrogen to grow well. Most of the phosphorus in the environment is stored in rocks and soil. When soil erodes and is washed into streams and rivers, it carries with it precious phosphorus supplies. Phosphorus in waterways encourages the growth of nuisance aquatic plants and algae.
On the farm, phosphorus is supplied to the soil from:
Most of the phosphorus in the environment is stored in sediment and rock. To aid pasture growth phosphorus is usually added to farm soils in fertiliser.
Regular soil testing allows you to check your soil’s Olsen P levels and avoid unnecessary spending on fertiliser if soil phosphorus is already at optimum levels. Monitoring your soil nutrient status can help improve both the economic and environmental performance of your farm business.
Phosphorus isn’t naturally found in the air. Unlike the nitrogen cycle, there isn’t an atmosphere component to the phosphorus cycle. Check out the phosphorus cycle below.
The pie charts below give you a rough idea what happens to phosphorus on a typical dairy or drystock farm. The pale green area on each graph is the amount of phosphorus going off the farm as product.
Phosphorus behaves very differently to nitrogen. It binds with soil and only dissolves slowly in water over time. Although it doesn’t readily leach into ground water, phosphorus can still pollute waterways via:
During soil erosion, valuable soil and phosphorus are lost from a farm. They often end up in waterways where they affect water quality.
During wet periods, when you can see dirty runoff going into waterways, it is likely to contain soil particles and phosphorus. Phosphorus is found at very low levels in the natural environment, so small increases in the amount of phosphorus going into waterways can have a big effect.
Phosphorus in waterways encourages nuisance weed and algae growth, which:
The table below shows the percentage of phosphorus entering the Waikato, Piako and Waihou rivers from:
|Source||Waikato River (1990-96 data)||Piako River (1993-97 data)||Waihou River (1993-97 data)|
|Agriculture||30 to 60%||25%||51%|
|Background levels||10 to 40%||13%||18%|
Reduce the amount of phosphorus entering waterways by:
Reduce the amount of runoff and erosion on your farm by:
Remember that many pasture soils in the Waikato region are over-fertilised.
Check out the NZ Fertiliser Manufacturers' Research Association Code of Practice(external link) for Fertiliser Use.
Find out more about algal blooms in the Waikato River.