Page 4 – Superphosphate pros and cons
Superphosphate has been a reliable and cost-effective fertiliser in New Zealand since 1882, especially on sheep and dairy farms. Once it was known that superphosphate was widely suitable for New Zealand pastures, suppliers struggled to meet the demand.
In the 1920s and 1930s the government promoted superphosphate, backed with research.
Developing hill country
After the Second World War (1939–1945), New Zealand developed aerial topdressing. Fertiliser could now be efficiently and easily applied from the air, rather than by hand. There was strong support from soil conservation authorities, such as catchment boards that had been formed to combat erosion in hill country. The result was a huge increase in hill country farming, both privately and through government schemes.
Research revealed the need for other nutrients such as potash, cobalt, boron, selenium and molybdenum, and farmers were able to add these to the superphosphate as required.
There are alternatives to superphosphate as a source of phosphorus for crops and pastures. Superphosphate is not used on organic farms because it is a manufactured (chemical) fertiliser. Organic farmers use unmodified rock phosphates that will react with the natural acids in the soil to release phosphorus.
As with all chemical fertilisers, there are some features of superphosphate that need to be treated with caution.
- Damaging seeds. Superphosphate and other very soluble fertilisers can damage nearby germinating pasture seeds. To avoid this, each seed can be protected in a lime pellet, or fertiliser should be sown separately.
- Fluorosis. Superphosphate contains fluorine, which can cause sickness (fluorosis) in animals that ingest it directly from recently fertilised pasture. The best practice is to ensure that animals do not graze pastures recently topdressed with superphosphate until rain has washed the foliage clean.
- Cadmium. Cadmium, contained in most rock phosphates, is regarded as toxic to humans. New Zealand manufacturers take care to ensure that cadmium does not exceed 280 milligrams per kilogram of phosphorus. This is to avoid any excess cadmium entering the food chain.
- Impact on waterways. Phosphate is strongly held in the soil by clinging to clays. It is then released slowly as plants require it for growth. Very little is carried by runoff water into streams. The phosphate that gets into streams and damages the environment is mostly attached to fine sediments. So if the loss of these sediments can be reduced, then the amount of phosphate entering waterways will also be reduced. But if superphosphate is dropped straight into water it will release phosphate rapidly.This could raise the levels of the phosphate in farm waterways or dams. So it is important that fertiliser is applied in a way that does not allow this to occur.