CONVENTIONAL wisdom that excess soil phosphate fertiliser is "money in the bank" for future crops is under fire as environmental issues come to the fore.
In Northern Ireland very small amounts of phosphate from the predominantly livestock industry are said to be ruining water quality in some lakes. Such events mean phosphate is likely to get as much attention as nitrate in future, delegates to a Fertiliser Society meeting heard last week.
While no-one openly mentioned phosphate-vulnerable zones, the possibility of some form of restriction was never far from the discussions. International agreements are already used to protect coastal waters in the North and Baltic seas, according to DANI ecologist Dr Bob Foy.
Recognising the need for phosphate as a nutrient, Dr Foy said it "does its job far too well in fresh water." Concentrations of 15µg/litre caused few problems, but 50µg/litre resulted in an algal scum.
The key was that it required only minute amounts – about 0.2kg/ha (0.16 units/acre) – to escape from soils through run-off or leaching to trigger a rise of that level. "Farmers are generally talking of adding 10-30kg/ha," he noted.
Defining standards to avoid environmental damage was tricky, because lakes varied in their capacity to absorb P before it caused excessive algal growth. "Id argue that it would be unwise to adopt a single standard for phosphate as for nitrate." Any figure would have to be set according to the type of water source and its potential use, he explained.
Although there had been no appreciable increase in the provinces use of phosphate fertilisers over the years, imported feed had boosted the intensity of livestock farming and meant soil P levels were rising. This was reflected in drain flow monitoring which showed concentrations increasing by about 1µg/litre a year, said Dr Foy. The rise was enough to "wipe out" the benefits of phosphate removal from point sources such as sewage works.
He estimated that Northern Ireland used about 18,000t of phosphate inputs each year, of which only 58% were as fertiliser. But only about 6000t left the system in outputs. "Its not very efficient," he commented. "And this situation has been going on for years."
Although only a very small proportion of the "accumulated" soil, P found its way into lakes it was enough to "wreck the system".
Apart from making low- or zero-P compound fertilisers more readily available, the best answer lay in encouraging more soil testing to allow farmers to make better use of their reserves, he suggested.
• According to the Fertiliser Manufacturers Association one of the reasons for including phosphate in compounds is that it is a useful "glue" to bind other nutrients. Although there are some low P products on the market, the main alternatives are blends.
Translating Northern Ireland experience to the rest of the UK is far from simple, said spokeswoman Jane Salter. Phosphate movement from the soil is "very complex" and varies widely with site and soil, she explained. *