Phosphate is essential to create food. All plant and animal life need phosphorus to live. Phosphate rock is a finite source so must be managed carefully. The rock is either igneous or sedimentary. Sedimentary forms, which tend to be sources for New Zealand, are derived from old ocean seabed deposits so include fossilised and mineralised fishbones and teeth. The rock often needs to be treated so the phosphate becomes available to the plant when needed.
Plants absorb phosphorus for their growth, root formation and other processes. There are no substitutes or phosphate replacement technologies that add to the total store of phosphate in the soils.
New Zealand is a largely grass-based (pastoral) farming nation and because phosphorus is important for pasture production, there is a requirement for supplementing naturally occurring low phosphate soil levels. The removal of produce from farms for sale further extracts nutrients including phosphorus and this must be replaced to maintain soil fertility.
New Zealand is dependent on imported phosphate in order to feed its population and export valued food and fibre products. This led to two co-operatives (Ballance Agri-Nutrients and Ravensdown) being set up by farmers to secure a reliable supply of nutrients like phosphate from around the world.
As well as needing to continually replace phosphate, New Zealand soils are also typically low in sulphur, another element essential for plant life. High levels of sulphur are needed to enable clover growth to compete successfully with grasses within a pasture. Clover increases the natural ability of pasture to capture nitrogen from the atmosphere.
After more than 130 years of being made in New Zealand, superphosphate remains the country's most widely and effectively used fertiliser for pastoral, cropping and horticultural uses.
Its use on grazed pastures promotes organic matter, which returned to soils through excreta, uneaten herbage and root turnover, stimulates soil microbes and worms, improving soil health and fertility.
Without additional nutrients like phosphate, agricultural production would be estimated to be about half of what it is today. Over the decades, its use has benefitted the entire economy to the tune of billions of dollars.
The countries with the largest reserves of rock phosphate are Morocco, China and Algeria, with Morocco owning 70% of known reserves. There are only about a dozen countries that export phosphate rock, because the rock is often produced and kept by countries like China and the USA to produce food to feed their own people.
For superphosphate in New Zealand, the total phosphate in the fertiliser needs to be at least 9% as the cost effectiveness of spreading per kg of phosphate will affect farmers' bottom lines. Grinding ability and its reaction to sulphuric acid are factors for manufacture. There are controls on odour and fluoride emissions during manufacture, and these can be impacted by different rock sources. Oxides in some rocks can impact on granule strength. The industry also has a strict policy to ensure that rock sources have low levels of contaminants, particularly cadmium.
Sourcing rock is all about striking the right balance, whether it is the potential dust or cadmium in one rock source or the lack of solubility of another source of rock, the reality is that there are no 'perfect' choices available. With some countries, the issue is less about the rock, but the infrastructure or political environment that can potentially disrupt the flow of imports.
Stable long-term relationships and security of supply is essential when considering sources of phosphate rock for New Zealand farmers.
Recycling human and animal effluent is possible to support the circular economy to make use of all available resources. In practice; the handling, biosecurity, health, cultural and processing issues can be complex. Critically, sources of organic material in New Zealand only supply a fraction of what is required.
Reactive phosphate rock (RPR) fertilisers can take a long time to have an effect when applied, because they are unprocessed. They rely on the soil's natural acidity to slowly make the phosphorus available, and so cannot be used where soil pH is too high, or where a rapid response is needed. While there are many more countries supplying Diammonium Phosphate (DAP), this product contains nitrogen but no sulphur. Having DAP as a sole source of phosphate would mean many drystock farms would begin using nitrogen, where previously they relied solely on clover fixation to supply nitrogen for pasture. DAP is also more expensive. Manufacture of DAP creates large amounts of gypsum as a waste product-often stockpiled or disposed of at sea. Manufacture of DAP also results in a higher greenhouse gas footprint than other fertilisers.