Setting Olsen P targets

The article is entitled ‘A Global Perspective on Integrated Strategies to Manage Soil Phosphorus Status for Eutrophication Control Without Limiting Land Productivity’, published in Journal of Environmental Quality, July 2019.

It was co-written by 15 scientists from several countries, including Lucy Burkitt from Massey University, and Murray Hart, formerly a senior research scientist for Summit-Quinphos before moving to Australia.

Here are a few direct quotes from the article:

‘Phosphorus losses from agricultural soils are hard to mitigate because they occur in both dissolved and particulate form, and are transported from highly variable source area…. Managing soils and soil P status represents an important strategy for the mitigation of eutrophication’.

‘Modern agriculture has come to rely on maximising the availability of P in soils through [so-called] insurance application rates of highly soluble P fertilisers to build up soil P fertility and minimise risks to productivity. This makes it harder to manage resulting soil P losses’.

‘Legislation to help reduce (loss-inducing) threshold soil P test values has been generally slow to implement despite widespread eutrophication problems’.

‘Our analysis spanned three continents and catchments with different ago-hydrochemical functioning. The study areas were expected to represent variable run-off response, soil P response patterns and erosion vulnerability’.

‘All data sets showed a highly significant effect of soil P status on P runoff concentrations’.

‘Future sustainable P management within the food system must reconsider old philosophies that place undue emphasis on maintaining an artificially high level of P fertility dependent on inputs of highly soluble manufactured fertilisers towards new philosophies that concentrate on precision feeding of the crop not the soil’.

‘Our meta-analysis across three continents suggests agronomic optimum P concentrations need to be in the range of 10-20 Olsen P to match challenging eutrophication control targets being set for freshwaters (around the world)’.

In New Zealand, where all our soils are mildly acidic, we have the enormous advantage that we could maintain high pasture production levels without Olsen P levels needing to be pushed above 20, simply by avoiding the use of soluble P fertilisers such as superphosphate where sustained-release P fertilisers like reactive phosphate rock (RPR) will maintain at least as high production, with much less P run-off and leaching. 

Dairy farmers, you need to figure this reality out for yourselves soon, before you are forced to destock. Unfortunately, vested interests in a small country like ours prevent the fair and open discussion of scientific knowledge.

• Dr Bert Quin is managing director of Quin Environmentals (NZ) Ltd.