Phosphate News: Is the Olsen P test still fit for purpose?

New research published in Global Change Biology by scientists from AgResearch and Lincoln University raises important questions about the reliability of the Olsen P test – New Zealand’s long-standing method for estimating plant-available phosphorus (P) in soils.

Key Reading:

  1. Loss of P Fertilizer Effectiveness in Raising Soil P Availability in a Grazed Grassland Enriched with CO₂ for 24 Years
  2. Could Elevated CO₂ Lead to Over-Application of Phosphate Fertiliser for Pastures? Implications for P-Limited Ecosystem Responses to eCO₂

For decades, New Zealand farmers have relied on the Olsen P test to assess soil phosphorus (P) levels and guide fertiliser decisions. However, as lead author Zac Beechey-Gradwell explains, new data from AgResearch suggests that this test may significantly underestimate the actual availability of P for plant uptake—particularly under elevated carbon dioxide (CO₂) conditions.

The long-term CO₂ enrichment experiment revealed a substantial and sustained reduction in Olsen P levels in topsoil before annual P fertiliser application. Moreover, the effectiveness of added P in raising Olsen P readings was notably diminished.

So, What’s Wrong with the Olsen P Test?

Technically, nothing – when it’s used in the context it was designed for. The Olsen P test was developed by Dr. Carl Olsen in North America to measure plant-available phosphorus in alkaline soils (pH 7 and above). Research from Perry Ag Lab (USA) confirms that for soils with a pH >7.0, Olsen P provides a reliable measure of available P. For more acidic soils (pH <7), the Bray P2 is more accurate.

At Top Soils, using Albrecht’s system of soil fertility, we adopt either the Bray P2 or Olsen P test depending on the soil’s pH.

In New Zealand, according to Ballance Agri-Nutrients, phosphorus levels are routinely measured using the Olsen P test. It is recommended as providing the best correlation between soil test results and plant growth response across most NZ soils. However, there is a notable exception: Reactive Phosphate Rock (RPR).

RPR and Olsen P

Olsen P is unable to detect residual P from RPR. As a result, both Ballance and the Fertiliser Association of New Zealand (via Massey University) recommend multiplying the Olsen P value by 1.5 to 1.7 to estimate the actual available phosphorus in soils treated with RPR. Massey University specifically advises using a factor of 1.69, as Olsen P underestimates yield response in soils fertilised with RPR compared to those treated with soluble P.

Implications for NZ Soils

Most New Zealand soils are acidic (<pH 7). Yet even in alkaline soils that have been recently limed or topdressed with Dolomite, RPR, or Guano, the Olsen P test can produce misleading results – underestimating the actual level of plant-available phosphorus.

For example, a soil test showing an Olsen P level of 13 (mg/L), when adjusted with a 1.69 factor, reveals a corrected value of 21.9 (mg/L). Based on the unadjusted result, one might conclude that more fertiliser is needed—when in fact, the soil may already have sufficient P.

When we’ve asked soil scientists, co-op fertiliser reps, and farm advisers why the Olsen P test remains the industry standard despite its limitations, the most common response is simply: “Because it’s the industry standard.” But this raises a serious question: Why do we continue to rely on a test that overestimates available P in acidic soils and underestimates it in alkaline soils, especially when evidence shows it’s no longer reliable for New Zealand soils. Often, the response is silence.

Donald L Hart