For a long time now, the Olsen P test has been considered the “official” New Zealand test to measure available phosphorus. The decision to use the Olsen test was made some decades ago, based on local field trials. However there are a number of concerns in relation to the Olsen P test. This may be the “official” test that does not mean to say that it is always the best test to use. The concerns relate to the Olsen P test at low soil pH levels, when soils are tested where lime has been applied in the months prior, and where Rock Phosphate has been used as a phosphorus input.
For now, I just want to talk about the situation where Rock phosphate has been used. The use of Rock Phosphate (often called RPR – Reactive Phosphate Rock) has increased significantly in the last few years. Indeed, virtually all our clients are now using RPR in their base fertiliser (instead of more soluble forms of phosphorus). The use of RPR is more sustainable, reduces leaching, RPR can be used by organic growers, and it is more compatible with the promotion of beneficial mycorrhizal fungi in the soil than soluble phosphorus, especially applied at higher rates. More about the use of RPR in another Blog one day. This time of the year most growers are doing their annual soil tests. We increasingly see not shown the type of increase expected. Yet petiole and blade testing confirms the improved phosphorus status. See for instance the screenshot below (Vineyard names and block names have been blocked out). The green dot well below the target line (100%) represents the Olsen P result of 7 ug/L; well short of the target. The yellow brown line at the top is the Bray P level, well above target. The same applies to the blue line; representing petiole phosphorus at flowering over a number of years; well above target.
Integrape screenshot – phosphorus profile.
RPR has been used in this ( Marlborough) vineyard for the last 7 years or so. This is typical in many parts of Marlborough, where the soil test may show Olsen P levels of only 6 or 8 ug/L of plant available phosphorus (medium range 15-40 ug/L), yet petiole and blade levels are well into the medium range. I have a test in front of me right now from an organic vineyard in Hawkes bay; Olsen P is well below the medium range at 12 ug/L, however the Resin P test from the same sample shows a level of 56 mg/kg in the middle of the medium range (40-75 mg/kg). The problem with the Olsen P test in this case is that the test does not pick up plant available P compounds from RPR like the plant can. The test underestimates the quantity of PLANT-available phosphorus. The more RPR has been applied, or the longer a grower has been using RPR, the bigger this discrepancy can be. Thus is all well documented, and should be recognised by any competent consultant. There has been some talk about introducing correction factors, but that was not the answer. In our opinion, growers should simply ask the laboratory to do a Resin P test in addition to the standard soiltest. The resin P test works on a different principle, and much better reflects the levels of available phosphorus where RPR has been used. The Bray P test used by some laboratories is probably also more reliable as an indicator than the Olsen P test in these situations. Let’ put this in context; there is no perfect soil test for any element. The resin P test, the Mehlich 3 and the Bray P all have their limitations, just as the Olsen P test has. However in these circumstances (where RPR has been used), it makes a lot of sense to ask for a second opinion” using one of the more suitable tests for soils where RPR has been used. Quite a bit of work has been done with the Resin P test, and there are even some voices for the resin P test to take over as the “official P test”. To me, at this point, it just makes sense to test both; especially if you have applied RPR.
