In this file a collection can be found of contributions to a discussion on the development of a environmental P test, based on a water extraction. The discussion started in the framework of COST 832, 'Quantification of agriculture to eutrophication', working group 2: Phosphorus losses at the field scale. It started during a meeting in Cordoba, Spain, May 1999, where Turner and Haygarth presented a provisional protocol for a test method (see below). Contributions to a discussion on this paper, via email or elsewhere, will be collected in chronological order. At the end of this file a list of relevant publications can be found.
Contributions to the discussion can be sent to

Index (day-month-year)

13-05-1999, Provisional protocol, Turner & Haygarth
01-12-1999, Chardon, comment on provisional protocol
30-12-1999, Hartikainen: additional comment on drying soil samples
26-01-2000, Reaction Turner on comment
26-01-2000, Reply Chardon on reaction Turner


Last update 18-2-2000

Provisional protocol for potential P solubilization (PPS)

Benjamin L. Turner & Philip M. Haygarth

Soil Science Group, Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, U.K.

Discussion paper. Source: Turner & Haygarth (1999), see end of text

We decided that displacement of in situ soil water is not a satisfactory approach to PPS, because in dry conditions, soils may contain little or no water. We therefore prefer an extract method, outlined below:

A PPS method such as this could provide a particularly effective risk assessment for P transfer in rapid flows, such as overland or preferential, associated with all forms of P. Alternative, more traditional chemical agronomic soil P tests, may provide a more refined means of assessing ‘leaching’ potential, which will occur during the slower water flows (discussed elsewhere in WG2).

1-12-99, W.J. Chardon, Comment on provisional protocol

A soil water extract can be seen as a suspension of both colloidal bound and molecular P forms. This holds both for P in the extract after primary filtration as for the fraction < 0.45 μm (Haygarth et al., 1997; Sinaj et al., 1998; Hens, 1999). At least part of colloidal P will be measured, both by e.g. Murphy & Riley’s method (reactive P) and as total P.

General comment:

A number of variables in the method will influence the amounts of colloids detached / released during the extraction procedure:

  1. shaking strength, which is the result of: solution:soil ratio (SSR); position and size of shaking bottle; empty space in bottle; way of shaking (end-over-end or reciprocating); speed of shaking apparatus
  2. ionic strength and (cationic) composition of shaking solution, which is mainly determined by the SSR

In my opinion, a method that aims to give identical results when applied in different countries must prescribe not only the SSR, but also the other variables mentioned under 1.

Specific comment:

sample depth: sampling soil surface layers will be appropriate if one is interested in surface runoff, or in case preferential flow occurs; however, in case vertical matrix flow is most important, sampling near the mean highest groundwater level may be more appropriate (Chardon & Van Faassen, 1999);

sample preparation: I understand that it is proposed to use non-dried soil samples; however, preparing sub-samples, and exchanging them for methodological research, will then be complicated. Also, large sampling campaigns, e.g. for detecting critical source areas, will also be easier when air-dried samples are used.

extract ratios: the narrow SSR of 1.2:1 and the wide SSR 15:1 seem to de derived from Chapman et al (1997); however, in practice much wider ratio’s are used (see also Kuo, 1996): 60:1 (v:v, Sissingh, 1971), 100-250 (v:w, Yli-Halla et al., 1995). In my opinion, a wide ratio must be based on the ratio found in case of either surface runoff or preferential flow, and in most cases this will be higher than the proposed 15:1;

gentle shaking: this has to specified, see general comment;

filtration: the measured amount of ortho-P will be influenced by the time desorption can proceed, especially at a wide SSR; at a (very) narrow ratio an equilibrium may be assumed when the soil is sampled, or shortly after water is added. Desorption rate will be highest between addition of water and primary filtration, and will proceed -but become slower- until the final determination. Thus, when analysing large soil series at a wide SSR the time taken for filtration may become critical, especially when a short shaking time is chosen;

PPS determination: refer to Rowland and Haygarth (1997) for the persulphate digestion.

30-12-99 Helinä Hartikainen: additional comment on drying soil samples

If non-dried samples are recommended, instructions for the storage conditions should be given. This is of importance, because e.g. in Nordic countries it is not possible to take soil sample during winter time when the soil is frozen and/or covered by snow. Because of the work load in laboratories it is not possible to carry out all the soil extraction tests during summer and autumn, wherefore it is store samples for future analyses.

26-1-00, Reaction Ben Turner on comment

I think that using dried soil is definitely to be avoided; by that I mean extracting dried soil directly with water is to be avoided, because I believe strongly that the soil must be dried after sampling in order to be stored and posted etc, as Helina suggests. However, I believe it MUST be moistened for some time period (we suggested 24 hours) before extraction. I have data showing that significant (up to 20X) more P is released from dried soil than 'field moist'. This P is mainly organic P: microbial cell lysis contributes perhaps 50% and breakdown of organic matter the rest (I suggest this on the basis of direct cell counts and on specific organic P compounds that I measured in extracts of moist and dried soils). In extracts of moist soils, most organic P is di-ester-P (derived from bacterial sources, I guess). The problem of P release from dried soil doesn't exist for measuring reactive P, because there is little change in this fraction. However, it is highly significant for organic P measurement. This is why we originally advocated a 24 hr remoistening period, to allow the microbial populations to recover after drying and attain some sort of 'equilibrium'. P release is then much more realistic.
As for the extract ratio - this is a double edged argument - when I look back at the original document, we seem to be suggesting that we should use wide ratio to predict surface runoff P but low ratio to predict leachate P. But we MUST remember that we are not trying to predict P transfer, but determine the potential for P release from the soil to solution. Nothing more. The next stage (whether P is lost from the field) is where the hydrology comes in. This was the whole problem with trying to get people to accept the need for the water P test at our meeting in Cordoba. Therefore, I now perhaps feel that a more useful thing will be a standardized extract to compare between soils, management etc. Not sure what the ideal ratio should be though!

26-1-00, Reply Wim Chardon on reaction Ben Turner

About drying soil: Maybe I have read your protocol wrong: you talked about minimal soil disturbance and 'soil moisture conditions', and I interpreted this as not drying the soil at all. So we seem to agree that drying after sampling should be done, mainly for practical reasons, but incubation with water before the extraction then becomes essential, as suggested in the method of Sissingh (1971) and Van der Paauw (1971).

About the extraction ratio: I did not mean that you suggested that a wide ratio predicts runoff, because we all know that it doesn't. However, we might say a wide ratio is more relevant for characterizing soils in case of runoff or preferential flow, and a low ratio is most relevant in situations where matrix flow is the way of P transport. The ratios chosen may depend both on what is practical in the lab and the experiences people now have with e.g. correlating P in runoff and in some kind of water extract.


Chapman,P.J., Edwards,A.C. & Shand,C.A. 1997. The phosphorus composition of soil solutions and soil leachates: Influence of soil:solution ratio. Eur. J. Soil Sci. 48: 703-710

Chardon,W.J. & Van Faassen,H.G. 1999. Soil indicators for critical source areas of phosphorus leaching. Report Program Integrated Soil Research, vol. 22, Wageningen, The Netherlands

Haygarth,P.M., Warwick,M.S. & House,W.A. 1997. Size distribution of colloidal molybdate reactive phosphorus in river waters and soil solution. Water Res. 31: 439-448

Hens,M. 1999. Aqueous phase speciation of phosphorus in sandy soils. Thesis, Kath. Univ. Leuven

Kuo,S. 1996. Phosphorus. In: D.L. Sparks et al. (ed.) Methods of soil analysis, Part 3. Chemical Methods. SSSA Book series 5. SSSA, ASA, 1996, Madison USA, pp. 869-919

Rowland,A.P. & Haygarth,P.M. 1997. Determination of total dissolved phosphorus in soil solutions. J. Environ. Qual. 26: 410-415

Sinaj,S., Mächler,F., Frossard,E., Faïsse,C., Oberson,A. & Morel,C. 1998. Interference of colloidal particles in the determination of orthophosphate concentrations in soil water extracts. Comm. Soil Sci. Pl. Anal. 29: 1091-1105

Sissingh,H.A. 1971. Analytical technique of the Pw method, used for the assessment of the phosphate status of arable soils in the Netherlands. Plant Soil 34: 483-486

Turner, B.L. & Haygarth, P.M. 1999. Influence of soil processes on solubilization of P forms: A review of experimental data and a provisional protocol for potential P solubilization (PPS). Paper presented at the COST 832-WG2 Workshop, Methodologies for Estimating the Agricultural Contribution to Eutrophication, ‘Phosphorus Losses at the Field Scale’, Cordoba, Spain 13-15 May 1999

Van der Paauw,F. 1971. An effective water extraction method for the determination of plant available soil phosphorus. Plant Soil 34: 467-481

Yli-Halla,M., Hartikainen,H., Ekholm,P., Turtola,E., Puustinen,M. & Kallio,K. 1995. Assessment of soluble phosphorus load in surface runoff by soil analyses. Agric. Ecosyst. Environ. 56: 53-62