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Determining plant available phosphorus

The goal of a laboratory soil test for phosphorus (P) is to estimate how much soil P will become available to plants during the growing season. To put it more simply, the laboratory test must try to duplicate, in about one hour, what plant roots will have access to over several months during the growing season.

P availability can be assessed by measuring phosphate concentration in the soil solution, and the soil’s ability to maintain the soil solution concentration. The quantity of P in the soil solution is only in the range of 0.3 - 3 lb/ac. Rapidly growing crops will absorb about 1 lb/ac of phosphate per day. Therefore, soil solution P is constantly being replenished by the “labile” pool of soil P. Labile P is a pool of soil P that is less available to plants but can undergo rapid chemical or biological changes to replenish the plant available soil P pool.

Soil tests cannot predict with 100 per cent accuracy when crops will respond to added phosphate fertilizer because the frequency of crop response can be strongly influenced by environmental conditions, particularly soil temperature and moisture early in the growing season. Also, different crop types use different mechanisms to aid in taking up soil P.

Various soil test methods have been used to estimate plant available soil P to make phosphate fertilizer recommendations for crops in Western Canada. Extensive field-testing in the 1950s led to the establishment of the Olsen method (Olsen et al. 1954) in the U.S., also referred to as the sodium bicarbonate or “bicarb” method. The Olsen method was developed specifically for higher pH, alkaline soils (soil pH >7) and was not intended for use on acid soils. This method has been widely used as the basis for making P fertilizer recommendations in Saskatchewan and Manitoba (Cowell and Doyle 1993).

Early work by the University of Alberta led to the use of the Miller-Axley method as the basis for P fertilizer recommendations in Alberta (Robertson 1962). This was an acid extractant that worked satisfactorily with lower pH, acidic soils (soil pH <7) but worked very poorly with alkaline soils.

The Kelowna method was developed at the B.C. Ministry of Agriculture soil testing lab in Kelowna in the mid-1980s (Van Lierop 1988). The method was analytically convenient and worked well to determine plant available soil P over a broad soil pH range with both calcareous and non-calcareous soils.

By the early 1990s, two modified versions of the Kelowna method were developed by Qian et al. (1994) at the University of Saskatchewan, and Ashworth and Mrazek (1995) at Norwest Labs. Acetic acid was added to the Kelowna extract by both groups to allow extraction of both P and potassium (K) with one extraction solution. The concentrations of ammonium acetate and acetic acid are slightly stronger in the Ashworth and Mrazek modification.

In the 1980s in Alberta, it was recognized that the Miller-Axley method performed poorly, particularly on alkaline soils. From 1991 to 1993, Alberta Agriculture led a major soil P calibration study at about 450 sites in the province to correlate P fertilizer response of wheat, barley and canola with a number of different soil test P methods (McKenzie et al. 2003). The Kelowna method and both modified Kelowna methods had the best correlation with P fertilizer crop response. The Kelowna method and the two derivatives of the soil test P methods were highly correlated, but there were slight differences. The two derivatives of the Kelowna method extracted slightly less P than the Kelowna method, but were still very highly correlated with the Kelowna method and each other. Generally, the modified Kelowna methods will extract more soil P versus the Olsen method, and perform much better over a wide soil pH range.

McKenzie et al found the modified Kelowna methods were well correlated for all soil types in Alberta and these have been the recommended test methods for Alberta soils since 1994. In Manitoba and Saskatchewan, most of the P soil test correlation has been with the Olsen method. Today, the Olsen P soil test method is recommended for growers in Manitoba, while in Saskatchewan both the modified Kelowna or Olsen methods are used.

Most Western Canada soil testing labs have adopted one of the two modifications of the Kelowna method. Most labs will also determine P by the Olsen method when requested. Exova and Farmers Edge soil testing labs use the Ashworth and Mrazek (1995) method and ALS lab uses the Qian et al method (1994).

Soil test P correlation work across the Prairies has been with 0 to 6 inch sampling depth. Ensure to sample the 0 to 6 inch depth separately from deeper depth samples to accurately determine P fertilizer requirements.

A new method to test for a wide range of nutrients including plant available P is the Plant Root Simulator (PRS) probes (Qian and Schoenau 2002). Originally developed at the University of Saskatchewan, PRS probes are ion exchange resin membranes held on plastic stakes that are inserted into a moist soil sample to estimate nutrient supply. There are two probes, one to adsorb cations and one to adsorb anions. One limitation of this method is there has been less field calibration with P fertilizer research trials versus the modified Kelowna and Olsen methods.

Some fertilizer dealers and agronomists use soil testing labs in Eastern Canada or the U.S. that determine soil P using other soil test P methods, such as the Bray method (Bray and Kurtz 1945). This method uses unbuffered ammonium fluoride and hydrochloric acid. Bray and Kurtz developed the method in the 1940s specifically for acid soils but not for use on alkaline soils (pH > 7.0). The test is not recommended for alkaline soils because it results in very inaccurate available soil P estimates. Because of the inaccuracies of this method, it has not been calibrated to western Canadian soils and its use is not recommended for making P fertilizer recommendations.

Ashworth, J. and Mrazek, K. 1995. “Modified Kelowna” test for available phosphorus and potassium in soil. Commun. Soil Sci. Plant Anal. 26: 731–739.

Bray, R.H., and Kurtz. L.T. 1945. Determination of total, organic and available forms of phosphorus in soils. Soil Sci., 59:39-45.

Cowell, L.E. and Doyle, P. J. 1993. The changing fertility of prairie soils. Pages 26–48 in D.A. Rennie, C.A. Campbell, and T.L. Roberts, eds. Impact of macronutrients on crop responses and environmental sustainability on the Canadian prairies. Canadian Society of Soil Science, Ottawa, Ont.

McKenzie, R.H., Bremer, E., Kryzanowski, L., Middleton, A.B., Solberg, E.D., Heaney, D., Coy, G. and Harapiak, J. 2003. Yield benefit of phosphorus fertilizer for wheat, barley and canola in Alberta. Can. J. Soil Sci. 83: 431–441.

Miller, J.R. and Axley, J.H. 1956. Correlation of chemical soil tests for available phosphorus with crop response, including a proposed method. Soil Sci. 82: 117–127.

Olsen, S.R., Cole, C.V., Watanabe, F.S. and Dean, L.A. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. US Department of Agriculture, Washington, D.C. Circ. 939.

Qian, P., Schoenau, J.J. and Karamanos, R.E. 1994. Simultaneous extraction of available P and K with a new soil test: A modification of Kelowna extraction. Commun. Soil Sci. Plant Anal. 25: 627–635.

Qian, P. and Schoenau, J.J. 2002. Practical applications of ion exchange resins in agriculture and environmental soil research. Can. J. Soil Sci. 82: 9-21.

Robertson, J.A. 1962. Comparison of an acid and an alkaline extracting solution for measuring available phosphorus in Alberta soils. Can. J. Soil Sci. 42: 115–121.


February 17, 2016
By Ross H. McKenzie PhD P.Ag.

Timothy response to annual broadcast P fertilizer in southern Alberta. The goal of a laboratory soil test for phosphorus (P) is to estimate how much soil P will become available to plants during the growing season.