9B Bicarbonate-extractable P (Colwell-P)

This empirical method favours both readily available and the quantity component of soil P. It provides an estimate of P fertility via correlations with crop and pasture response to P on acid, neutral, and alkaline soils in several Australian regions (e.g. Colwell 1963, Rayment and Helyar 1980). It has also been used to assess the ‘available’ P status of many Australian soils (Stace et al. 1968), and is one of two soil P tests required for the final calculation of PBI_+ColP.

Ratings for bicarbonate-extractable P relevant to north-eastern Australia (Bruce and Rayment 1982; Table 9.4) provide guidance on levels typical of many natural surface soils. Soils with a significant past history of fertiliser applications could contain three or more times the upper value in Table 9.4, while the vast majority of unfertilised Australian soils would rate as very low to low.

The bicarbonate extracting solution [0.5 M NaHCO₃ at pH 8.5 (with NaOH)] is best when freshly prepared on the day of use. Moreover, there is no inclusion of activated C prior to soil extraction for 16 h at a soil/solution ratio of 1:100 (Colwell 1963). The manual analytical finish of Colwell (1963) is based on Murphy and Riley (1962), following acidification of the bicarbonate extract with H₂SO₄. This acid is preferred to HCl for neutralisation of the bicarbonate extract, since the latter lowers the sensitivity of spectrophotometric methods involving the molybdophosphoric blue complex (e.g. Table 7-1 of Jackson 1962).

The alternative, automated analytical finish of Colwell (1965) is no longer recommended, as it includes chemical modifications sufficient to sometimes affect the outcome, one being the use of HCl for neutralisation and the other involving strong heating. Papers by Murphy and Riley (1962), Watanabe and Olsen (1965), Salt (1968), Dick and Tabatabai (1977) and Tarapchak (1983) present evidence that hydrolysis of organic P is enhanced by long time periods for colour development, and by heating. Excessive acidification has also been shown (John 1970) to elevate apparent P concentrations. McLeod (1982) provides further information on NaHCO₃-soluble P.

Table 9.4. Ratings by Bruce and Rayment (1982) for bicarbonate-extractable P and acid-extractable P (mg P/kg) in air-dry surface soÕs from north-east Australia.