Critical Olsen P and CaCl₂‐P levels as related to soil properties: results from micropot experiments

Abstract

The usefulness of soil phosphorus (P) tests used in routine soil analyses is limited by the fact that a single measurement cannot encompass all P-related factors potentially affecting plant performance. In this work, we performed micropot (15 mL) experiments to test the hypothesis that the predictive value of two common soil P tests (Olsen P and CaCl 2 -P) can be improved by considering properties commonly measured in laboratory analyses. Forty-nine sets of soils ranging widely in properties were used for this purpose, each set consisting of samples with similar properties but differing in P status. Ryegrass and turnip were grown in a chamber for 30 days in two separate experiments and their yields at harvest recorded. The critical Olsen P and CaCl 2 -P levels, which were taken to be those corresponding to 95% asymptotic yield as calculated from data fitted to a Mitscherlich equation, were greater for turnip than for ryegrass, probably as a result of the difference in yield (49 and 160 mg dry matter/micropot on average for ryegrass and turnip, respectively) and hence in P requirements between the two species. Critical Olsen P spanned narrower ranges than critical CaCl 2 -P in both crops and is therefore seemingly the more robust of the two tests. Both critical P values exhibited moderate correlations with soil properties. Thus, critical Olsen P was (a) lower in soils with a medium pHwhich is consistent with the fact that the bicarbonate solution method tends to overestimate plant-available P in strongly acid and calcareous soils; (b) positively correlated with pH and carbonate content in calcareous soils; and (c) uncorrelated with soil properties in noncalcareous soils. On the other hand, critical CaCl 2 -P in some soil groups was negatively correlated with some properties increasing the P buffering capacity of soil (e.g. Fe oxide content). Taken together, our results suggest that routinely measured soil properties help to predict critical Olsen P better than critical CaCl 2 -P.

Calculation of threshold Olsen P values for fertilizer response from soil properties

“…There is not much information on soil properties controlling this threshold P concentration in solution. In other plant growth experiments, carbonate content and pH were the soil properties affecting Olsen P threshold values (Sánchez-Alcalá et al 2015). Recena et al (2015) demonstrated that the Olsen P predictive value for plant P uptake is affected not only by those soil properties influencing the equilibrium between inorganic phosphate in solution and that in solid phase but also by factors related to organic P dynamics.…”

“…This extraction was performed in duplicate and at a soil:extractant ratio of 1:10 using 2 g of soil in 20 ml of extractant in polyethylene falcon tubes that were end-over-end shaken at 2.5 s −1 for 30 min. The extract was centrifuged at 1000g for 10 min, and then a portion of 2 ml centrifuged in Eppendorf tubes at 19,000g to remove soil particles with a diameter >0.05 μm (Sánchez-Alcalá et al 2015). In the remaining solution, molybdate reactive P was determined according to Murphy and Riley (1962).…”

“…However, although soil P tests are deemed to be useful for assessing P fertilizer needs, they can be inaccurate because the definition of their threshold values is not always precise. In fact, actual threshold values for a given test can range widely among those soils for which its use is recommended, e.g., for Olsen P (Delgado et al 2010;Sánchez-Alcalá et al 2015). Thus, Olsen P is not always well related to plant P uptake (Delgado and Torrent 1997;Kulhánek et al 2007;Tandy et al 2011).…”

“…These threshold values were estimated considering that plants can absorb sufficient P above a given P concentration in solution. However, recent studies have revealed that P concentration in soil solution for a sufficient P supply may differ from one soil to another (Sánchez-Alcalá et al 2015). There is not much information on soil properties controlling this threshold P concentration in solution.…”

New perspectives for the design of sustainable bioprocesses for phosphorus recovery from waste

“…However, by combining extraction methods, more information on the different P fractions can be obtained (Sanchez-Alcala et al, 2015). Sequential chemical extractions can be used to investigate the transformations of fertiliser P and understand its availability to plants (Nanzer et al, 2014).…”