During cultivation, the internal phosphorus cycle of Mollisols (Chernozems) of the Canadian
Prairies is perturbed by crop sequences including wheat phases, tillage practices, and regular
applications of fertilizers. To monitor these changes, a proximate sequential phosphorus (P)
fractionation procedure was developed by Hedley et al. (1982) to extract inorganic and organic P
fractions as very labile (resin-P), labile (NaHCO3-P), slowly available (NaOH-P), and very slowly
available (HCl-P) pools. Models used so far to monitor P pools do not address the interactive
behaviour of P fractions constrained to a closed compositional space. Compositional data analysis
using isometric log ratio (ilr) coordinates is appropriate for modelling the interactive P pools using
sequential binary partitions of P pools. Our objective was to model changes of P pools in Mollisols
in response to management and time using ilr coordinates. We used a dataset with treatments and
another where a Mollisol was analyzed at time zero and 4, 65, and 90 yr after sod breakup. Seven P
fractions were assigned to P reactivity groups to compute six ilr coordinates. The ilr2 contrasting
inorganic (geochemical) and organic (biological) P pools and ilr4 between the most readily available
and less P bioavailable pools were the most sensitive to crop sequence and fertilization. Using
composition at time zero as reference, the Aitchison distance reached a plateau after the 4th year in
the Bm horizon compared to continuous change in the Ah horizon. Time changed the P balance of
cultivated Mollisols primarily in the inorganic vs. organic P pools. The risks of yield loss and
environmental damage can be minimized using soil tests that quantify the rapidly bioavailable
inorganic P pools and crop management strategies that promote biological P pools
