Understanding of the risk of phosphorus (P) loss to the environment is crucial to monitor soil P and implement policies for P management. We assessed P sorption characteristics and adapted a P saturation index (PSI) for silage corn and blueberry fields in south coastal British Columbia (BC), Canada. We used 284 composite soil samples with contrasting P levels collected from eight silage corn and 23 blueberry fields across south coastal BC. The P sorption maximum (Smax) varied between 982 and 2532 mg P·kg−1 and was influenced by aluminum concentration and organic matter content. The degree of P saturation was related to water-extractable P (Pw) by a quadratic regression with R2 = 0.85. A critical Pw = 3.7 mg·kg−1 was established across the two cropping systems. The silage corn fields with pH > 5.5 had critical PSI value of 10.4%, and blueberry fields with pH < 4.7 had critical PSI value of 18.0%. These results showed that the risk of P loss from soil in the silage corn was high, but it was low for blueberry because the critical PSI for silage corn fields was lower than for blueberry fields, and therefore, saturation would be more easily reached, even though more P is applied to blueberry fields. The combination of a critical PSI and Pw as agri-environmental indicators will help farmers and professionals to identify fields with risk of P loss, to implement a nutrient management plan, and to monitor how this risk changes with time.

Phosphorus (P) management using mineral and manure P in excess of plant needs contributes to legacy soil P in coastal British Columbia, Canada. The objectives of this study were (a) to assess the effects of starter P at early growth stages and their carryover through corn maturity, and (b) to determine how corn yields relate to soil P saturation indicators. Eleven 1-year experiments were conducted on sites with contrasting legacy soil P in 2018 (three sites) and 2019 (eight sites). Six combinations of starter and manure P rates (T1: 0P starter + 0P manure ; T2: 0P starter + 35P manure ; T3: 5P starter + 30P manure ; T4: 10P starter + 25P manure ; T5: 15P starter + 20P manure ; T6: 20P starter + 15P manure) were assigned to a RCBD with four replicates. In 2018, corn dry matter weight (DMY) at the 6-leaf stage was affected by starter P in all sites, and the response curve was described by a linear-plus-plateau model with critical starter P of 5.0 and 7.5 kg ha −1. In 2019, corn DMY at early growth stages was not affected by starter P. Corn DMY at harvest varied between 15.2 Mg ha −1 in sites with low initial soil P and 27.2 Mg ha −1 in sites with excess P. Four sites exhibited a trend of high DMY with treatment comprising low starter P and high side-dressed manure P additions. The degree of P saturation (DPS, 14.0 to 38.0%) and P saturation index (PSI, 3.0 to 14.0%) were related to corn DMY by quadratic functions (R 2 = 0.76-0.94). From these results, we can conclude that the effect of starter P on young corn plants varies with year, but does not carryover through harvest. Moreover, current starter P (30-40 kg ha −1) recommended for corn can be reduced by up to 75% (5.0-7.5 kg ha −1) without affecting yield, thus reducing annual P inputs and farmers' production costs. Finally, PSI as a proxy of DPS is a possible agro-environmental indicator that, coupled with reduced starter P fertilizer, can further contribute to reducing the risk of P transfer from agricultural soils in coastal British Columbia.