This study was divided into two chapters that evaluated various organic soil amendments as alternative Phosphorus (P) sources under field and greenhouse conditions. The first chapter was a field study that evaluated the effect of a long-term P mitigation strategy on P sufficiency and arbuscular mycorrhizal fungal (AMF) infection in flax (Linum usitatissimum) grown in a long-term field study in Manitoba. The two main objectives of the study were to test if manure addition improved plant performance in terms of P tissue concentration, AMF, and biomass and whether the changes in AMF were related to the plant’s P uptake. Flax was planted after wheat (Triticum aestivum) in a 4-year – alfalfa (Medicago sativa) – alfalfa – wheat rotation under three treatments: organic manured (ORG M), organic non-manured (ORG NM), and conventional (CONV). Under wet soil conditions, adding manure to the organic plots increased biomass by 33%. Manure did not have a significant effect on flax P tissue concentration under both dry and wet soil conditions and neither did it improve flax P sufficiency; ORG NM treatment presented 8% more P tissue concentration than the ORG M. AMF abundance was not significantly different in organic manured and non-manured treatment under wet soil conditions. The second chapter investigated various organic soil amendments as alternative P sources under greenhouse conditions. The general aim of the study was to test the performance of different organic soil amendments from multiple sources including livestock, insects, and food waste to expose farmers to more diverse options for their organic soil nutrients. Novel manures (rabbit and black soldier fly) were compared with traditional amendments on soybean (Glycine max L.) grown in low P soil in greenhouse experiments. Ground rabbit manure (GRM) had higher biomass and tissue P concentration than composted beef manure, anaerobic digestate, and monoammonium phosphate but similar to insect frass. GRM and insect frass demonstrated advantages in P supply associated with higher AMF.