The response of arsenate tolerant and non-tolerant Holcus lanatus L. phenotypes, where tolerance is achieved through suppression of high affinity phosphate/arsenate transporters, was investigated under different growth regimes to investigate why there is a polymorphism in tolerance found in populations growing on uncontaminated soil. The growth of tolerant plants differed from non-tolerants in response to phosphate (P) fertilization when grown on the uncontaminated soil from the population's origin: non-tolerants put more resources into tiller production and down regulated investment in root production while tolerants tillered less effectively and did not alter resource allocation to shoot biomass under P fertilization. The two phenotypes also differed in their shoot element status having greater concentrations of manganese (Mn) and selenium (Se), while suppressed uptake of arsenic (As) and phosphorus in tolerant phenotypes. Enrichment of As-amended soil with P induced growth of tolerant phenotypes, particularly the root biomass and did not alter As influx, but increased As uptake of non-tolerants compared to their corresponding status in As-amended soil. Growth and nutrition of tolerant and non-tolerant phenotypes did not differ in response to fertilization with nitrogen (N). By contrast, iron (Fe) addition affected root biomass production, shoot-root ratio, and As influx. Fe fertilization decreased the uptake of As in both tolerant and non-tolerant phenotypes. When tolerant and non-tolerant phenotypes were grown in different contrasting local soils, there was a significant interaction between soil and phenotype for tillering and root biomass production. Sometimes tolerants out performed non-tolerants, and vice versa, or there were no differences. Therefore, variation in growth matrix response could be the basis of the maintenance of polymorphism in H. lanatus population. The polymorphism was also widely present (40%) in other wild grass species suggesting an important ecological role for this gene that can be screened through plant root response to arsenate and can be transplanted to other crops to harvest safe agricultural products from As contaminated sites.