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The critical value of soil Olsen-P (mg Olsen-P kg-1) is the value of soil Olsen-P above which the probability of crop response to P fertilization is small or null. The aim of the present study was to determine the critical value of soil Olsen-P for potato tuber yield in volcanic soils. The data used in this study were taken from 41 experiments of P fertilization conducted in southern Chile from 1977 to 2015 by the National Potato Center at the Instituto de Investigaciones Agropecuarias (INIA). To identify the critical values of soil Olsen-P, relationships between relative yields and initial soil P-Olsen were assessed using three models (linear-plateau, boundary and Mitscherlich function). Across experiments, the average fresh tuber yield was 41.9 Mg ha-1 and varied between 4 and 89 Mg ha-1. Fresh tuber yield decreased up to 92% by P deficiency; however, yield reduction was concentrated between 20 and 40%. The critical Olsen P values determined were 41, 33 and 11 mg Olsen-P kg-1 obtained (P < 0.01) with linear-plateau, boundary function and Mitscherlich function, respectively. However, linear-plateau and Mitscherlich functions showed lower coefficient of determinations (R2 = 0.23 - 0.31) than the boundary function (R2 = 0.97). Therefore, we suggest that the critical Olsen-P value of 33 mg P kg-1 determined with the boundary function is more appropriate for potato growers than the critical value determined with the other models. This information will be valuable for optimizing strategies for P fertilizer management in potato production systems aimed at achieving optimal crop yield and minimizing potential environmental risk.

Microplants of 2 apple rootstocks (M9, M26) and one cultivar (Golden) were inoculated with VAM fungi during a very early weaning stage of acclimatization following micropropagation and supplied with nutrient solution at different P concentrations. Phosphate fertilization containing a high level of P (40 ppm) had no effect on the growth response of mycorrhizal apple plants. At the lower levels of 8 and 4 ppm P mycorrhizal plants maintained the same growth rate as with 40 ppm P. Phosphate fertilization had no influence on endomycorrhizal infection. No difference was observed in the mineral contents of mycorrhizal and nonmycorrhizal plants, or between plants receiving different levels of P. At the lower fertilization rates of P, endomycorrhizal infection not only improved growth but also homogeneity of Golden and M26 plants., Des microplantes de 2 porte-greffes de pommier (M9, M26) et d’un cultivar (Golden) ont été inoculés avec des champignons VAM pendant une phase très précoce de l’acclimatation, après la micropropagation, et ont reçu des solutions nutritives comprenant des doses variables de P. La fertilisation phosphatée à un niveau élevé de P (40 ppm) n’a pas eu d’effet sur la réponse de croissance de plantes de pommiers mycorhizées. A des niveaux plus faibles de 8 et 4 ppm (P) les plantes mycorhizées ont la même vitesse de croissance que celles recevant 40 ppm de P. La fertilisation phosphatée n’a pas eu d’influence sur l’infection mycorhizienne. On n’a observé aucune différence entre la composition minérale des plantes mycorhizées et celle des plantes qui ne l’étaient pas, ni entre celles de plantes ayant reçu différentes doses de P. Aux bas niveaux de fertilisation phosphatée, l’infection mycorhizienne a amélioré non seulement la croissance, mais aussi l’homogénéité de Golden et de M26.