A Comparison of the Phosphorus Nutrition Index Versus the Petiole Analysis as Tools to Assess the Phosphorus Nutritional Status of Potato

The objectives of our study were to (i) evaluate the association between the phosphorus (P) nutrition index (PNI) and total or soluble P in petioles (Ppe T and Ppe S , respectively) throughout potato’s ( Solanum tuberosum L.) growing season and (ii) evaluate the use of Ppe T and Ppe S as predictors of the P nutritional status and tuber yield. Five potato (cv. innovator) field trials were carried out during two growing seasons, where four different P rates were evaluated. Whole plants were harvested, and the P content was determined and used to calculate the PNI. Also, petioles were harvested to determine Ppe T and Ppe S . At crop maturity, total and relative yield (RY) were determined. From all evaluated dates, Ppe T and Ppe S presented a stronger capacity to predict RY at 70 days after planting ( r = 0.65 and 0.66, respectively), when Ppe T and Ppe S presented a 2.96 and 1.48 g kg −1 threshold below which RY was negatively affected, respectively. The PNI was mostly better associated with Ppe T than with Ppe S . The PNI presented a moderate capacity to predict the potato tuber RY which was stable throughout the growing season and presented an average critical threshold of 0.96. Depending on the sampling moment, all three evaluated indexes (PNI, Ppe T , and Ppe S ) resulted to be suitable in-season plant analysis to assess the P nutritional status of potatoes. However, based on the simplicity of the quantification and the observed predictive capacity, the methodologies based on petiole analysis are advantageous as compared with PNI.