In potatoes, the petiole of the fourth leaf from the top of the plant (see figure below) is generally used to determine plant nutrient status. It is important to consistently sample the fourth petiole since analysis of samples taken from higher or lower leaf positions will produce significantly different results that are not calibrated with the crops nutritional status. Petiole sampling usually begins at tuber initiation and is continued on a weekly basis throughout most of the tuber growth period.

Usually 50-60 petioles are collected from representative areas of the field consistent with the soil sampling procedure. Alternatively, separate samples can be collected from selected areas or management zones differentiated by soil type, crop history, topography, etc. All leaflets should be stripped off the petiole immediately after sampling. The petioles should then be placed in a clean container or paper bag and quickly dried at 150 F or kept cool (<40 F) until submitted to the lab. This procedure should minimize changes in nutrient concentrations that can occur in warm, moist tissue samples.

While there are numerous ranges for petiole nutrient concentrations based on their relationship to growth and yield, these ranges can vary according to growth stage and cultivar, particularly for nitrate nitrogen concentrations which can vary widely over the course of the growing season. Use of nutrient concentration, ranges is generally adequate for vine and tuber growth at the time of sampling. However, trends observed from weekly petiole samples are more reliable in anticipating and detecting nutrient deficiencies.

Diagrams of Vegetative shoot with petioles (a) and full potato plant with a floral spike (b).  The fourth petiole is used in tissue analysis for determining plant nutrient status.