Once the sampling site has been identified, sampling operations are ready to start. Consistency in the sampling technique is the most important source of error in the sample collection process. The nutrient level within a tissue sample will vary depending on which part of the plant is being analyzed. In the case of potatoes, the part of the plant that should be used for tissue analysis is the 4th petiole from the top (see figure 3). The goal is to get the youngest/latest fully expanded leaf.
Older or younger leaves will have different nutrient concentration which will affect test results. If you are unsure which leaf to sample, choose an older leaf since a leaf which is not fully developed will have a lower nutrient concentration.
It is essential that the plant tissue is not contaminated with any foreign material, including soil. For consistency, it is recommended that the samples are taken at the same time each day (e.g., if grower A’s field is sampled at 9:00 AM, you should aim to always sample grower A’s field around 9:00 AM to provide similar conditions from week to week.)
If you cannot avoid petioles with soil on them, RINSE briefly with clean water; DO NOT wash them with soap or detergent and do not scrub them as the nutrient composition of the leaf can be altered. Try to avoid sampling fields within 24 hours of rainfall and within 3‐5 days after being sprayed with fungicide, pesticides, or foliar nutrient solutions.
Once collected, KEEP the petiole and leaflet intact and place the 30‐40 leaves in a properly labelled PAPER bag until ready to scan. Never store fresh samples in sealed plastic bags as it might cause them to mold, thus altering the nutrient composition of the sample.
The potato petioles samples are collected every 7‐10 day, 30 days after emergence of the plant. Sampling continues up to 90‐100 days after emergence to obtain a complete seasonal profile of treatment effect. Each sample is made up of 30‐40 leaves collected in the sampling area.
We highly recommend preparing your labels and sampling bag in advance. Labels can be mass printed with a basic printer and sticky labels available at Staples or other office supply stores. This will always ensure your scouts have the proper supplies to adequately sample your customers' fields and will prevent any mix‐ups. We recommend that each label contains the following information: