For grapes, there are two times of the season recommended for testing – full bloom (mid to late June) and early veraison (mid July to mid August). While both sampling periods have established sufficiency levels, the mid-July to mid August timing is generally thought to provide a more accurate assessment of grapevine nutrient status.
At least 25 to 50 vines should be selected for sample collection. Vines not typical of the vineyard should be avoided.
Vines should be of the same age, variety, and rootstock, growing on a relatively uniform soil of the same fertility. If these conditions are not met, divide the vineyard into uniform blocks and sample separately.
For full bloom time sampling, take the petioles from leaves opposite the bottom flower cluster (Fig. 2). For samples collected in mid July to mid August, take petioles from the most recently developed matured leaf (Fig 3).
Collect a total of 75 to 100 petioles (more may be required from cultivars with short petioles) from 2 or 3 leaves on the vine. Do not pick more than 1 leaf from any one shoot. Discard the blade and keep the petiole. Leaves showing insect, disease, or mechanical damage should not be selected for sampling.
If you are trying to diagnose specific symptoms, send in two samples – one set of 75-100 petioles from vines showing the symptoms and another set from vines not showing symptoms. This can be done at any time of the growing season.
If petioles are dusty or dirty, they can be rinsed in distilled or deionized water. Do not let petioles soak in water, because nutrients will leach out. Dried petioles should not be washed.
Place petioles in a clean paper bag and dry at room temperature or send immediately to a laboratory. Do not use plastic bags unless samples have been previously dried.