Fertilizer needs can be determined by 3 ways: observing visual symptoms, using soil tests, and using tissue (petiole) analysis. Because each method has advantages and limitations, all 3 should be checked on a regular basis. Petiole analysis is one of the best tools available to monitor the nutrient status of your grape varieties. Unfortunately, grape producers do not often use it. Tissue analysis serves two purposes: determine the nutrient status of the vine; and identify a suspected nutrient deficiency observed in the vineyard. By using tissue analysis, growers have a better handle on their fertilizer program and usually end up cutting costs since fertilizers are applied only as needed. Here are answers to commonly asked questions about petiole analysis:

Why petiole test if I did the soil test?
Soil testing is important, before planting and for established vineyards, to determine soil pH and monitor the overall balance of major nutrients. However, there is a poor relationship between soil and plant nutrient levels. Simply put, a soil may be high on a nutrient, but the same nutrient is deficient in the vine; or the vine may show sufficient level of another nutrient while the soil test indicates a deficiency. For these reasons, petiole tests are generally more useful and more reliable for judging nutrient status of the plant than soil tests.

When should I do the test?
"Trouble-shooting": "If you observe leaf symptoms and suspect a nutrient deficiency, collect petiole samples from vines showing leaf symptoms and from vines without symptoms (healthy or normal). The two samples are sent and analyzed separately for comparison purposes. This will allow you to diagnose whether or not the problem is related to the nutrient status of the vine. In the case of trouble-shooting, samples are collected anytime leaf symptoms "show up" during the growing season.

"Routine assessment": In the Midwest, mid-July through mid-August is the ideal time for petiole sampling for several reasons. Sampling during this time period gives a better measure of Potassium (K). Potassium levels are especially critical for wine grapes since they are correlated with wine quality (high fruit K leads to high pH and thus unstable wines); there is less vineyard activity (if there is such a thing!) during this time of year and more time for petiole sampling; and most importantly, the standard optimum level of each nutrient is based on research of nutrient levels at this time of year. It is recommended collecting petioles about 10 weeks after bloom but before harvest.  

What are the most common nutrient deficiencies in vineyards?
During the past two years, potassium (K) and magnesium (Mg) are the most common nutrient deficiencies observed either visually or by petiole analysis. Other nutrient problems occur occasionally and include nitrogen (N), boron (B), zinc (Zn), and iron (Fe).

What can I expect from the lab?
Petiole analysis is usually performed for nitrogen, phosphorus, potassium, magnesium, calcium, manganese, iron, zinc, boron, and copper. Laboratory results will show that each nutrient is "adequate", "deficient" or "excessive".

Take Home Message
Be proactive not reactive: Do not wait until you see visual symptoms to correct a nutritional problem. If you see it, the problem already exists and the damage has already been done on growth, yield, and fruit quality. The goal of fertilization is to prevent nutrient deficiencies from occurring in the first place.