By Erica Lundquist, Ph.D.
Former LCWC Viticulturist

The most effective way to determine grapevine nutrient requirements is plant tissue analysis, and in California the standards for determining grapevine nutrient status are based on leaf petiole nutrient levels at bloom. Much of the work on vine mineral nutrition has been done by Dr. Peter Christensen of University of California Cooperative Extension, and the following summary is based on several of his reports and publications.

Plant tissue analysis tells you directly what nutrients your vines are able to derive from the soil. Factors such as variety, rootstock, soilborne pests and root condition, soil depth, and the rate of nutrient release from a particular soil will influence nutrient uptake by grapevines, and so soil tests are not reliable indicators of vine nutrient status. Soil analysis does have a useful place in the diagnosis of some chemical imbalances and in determining the suitability of a soil for winegrape production.

The time to sample is at full bloom, or when about two-thirds of the caps have fallen from the flowers. The sampling area may be a given vineyard block. If there are substantial differences within the block in vine vigor, rootstocks or varieties, you may want to sample each of these different areas separately. Sample 75-100 petioles distributed uniformly over the sampling area. (The higher number is needed for varieties with small petioles such as Pinot noir or Gewurztraminer.) Sample the leaf petioles (stems) opposite the grape flower clusters, retaining the petiole and immediately discarding the blade. Samples should be placed in a clean paper bag. Do not use plastic bags because they will retain moisture, possibly causing molding. Do not sample after a nutrient spray unless you are not analyzing for nutrients present in the spray or have made arrangements with the lab for sample washing. Record the sample date, variety, location and all pertinent information on the sample bag and in your own records. Send the petioles for analysis immediately, or if there is a delay, keep them in a warm, dry location to prevent molding.

In general yearly analyses for nitrogen (N), phosphorous (P), potassium (K), zinc (Zn) and boron (B) are adequate. After several years of testing you may be able to reduce the frequency of testing to once every few years. Elements such as magnesium (Mg) and manganese (Mn) are rarely deficient, and testing for them may be necessary only for leaf symptom diagnosis.

Levels indicating mineral nutrient deficiency or adequacy at bloom are given in Table 1. Levels of many of these nutrients change depending on vine growth stage and plant part, and the levels in this table apply only to petioles sampled at bloom. In general tissue levels in the adequate range or above indicate that the current fertilizer program and/or soil nutrient supply capacity are supplying adequate levels of the nutrient to vines. Some exceptions are discussed below.

Table 1: Guide for Mineral Nutrient Levels in Bloom Time Petiole Samples.

Nutrient	Deficient	Adequate
NO3-N (ppm)	50-350	350-1000+
Total N (%)	0.65-0.9	0.9-1.2+
P (%)	0.1	0.15
K (%)	1.0	1.5
Mg (%)	0.2	0.3
Zn (ppm)	15	26
B (ppm)	25	30
Mn (ppm)	20	25

Nitrogen levels in tissue analysis are the most difficult to interpret because critical N levels vary depending on rootstock, variety and local growing conditions. (See Table 2 summarizing rootstock differences in nutrient levels below.) Extensive trials comparing both petiole and leaf blade concentrations of Nitrate-nitrogen, (NO3-N) and total N in eight varieties and several locations found that NO3-N and total N in petioles sampled at bloom were most responsive to N availability. However, there are large ranges in deficient and adequate levels due to differences in variety, rootstock and location. Because of the limitations of tissue N analysis, assessment of vine vigor should be taken into account in N fertility decisions. Where canopy size and vigor is adequate or excessive, low or no N fertilizer applications are needed. It is also important to take into account other sources of nitrogen to vines. Irrigation water may contain substantial levels of NO3-N, and legume cover crops, composts and manures will contribute N to the soil.

Phosphorous deficiency is rare in California. When seen, it is found on highly weathered and acid soils, including some soils of volcanic origin.

Potassium deficiency symptoms frequently show up after veraison because of the large demand for K during fruit growth and maturation. Vines that have questionable potassium levels at bloom (between 1 and 1.5% total K) should be checked again at veraison. At veraison, petioles from recently matured leaves should be sampled, usually the sixth to seventh leaf from the growing tip. Deficient and adequate K levels at veraison are 0.5 and 0.8 % K, respectively.

Zinc deficiency is associated with poor fruit set, and so the critical time for Zn application is prior to bloom. Varieties that are particularly susceptible to Zn deficiency, such as Merlot, should be maintained above the adequate level of Zn.

Both Boron deficiency and toxicity can be problems. A bloom petiole level of 100 ppm B is considered excessive and greater than 150 ppm is considered toxic.

There are no recommendations for calcium (Ca) tissue levels, as Ca deficiency has not been documented in California. In soils derived from serpentine minerals, however, there is frequently a calcium to magnesium imbalance, and vines respond positively to Ca addition.

There is no relationship between tissue levels and Iron (Fe) deficiency, so critical levels have not been established. Iron deficiency occurs in soils with a high lime content, and use of lime tolerant rootstocks is the best solution to this problem.

To determine whether your vine nutrition program is effective, you must sample consistently from year to year (same sampling area, as close to full bloom as possible, etc.). Therefore you should keep a record of where, when and how you sample. Retain records of all tissue analyses and fertilizer applications (including compost and manure) in order to assess the effectiveness of your vine nutrition program.

Table 2: Ranking of Vitis rootstocks for average bloom petiole values of NO3-N, P, K, and Zn.

	High	Medium	Low
NO3-N	039-16 Freedom St. George Ramsey	101-14 Mgt 5BB 1103P 3309C Schwarzmann 44-53M 110R	Harmony 5C 1616C 420A
P	110R 1103P Ramsey Freedom	Harmony 5C 5BB 039-16 Schwarzmann	St. George 420A 101-14Mgt 3309C
K	Freedom St. George Schwarzmann 44-53M 1616C Harmony 039-16 101-14 Mgt	5C 5BB Ramsey 3309C	1103P 140Ru 110R 420A 5A
Zn	Vitis vinifera (own root)	110R 3309C 101-14 Mgt 5BB 5C 1103P 420A	039-16 Freedom Ramsey Harmony

Table from Monitoring and Interpreting Vine Mineral Nutrition Status for Wine Grapes, Peter Christensen, 2001.

Christensen, Peter, "Analyze Tissue to Know Your Grapes, Process More Reliable Than Soil Analysis", Grape Grower Magazine, January 2001.

Christensen, Peter, Monitoring and Interpreting Vine Mineral Nutrition Status for Wine Grapes, 2001.

Christensen, Peter, Vineyard Tissue Sampling Guide for Plant Analysis, 1989.