By Joe Traynor
Grape Grower Magazine, February 2003


Nitrogen fertilization is a very important input in grape culture. Too much nitrogen and fruit - and by extension, wine - quality suffers; too little, and yields fall off significantly. Grapes have a relatively low nitrogen requirement compared to almost any other agricultural crop. Sixty units of nitrogen annually are all that is needed in most cases. Extensive root systems, heavy winter pruning and wide row spacings contribute to this relatively low nitrogen requirement. With recent closer row spacings, we're seeing a modest increase in nitrogen fertilization rates.

Nitrate analysis of the stalks of leaves, or petioles, is used by most grape growers to fine-tune their nitrogen fertilization programs. Petiole samples are taken at full bloom and the analysis results are compared to standards developed by the University of California for Thompson Seedless and are shown in Chart 1.

Chart 1 is tidy and the six different categories give the impression that the nitrogen status of a given vineyard can be pinpointed by simply plugging petiole nitrate figures in the UC chart. Ag labs, consultants and farm advisors make extensive use of this chart - it dovetails with our desire for tidy solutions to perplexing questions. Unfortunately, it's not that simple, or tidy.

Chart 1
CATEGORY	NITRATE-N*, PPM
Deficient	below 350
Questionable	350 - 500
Adequate	500 - 1200
More than necessary	over 1200
Excess	over 2000
Possibly toxic	over 3000
* Divide PPM Nitrate by 4.4 to get PPM Nitrate-N

Some Limitations

First, the UC emphasizes that the chart is good only for Thompson Seedless. Second, and most important, cultural practices and weather conditions can have as much of an effect on petiole nitrate as can nitrogen fertilization.

The limitations of petiole nitrate analysis were recognized early on by UC workers. James Cook of the UC Davis viticulture department studied the subject extensively. In a 1966 review based on two UC studies, Cook said, "Perhaps the greatest drawback [of petiole nitrate analysis] to its universal application is the reaction to rainfall or irrigation. It has been shown that the petiole-nitrate level drops rapidly after an irrigation, requiring 10 to 14 days to recover." Citing his own work, Cook concluded,
" . . .bloomtime petiole nitrate level is correlated inversely with rainfall pattem from bud-break to bloom. Thus, high rainfall in the spring results in barely detectable nitrate levels in vines receiving abundant nitrogen application."

Unfortunately, Cook's reservations did not make it into the UC extension literature that accompanied the UC's petiole chart. Growers took - and still take - the chart as gospel.

Cook later collaborated with UC colleague Mark Kliewer to find a better method of delineating the nitrogen status of vineyards and they concluded that analysis of an amino acid in juice and canes called arginine was superior to petiole analysis. Kliewer and Cook implied the deficiencies in petiole nitrate analysis stating, "It also indicates that petiole nitrate differs only very narrowly between vines with low and high crop yields. For example, vines with petiole nitrate levels of 1000 ppm to 1200 ppm were almost always associated with low yields, whereas vines with 1400 ppm to 1600 ppm petiole nitrate had no reduction in crop weight."

The UC's arginine test created a flurry of interest in the 1970s but this interest has waned in ensuing years. Today, many consultants and vineyard managers still rely on petiole studies from the 1950s and petiole standards for nitrate are still provided by UC's extension service.

I used the UC's petiole standards when I first got into consulting in the 1970s and it wasn't long before I got into trouble. I took samples from several lush vineyards that used in excess of 100 units of nitrogen per acre and was confident that petiole analysis results would convince the grower to cut back on nitrogen. When the petiole analysis came back showing nitrate in the deficient range I was stunned. I called the lab and told them they probably made a mistake, but a re-run gave the same results, as did new samples split between two labs.

I found the same inconsistencies in subsequent samples and long ago abandoned petiole-nitrate analysis as a guide to the nitrogen status of vineyards - it just doesn't work. I now use total nitrogen in the leaves, sampled two or three times during the year. Total nitrogen is much more stable and not subject to the wild fluctuations of petiole nitrate.

The total nitrogen levels shown here in Chart 2 is not nearly as comprehensive as the petiole-nitrate chart, but it is a good guide when used in conjunction with field observations. So far it holds up for varieties other than Thompson.

Chart 2
OPTIMUM NITROGEN LEVELS IN MOST RECENTLY MATURED LEAF*
Bloom (early May)	Veraison (June-July)	Harvest (July-August)
3.3 - 4.0%	2.5 - 3.2%	2.1 - 2.7%
* Usually four or five leaves from the growing point (total nitrogen)

Total nitrogen analysis of leaves, and in some cases, leaves plus petioles, is used extensively in France and other European countries and also in South Africa and Australia. I have found it to be far more reliable than petiole-nitrate analysis. The vineyardist should also rely on what his vines tell him: lush growth indicates excess nitrogen; sparse growth and low yields may indicate insufficient nitrogen. Use this visual information along with leaf analysis for nitrogen to come up with a suitable nitrogen fertilizer program.


More Nutrition Work

UC extension viticulturist Pete Christensen worked extensively on vineyard nutrition in the 1960s, 1970s and 1980s. He also showed "... wide year-to-year variations ..." in petiole nitrate over a four-year period from 1964 to 1967. In a subsequent three year study from 1979 to 1981, Christensen compared total nitrogen analysis of leaves with petiole nitrate and conceded that, "Total nitrogen levels were much more stable, especially during the bloom period," but expressed reservations about using total nitrogen as a diagnostic tool partly because he felt that a portion of the total nitrogen was in a form that was not available for assimilation by plants. (*note - total nitrogen is used extensively in orchard leaf analysis and this question has not been raised by orchard scientists).

Part of the UC's reluctance to recommend total nitrogen in leaves as a diagnostic tool for grapes could be because petiole analysis is superior to leaf analysis for monitoring potassium status in grapes and it is much more convenient - and economical - to sample one plant part only.

The UC's James Cook studied vineyard nutrition as much as any individual and his 36 page review in 1966 is a tour de force on the subject. Cook's review is a thorough discussion of all phases of vineyard nutrition with emphasis on plant analysis and nitragen status, including over 200 references, 10 of which carry Cook's name as the principal author. Regarding nitrogen nutrition, Cook's frustration at the inadequacy of diagnostic tests as a definitive tool is apparent through-out his treatise. As Cook put it, "... whether visual symptoms, soil analyses, or tissue analyses - singly or in combination - are used as diagnostic tools, the proof of their usefulness must be established by well planned, carefully conducted field trials. Survey data can provide a range of values - high, medium and low - for whatever reference analysis system may be established, but calibration of that system to determine critical response levels must be on the basis of actual field-trial data. Without such calibration, statements that 'low' values are deficient are only speculative at best, and in many cases may be entirely in error." A number of such "speculative statements" are being made today on the basis of petiole-nitrate analysis.

Leaf analysis for total nitrogen is not a definitive tool for assessing the nitrogen status of a vineyard, but it can be quite useful. Use it along with field observations and yield data, and be skeptical of using petiole-nitrate analysis as a diagnostic tool; this particular emperor has no clothes.

Joe Traynor is a certified professional soil scientist, crop scientist and agronomist listed with the American Registry of Certified Professionals in Agronomy, Crops and Soils, Ltd. He holds multiple degrees from the University of California, Davis, is a member of the American Society for Horticultural Science, and is the author of Ideas in Soil and Plant Nutrition, published by Kovak Books.

References

1. Cook, James A. (1966). Grape Nutrition. Chapter 23, pp. 777-812, in the book Nutrition of Fruit Crops, Norman Childers, Ed. Rutgers Horticultural Publications.

2. Kissler, James J. (1957). Nitrate fluctuations and petiole sampling techniques with grape vines. MS thesis, Univ. of Calif., Davis.

3. Cook, James A. and Lloyd Lider. Mineral composition of bloomtime grape petiole in relation to rootstock and scion variety behavior. Proc. Amer. Soc. Hort. Sci. 84:243-254.

4. Kliewer, Mark and James A. Cook. (1974). Arginine levels in grape canes and fruits as indicators of nitrogen status of vineyards. Amer. J. of Enology and Viticulture 25:111-118.

5. Christensen, Peter (1969). Seasonal changes and distribution of nutritional elements in Thompson Seedless grapes. Amer. J. of Enology and Viticulture 20:176-190.

6. Christensen, Peter (1984). Nutrient level comparisms of leaf petioles and blades in twenty-six grape cultivars over three years. Amer. J. of Enology and Viticulture Vol. 35, No. 3.