“Drip irrigation is probably the single most important step that’s been made in changing vineyard nutrition practices,” says a University of California, Davis emeritus viticulture specialist.

Speaking at the 2008 San Joaquin Valley Table Grape Seminar in Visalia, Peter Christensen, in reviewing fertilization principles, said research associated with drip irrigation developments in the past few decades has taken much of the guesswork out of vineyard mineral nutrition.

The result has been improved availability of nutrients in the soil profile, greater efficiency of application, and improved timing and rates of fertilizers, particularly nitrogen and potassium.

Along the way, more was learned about how different scion and rootstock varieties respond and how best to use foliar application of nutrients.

Nowhere, he added, has drip been more instrumental than in refining the timing of measured amounts of nitrogen. The practice minimizes leaching as long as irrigation does not exceed plant water use.

Research has shown that grape vines depend heavily on nitrogen stored in roots and other permanent vine parts for early growth. “This minimizes the need for soil nitrogen at bud break,” he said. “Consequently, nitrogen is best applied in late spring after the frost danger period and when vine uptake and demand are increasing.”

Noting that the main goal of nitrogen fertilization is to promote crop development and not vegetative growth, Christensen said about three pounds of nitrogen is removed for each one ton of fruit, making a basic requirement of about 30 pounds per acre for a ten-ton crop. Other nitrogen sources include that from irrigation water, crop residue, mineralization of soil organic material, and legume cover crops.

Irrigation studies showed that with drip, applied nitrogen could be reduced from 30 percent to 50 percent and timed precisely for needs of the vines.

One of the main things to remember in nitrogen application is that too much of it is more often a problem than too little, Christensen added. “It’s a good idea to monitor your nutritional analyses on an annual basis to see where you’ve been and where you are going.”

Turning to rootstocks, he said 3-year average petiole nitrate-nitrogen levels determined in Kern County table grape trials differ greatly by rootstock. For example, Freedom had 2,220 parts per million (ppm), while own rooted vines showed only 635 ppm. Therefore, the high-vigor Freedom’s nitrogen needs are modest to nil.

In planting a new vineyard, growers should consider the research that’s been developed on rootstocks to get an idea of the responses to nutrition to expect from the various rootstocks.

Excessive nitrogen can lead to adverse effects of poor bud fruitfulness, fruit set, ripening, and disease pressure. These hinge on soil and root conditions, vineyard design, fertilizer history, and above all, Christensen said, on the grower’s observation and judgment.

In foliar applications, he said, stay with a product that exclusively corrects the deficiency you have when there are no other deficiencies.

“One problem I’ve seen through the years is that when growers have a zinc deficiency they will use a product with some zinc in it. The product often has a little bit of everything but not enough of any one element to correct the target deficiency.

“These products add cost, and they can tip the nutritional balance, creating greater problems than they are worth. Before you use these products on a broad scale, set out some trials and see if you get any good out of them.”

In describing three typical nitrogen disorders, he said no clear solutions exist, although judicious use of nitrogen and avoiding nitrogen applications near bloom and veraison are known to be critical in preventing problems.

“Use only what is needed to maintain a calm, uncongested, healthy leaf canopy with daylong flecks of light penetration into the canopy and fruit zone,” Christensen recommended.

Among common nitrogen-induced disorders, so-called “spring fever” can occur in Thompson Seedless and Flame Seedless vineyards when springtime temperatures fluctuate wildly, particularly a cool, cloudy period followed by a warm spell.

Leaf edges usually curl upward, may die, and may resemble an early season potassium deficiency. Affected leaves show elevated, toxic concentrations of putrescene, an amino acid.

Another disorder, early bunch stem necrosis, can appear in vigorous vineyards with dense canopies. Fruit set is greatly reduced, and some clusters may dry completely. Some research indicates the cause to be high tissue nitrate and ammonium levels and poor carbohydrate availability. Studies have demonstrated the USDA rootstock 10-23B has tolerance to this disorder.

Late bunch stem necrosis, also known as “waterberry,” affects ripening berries during veraison and fruit maturation, initially as dark, necrotic areas on cap stems. Afterward, berries shrivel and dry. Researchers have traced the cause to elevated levels of ammonium in the rachis and presence of putrescene. Shading also is linked to this disorder.

Christensen said tissue analysis is the most effective and reliable method to survey and monitor a vineyard’s nutritional status because it directly measures the concentration of nutrients accumulated in the vines.

Soil analysis, on the other hand, indicates the nutrients in the soil and cannot account for plant availability or demand.

Nevertheless, he added, since nitrogen levels vary according to variety, tissue nitrogen levels should be only a general guide for tracking year-to-year trends. Nitrogen fertilization should be primarily based on knowledge of nitrogen inputs and assessment of vine growth and vigor.

Bill Peacock, who is retiring after more than three decades as a Tulare County farm advisor, traced the developments of drip irrigation that made it popular among San Joaquin Valley table grape growers.

Along with the benefits in fine-tuning vines’ nutritional status, improved irrigation efficiency brought about new varieties and advancements in trellis design and canopy management, he said.

“Thirty years ago,” he added, “4 acre-feet were commonly applied to table grape vineyards that produced about 400 boxes per acre. Today, [with drip irrigation] we average more than 1,000 boxes per acre using a little over 2 acre-feet.”

An historical perspective for post-harvest handling of table grapes was offered by Don Luvisi, emeritus Kern County farm advisor, who warned no replacement for sulfur dioxide is expected in the near future.

Luvisi urged growers to be wary of products that claim to be a replacement for the traditional fumigant, but do not have sufficient testing to support the claims.

Although testing of alternative materials is being done, he said, table grapes are limited because they cannot be dipped in or sprayed with materials to combat spoilage and because sufficient penetration of a material throughout clusters is difficult to achieve.