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Grape fertigation handbook nears release

McGourty is project leader for development of a University of California fertigation handbook for wine grapes, now in final review by a panel of university specialists before it goes to press.

Fertigation is the practice of using drip irrigation to deliver precise, uniform amounts of water-soluble fertilizers to the crop.

McGourty outlined contents of the handbook during the recent Fertilizer Research and Education Program (FREP) Conference in Tulare, and said it will be a valuable tool for growers in developing environmentally safe and effective fertigation programs.

"Drip and micro-irrigation have brought a veritable revolution in wine grape grower in the last 10 years," McGourty said.

The 60-page publication is an extensive compilation of existing literature in a convenient package, tailored specifically to grape vines, to help growers catch up with that revolution.

Yield limitations

Use of fertigation, he predicted, will be an increasingly important issue for not only for North Coast growers but those throughout the state. "With the surplus of acreage we have, wineries are going demand that growers farm for flavor, and this is where nitrogen is going to make a big difference. I can guarantee that wineries are going to start putting yield limitations in contracts."

When it comes to putting extra things through drip systems, he said, "we have to pay attention to how to keep them working properly. So in the handbook we cover fertilizer materials and their suitability to drip."

In addition to detailing nutritional needs of grape vines, the book deals with design, selection of equipment, and installation, including backflow prevention, of systems.

Growers see fertigation as a means of rapidly augmenting nutrition to vines. However, McGourty explained, this is not the correct approach for all nutrients and is limited to materials that are soluble in water.

While nitrogen is easily adapted for such application, growers have to be cautious of environmental concerns and not exceed safe levels of nitrogen in groundwater. "It’s much easier to keep nutrients out of a water supply than it is to remove them."

The handbook has steps to key materials with vine petiole analyses. In some cases, soil or foliar applications of nitrogen may be preferable to fertigation.

Can clog emitters

When phosphorous, although rarely needed for a vineyard, is applied through drip, it can combine with calcium in the water to clog emitters.

"I try to stress to growers that they only apply nutrients that are really needed. And be sure that what you put on will not be in conflict with something else." And timing of an application is essential so vines take up nutrients when they do the most good.

Another issue is how a material applied will affect the soil. "It’s not just about applying N, K, and P. Grape vines also respond extremely positively to well-structured soil. Without a good root system, you don’t get the quality the vine is capable of providing."

The concept of "terroir" is, of course, as old as wine, but McGourty pointed out that it is gaining currency as the industry focuses on quality rather than quantity.

"It’s an old French term that has caught the imagination of American winemakers. Quite simply, it’s the expression of the environment of the vines. Wine is one of the rare agricultural products whose place and quality really make a difference in terms of flavor. Perception is also a big part of it."


Among the components of terroir are climate, sunlight, slope and aspect of the site, geology and soil characteristics, and water use during the growing season, all of which have to be fine-tuned to the variety used.

"Water use," he said, "is something we can address extremely well with a drip system, and drip has allowed us to grow grapes where we could not otherwise."

McGourty said grape growing on the North Coast is undergoing several other changes in concert with drip irrigation. On-farm composting is emerging as wineries search for ways to deal with leaves and other solid waste. More sophisticated use of cover crops governs nitrogen levels and provides beneficial insect habitat. Reduced tillage from use of cover crops also responds to run-off and endangered species issues.

Before the advent of fertigation, McGourty said, a lot of growers attempted to meet nutritional needs from a fertilizer bag after they chose very gravelly sites. "But it takes more than that to have the balanced grapes that wineries want to use."

A critical subject covered in the handbook is designing and installing systems with backflow prevention to avoid escape of materials into groundwater or streams. This will become more important, he said, since more-stringent EPA regulations will go into effect to avoid water pollution.

Storage, use

Listed too are best management practices for storage and use of fertilizers, in particular, concentrated materials.

Collaborating with McGourty on the handbook are UC irrigation specialists Larry Schwankl and Terry Prichard and UC viticulturists Pete Christensen and Bill Peacock.

FREP, a program of the California Department of Food and Agriculture, was established in 1990 and since then has funded more than 100 projects for a total of nearly $6 million. The funds are generated from a mill tax on sale of commercial fertilizers in the state.

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