Base fertilizer use on tissue analysis trends rather than a single test result

By Marni Katz

If you follow the old University of California critical use values for nitrogen and potassium in winegrapes, you may be overspending on fertilizer and promoting undesirable vegetative growth.

When deciding how much fertilizer to apply, don’t base your decision on one year’s sampling results. Instead, look for year-to-year trends, says University of California viticulturis Larry Williams at the UC Kearney Agricultural Center near Parlier.

He favors basing fertilizer decisions on laboratory testing results of tissue samples rather than soil samples because petioles or leaves reflect an estimate of nutrients absorbed by the vine.

“Here in California, we recommend growers use tissue analysis because the grapevine is integrating what is taken up from the soil,” Williams says. “And there are a lot of variables that may not reflect what is getting from the soil to the vine.

“The cost of a tissue analysis may be less than applying nitrogen or potassium fertilizers. And if you don’t use tissue analysis but apply fertilizer anyway, you may be overfertilizing, resulting in excess vegetative growth and possibly contributing to groundwater contamination.”

Bucking tradition

During the 2002 growing season, Williams analyzed petioles from 75 vineyard sites with 24 different raisin, wine and table grape cultivars on 13 different rootstocks for nitrogen and potassium.

His findings challenge fertility recommendations developed in the 1960s and 1970s for Thompson seedless vineyards in the San Joaquin Valley.

The old recommendations set sufficient nitrate-nitrogen critical values at 500 parts per million to 1,200 ppm; questionable at 350 ppm to 500 ppm; and deficient at 350 ppm. Critical deficiency values were 0.15 ppm for phosphorus and 1.5 ppm for potassium in grapevine petioles.

Based on his research, Williams says those numbers probably are high.

“I’m now of the opinion that a value for nitrate-N in petioles somewhere between 100 and 200 ppm is adequate,” he says. “Those old levels are still valid. But if you get a value of 250, you may be applying nitrogen when in fact you don’t need to.”

Similarly, he says bloom-time petiole deficiency levels of potassium also could be lower than the previously recommended 1.5 ppm.

“I think you could go lower for what is considered adequate, down to perhaps 1 ppm before you fertilize,” he says of potassium.

Williams’ research also found that rootstocks shouldn’t be a criteria for establishing critical deficiency levels.

“I’m of the opinion that critical values established for nitrogen, potassium and other nutrients, including micronutrients, will be good for whatever you grow, regardless of rootstock” he says.

In addition, he found that the location of the sampled leaf—whether in the canopy interior or exterior or near a cluster—didn’t appear to matter. What mattered was pulling a mature, fully expanded leaf.

“The bottom line is, don’t take young leaves that are still growing; take petioles from mature leaves,” Williams says.

Track nutrient levels over time

You should place less emphasis on critical nutrient levels and more on regular bloom-time sampling and tracking of year-to-year trends, Williams says, citing a cabernet sauvignon vineyard as an example.

In the first year of a study with cabernet sauvignon on three different rootstocks, the mean nitrate-nitrogen level in the petioles was 58 ppm (dry weight basis) with no deficiency symptoms, Williams says. The next year, the mean was about 1,300 ppm without applying any fertilizer, and the following year it decreased to 55 ppm.

“It’s this type of information that is making growers question established critical values,” Williams says. “I tell them is if they get a really low reading one year without deficiency symptoms, don’t apply nitrogen, and wait and see what the values are next year. If they are still low, then maybe it’s time to apply some N.”

UC Cooperative Extension farm advisor Jennifer Hashim agrees.

“Use these numbers as a guide,” says Hashim, who serves Kern County. “But if you’re not seeing any deficiency and the vines are growing just fine, you’ve got to take those numbers with a grain of salt.

“Numbers are not the end-all, be-al—we use these numbers to develop trends. Don’t rely on hard numbers so much as what is happening from one year to the next,” she says.

Historical database guides decisions

Dan Bosch, senior viticulturist for Constellation Brands’ Icon Estates Wines in the California Napa Valley, carefully tracks grapevine tissue analyses throughout the group’s vineyards.

Since 1987, Bosch has recorded yearly petiole nutrient levels for each block in a database that can be easily referenced. As a result, he can use trends going back 20 years to determine whether nutrient levels in individual blocks, or even certain varieties across the blocks, have changed over time.

Each year around bloom, Bosch takes petiole and leaf blade samples from healthy and symptomatic vines to compare. All petiole samples are tracked separately by variety and vineyard.

“We look for differences when we see symptoms show up and use that, in addition to our baseline, to draw conclusions about our fertility program,” Bosch says.

The information is used to avoid overapplying nitrogen, particularly to the red varieties. Bosch also correlates his deficiency baseline of 100 to 200 ppm nitrate-N with what he sees in the vineyard.

“If it’s continually dropping every year or if something in the field is different, say maybe we added a cover crop that is going to be cultivated, we would look at our N levels,” he says. “We want to be on the cusp of just having enough.”

With white varieties, such as sauvignon blanc, Bosch strictly follows critical values and typically applies nitrogen fertilizer when petiole levels fall below 200 ppm.

Establish a baseline

Hashim recommends annual tissue sampling for three to four years for new or recently purchased vineyards so you can establish a baseline to which to compare trends within each vineyard. As part of the baseline, she recommends a full panel-nutritional analysis that includes nitrate-N, total N, zinc, boron, chloride, sodium, potassium and phosphorus.

Once that baseline is established, annual sampling may not be necessary, although regular sampling will help track changes in the vine’s nutrient status, Hashim says.

“Then you should really only have to sample the vineyard for questionable nutrients that you might see deficient,” Hashim says. “If you’ve made a change in the vineyard, such as a trellising retrofit or observed a change in yield or vigor in the vine, then this would warrant further testing.”