May 3, 2003
Maintaining just the right amount of boron, an essential micronutrient for grapes, can be tricky for growers in some parts of California, but new research in foliar and drip-irrigation application helps in striking the knife-edge balance.
Bill Peacock, Tulare County farm advisor, says once boron amounts are determined by tissue samples and stabilized, a cornerstone for a good maintenance treatment with drip is one pound per acre every three to four years, or one-quarter pound to one-third pound each year.
He cautioned that annual applications at the one-pound rate for three or four years will cause excessive levels in the tissue.
Peacock told of his trials with boron on grapes from 1988 to 2001 during the recent San Joaquin Valley Table Grape Seminar at Visalia. Collaborating in the trials were Pete Christensen, University of California viticulture specialist emeritus, and Bob Beede, Kings County farm advisor.
The acceptable range for soil-content boron in grapes is indeed narrow, from deficient at less than 0.4 parts per million to toxic at 1 ppm or more. Irrigation water with a concentration of 1 ppm boron can also cause problems. In vine tissue, less than 30 ppm can be deficient, while anything greater than 100 ppm can be toxic.
Too little boron can lead to irregular ripening shot berries and poor production, even though vines later overcome symptoms of foliar speckling.
Too much, perhaps when applied through unneeded foliar or soil applications, causes toxicity with cupped, irregularly shaped foliage.
Natural boron low
Boron content of granite and other igneous rock of the eastern slopes of the SJV is relatively low. That leads to some deficiencies on that side of the valley, particularly in sandy soils, low spots, or around irrigation valves where excessive leaching occurs.
Toxicity along the east side is rare but may occur with over-fertilization with materials containing boron. Too much boron, however, can reach vineyards on the west side of the valley and in coastal counties where lakebed soils have accumulated material from sedimentary rock of the coastal range. Excesses in soil must be leached out by using high-quality, boron-free water.
Peacock said the studies on a mature Thompson Seedless vineyard in Tulare County showed that one pound of boron per acre could be applied safely through drip, even in a single application. Uptake of drip-applied boron is very rapid and response can be seen in about three weeks.