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46 WINES&VINES March 2016 GRAPEGROWING PRACTICAL WINERY & VINEYARD M ineral nutrients applied as fertilizers do not move in a straight line from the point of application into grapevines; their jour- ney is fraught with obstacles and detours. Moreover, the 13 essential plant nutrients may interact with one another along the way. Positive interactions occur when the presence of one nutrient en- hances the activities of another. Negative interactions occur when one nutrient interferes with or suppresses another. Some noteworthy mineral ion interac- tions are reviewed below. Nutrient interactions in vineyard soils Positively charged minerals (cations) interact with one another in soils as they compete for adsorption in the ion swarm near soil particle surfaces. Those cations with the greatest charge and smallest hydrated radius have a competitive advantage. As a consequence, calcium adsorption is greater than magnesium adsorption, and magnesium adsorption is greater than potassium adsorption. A significant number of California vine- yard soils developed from rocks that are comparatively rich in magnesium. Where magnesium is prevalent, quantities of ad- sorbed potassium are low and, correspond- ingly, the risk of potassium deficiency in grapevines is high. This is a case of magne- sium negatively interacting with potassium (see photo at top right). Fortunately, cation exchange reactions and cation availability to vines can be manipulated by applying mineral amendments and fertilizers to enhance potas- sium availability in high-magnesium soils. Interactions affecting nutrient uptake by grapevines Mineral ion interactions also impact nutrient uptake by grapevine roots. For example, potassium, calcium and ammonium can strongly depress magnesium uptake. In high- magnesium soils, grapegrowers can benefi- cially use these interactions, applying ammonium, potassium and calcium fertilizers to avoid excess grapevine magnesium. How- ever, in low-magnesium soils (including many vineyard soils in Lodi and the Sierra Foot- hills), excess application of these competing minerals must be avoided, while at the same time applying sufficient magnesium ought to be applied to meet grapevine needs. Ammonium also stimulates phosphorus uptake, which in turn increases vine phospho- rus concentration and promotes root growth. For this reason, ammonium phosphate-based fertilizers are effective during spring and autumn flushes of root growth. Nitrate, another common form of nitrogen in fertilizers, promotes calcium uptake. Given that both of these minerals easily move from grapevine roots to shoots, calcium nitrate is highly effective in stimulating shoot metabo- lism and growth. It is also an excellent in- season complement to early and late-season ammonium phosphate-based fertilizers. When applied at the same time, however, ammo- nium suppresses nitrate uptake, exposing it to potential losses before it has an opportunity to benefit grapevines. Nutrient interactions in grapevine tissues Within plant tissues, virtually all 13 mineral nutrients have the potential to affect one an- other—if not directly, then indirectly through effects on grapevine growth and/or metabolism. Of the documented interactions, those involving nitrate are especially important in vineyards. Considering Mineral Nutrient Interactions Examining the benefits and detriments of minerals on grapevines and vineyard soils By Stan Grant This leaf from a Chardonnay vine displays symptoms of excessive magnesium. A Zinfandel grapevine leaf shows mottled, creamy chlorosis, a symptom of boron deficiency. This Riesling vine displays symptoms of manganese deficiency in its leaves. SOURCE: PROGRESSIVE VITICULTURE LLC