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58 p r a c t i c a l w i n e r y & v i n e ya r d M a r c H 2 0 1 4 g r a p e g r o w i n g I mposing soil moisture deficits during certain phenological stages in grape- v i nes i s a com mon prac t ice i n California. Deficit irrigation strategies, like regulated deficit irrigation (RDI) or sustained deficit irrigation (SDI) have become commonplace practices adopted to control shoot growth and canopy devel- opment, without significant loss in yield, while improving fruit composition. Growers should be aware, however, that the soil moisture deficits not only influence vine water status, growth, yield and fruit composition, they also influence the soil environment. Soil moisture defi- cits can have effects on specific soil prop- erties such as thermal conductivity, soil temperature, soil hardness, microbial activity and mineral nutrient availability. Among all the mineral nutrients, the availability and uptake of potassium is the most responsive to changes in soil moisture levels. 1,2,3,6 In this article, the effects of soil moisture deficits on vine potassium status are reviewed. Influence of irrigation on vine nutrient status Soil moisture deficit can have an influence on the availability of mineral nutrients in soils, although not all nutrients are affected. The availability and uptake of potassium (K) and, to a lesser extent, phosphorus (P) appears to be more depen- dent upon soil moisture levels than the availability of other macronutrients such as nitrogen (N), Calcium (Ca), Magnesium (Mg) or micronutrients. For example, in a previous study of irri- gated and non-irrigated Cabernet Sauvi- gnon grapevines grown on calcareous soils in Spain, 6 petiole and leaf blade analysis showed clearly that vine K sta- tus was more responsive to irrigation than any other macronutrient (Table I) or micronutrient (Table II). There were small differences in macro- nutrient status at bloom between irri- gated and non-irrigated vines. The concentration of petiole N and P declined between bloom and véraison in both irri- gated and dry-farmed vines. On the other hand, petiole K declined in non-irrigated vines only, while it remained constant in irrigated vines due to higher soil mois- ture content. The concentrations of both Mg and Ca rose sharply in both irrigated and non- irrigated conditions between bloom and véraison, indicating adequate uptake of both nutrients. At véraison, both petiole and leaf laminae analysis showed a sig- nificantly higher K level in irrigated vines than non-irrigated grapevines. At harvest, the petiole K levels were three-fold higher in irrigated grapevines than in non-irrigated grapevines, while other macronutrients were not influenced by irrigation. The petiole levels of K at harvest were quite low in non-irrigated vines, indicating that these vines may be deficient, although foliar symptoms were not observed. In the same trial, the availability of micronutrients (iron, manganese, copper or zinc) was not affected by irrigation (Table II). Petiole samples taken at bloom, véraison and harvest, plus samples of leaf laminae at véraison, showed no difference in the iron (Fe), zinc (Zn), manganese (Mn), or copper (Cu) levels between irri- gated and dry-farmed vines. The concentration of these micronutri- ents was considered adequate, and no visible deficiency symptoms were observed. In short, the results of this trial demonstrate how soil water deficit could lead to a late-season K deficiency, while the availability of other nutrients was not impacted by a soil water deficit. Potassium availability in relation to soil moisture content Potassium is present in soils in several dif- ferent states: potassium dissolved in soil solution, potassium absorbed by cation exchange complex, potassium fixed by clay minerals and potassium minerals. The total potassium pool is usually much greater than the readily available potassium pool, which is usually consid- ered to be comprised of K in soil solution and K absorbed by the cation exchange complex. Grapevines, like all plants, obtain min- eral nutrients as cations or anions from the soil solution. The availability of all nutrients is governed to some extent by their solubility or amount in soil solution. Since mineral nutrients are dissolved in soil water, it is to be expected that soil moisture should have an impact on nutri- ent solubility. Michael Sipiora, viticulturist, Huneeus Vintners, Rutherford, Calif. BY Soil moisture and mineral nutrient status IMPACT ON GRAPEVINES Magnesium deficiency Late-season potassium deficiency