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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 59 g r a p e g r o w i n g Euro-Machines, Inc. West: Fairfield, CA 94534 - Phone 707-864-5800 East: Culpeper, VA 22701 - Phone 540-825-5700 info@euromachinesusa.com www.euromachinesusa.com A subsidiary of Scharfenberger Company Germany Euro-Machines offers high-quality equipment based on your vision to bring out the best of every vine, with our proven history in the careful handling of grapes and together our love of good wine. Europress 6-320 hl Europress-Cool with cooling jacket Euroselect berry selector Destemmer crushers Sorting equipment Conveyor belts Vibrating tables Custom-made tanks Pumps Braud grape harvesters and much more Get the best out of your grapes. The Europress exclusively by Euro-Machines There is evidence that the concentra- tion of K in soil solution increases as soil moisture declines (Table III). However, even though the concentration of K in soil solution may increase as soils dry, the total amount of K dissolved in a given volume of soil may be small. 2 Even though the concentration of K in soil solution may change in response to soil moisture deficits, previous research concluded that soil moisture levels have a greater impact on the mobility and, hence, availability of K to roots. 1,2,3 An understanding of the concepts of mass flow and diffusion as first proposed by S.A. Barber is essential for under- standing how soil moisture content influ- ences K availability. 1 Water content appears to have a greater influence on the mobility of K in the soil, rather than the solubility, which turns out to be the rate- limiting factor for K uptake by crops. 1,2,3 Potassium is relatively immobile in soils. Mineral nutrient availability depends largely upon the mechanism of transport to the roots: either mass flow or diffusion. 1 Some nutrients such as N, Ca, Mg and the micronutrients move to the roots primarily by the mechanism of mass flow. The concentration of these nutrients in soil solution is considered to be high enough that the flow of water from soil to the root surface caused by vine tran- spiration and water uptake (mass flow) is sufficient to supply nutrient to root surface. In essence, these nutrients move with soil water fluxes and are consid- ered to be mobile. On the other hand, potassium and, to a lesser extent, phosphorus, are not pres- ent in soil solution in sufficient quanti- ties for mass flow of the transpirational stream to provide enough nutrients at the root surface for uptake. Instead, potassium moves primarily by diffusion from cation exchange complex or miner- als to the root. Diffusion of nutrients (in this case potassium) occurs in the soil solution along a concentration gradient from high concentration (cation exchange complex) to the root surface, and is created by uptake of potassium. R. Kuchenbuch et. al. measured the effect of soil moisture on the K diffusion rates in soils. 2 For the soil used in their experiments, the effective diffusion coef- ficient declined by 61% as soil moisture content dropped from 34% water to 19% water by volume (Table III). Thus, soil moisture has a direct influence on the rate that K diffuses through the soil to the rhizosphere. In short, K is not con- sidered a mobile nutrient in soils and must move through the soil solution to the root surface. Figure 1. Effects of potassium (K) fertilization and supplemental (SUPP) or standard (STD) grower irrigation on petiole K (% dry weight) of Carneros Pinot Noir grapevines. 8