Nutrients for Healthy Vines and Good Wines - Soils for Fine Wines - page 128

vides the immediate source of these elements for plants (section 4.6.1). Interlayer
K in micaceous clays is a more slowly available source. Total reserves are there-
fore variable, reflecting the conditions of soil formation, but exchangeable Ca 2
content is usually 1,000-5,000 kg/ha, Mg 2 is 500-2,000 kg/ha, and K is ca.
1,000 kg/ha.
False K Deficiency . This disorder can occur on young vines in their third or
fourth season, and it often coincides with periods of fluctuating temperatures be-
fore flowering. The lower leaves lose color, and the margins curl and become
necrotic. The leaf K:N ratio, which is especially important in vines, is temporar-
ily out of balance. Leaves with relatively low K concentrations frequently have
high concentrations of the diamine putrescine, formed by breakdown of the amino
acid arginine (and release of NH 4 ions).
Forms of K, Ca, and Mg Fertilizers . The commonly available K, Ca, and Mg
fertilizers for vineyards are listed in table 5.8. KCl is the cheapest form, per unit
of K, but K 2 SO 4 is generally favored, especially in soils where salinity and high
Cl are a problem. KNO 3 is the most expensive of the K fertilizers and should be
used only where N as well as K is required. As with N, K must be controlled
within well-defined limits to achieve good wine quality (section 9.8).
Losses and Residual Effects . There are no gaseous losses of these elements. Apart
from crop removal, the main loss is by leaching; for example, Ca 2 is the main
cation accompanying NO 3 when it is leached from the soil. Many Australian
duplex soils tend to have high exchangeable Mg 2 contents in the subsoil, which
may lead to a K:Mg imbalance in deep-rooted vines. Conversely, a K-induced Mg
deficiency may occur in high K soils with a K-accumulating rootstock, such as
Ramsey.

Common K, Ca, and Mg Fertilizers Used in Vineyards Table 5.8

Element

Compound Formula Content (%) Comments

K

Potassium sulfate K 2 SO 4 41 Easier to handle than KCl; more

expensive per kg K than KCl, but
preferred on saline soils

Potassium nitrate KNO 3 38 Very soluble, supplies K and N

Muriate of potash KCl ca. 50 Very soluble in water; natural source is

from salt deposits

Ca a

Gypsum CaSO 4 .2H 2 O 18-22 Sparingly soluble in water; primarily

used to improve soil structure; contained
in SSP

Mg a

Epsom salts MgSO 4 10 A water-soluble form of Mg fertilizer

a Ca and Mg are also supplied in liming materials (see table 5.9).