Agriculture Reference
In-Depth Information
of sufficient Ca to bring soil pH to a desirable level at orchard establishment
may fail to maintain this, being followed by decreasing pH and a decline in
exchangeable Ca to inadequate levels (Bolton,
). Because
of its divalent charge, surface applications of Ca salts often result in little
downward movement in the soil.
In some important subtropical apple producing areas, e.g. Cape Province,
SouthAfrica(KotzeandduPreez,
; Korcak,
)andsouthernBrazil(BassoandWilms,
), the predominant soils are strongly acidic. Exchangeable aluminium
increases rapidly as pH (measured by the CaCl
method which records about
unit lower than the water-based method) declines below
.
(Terblanche
et al.
,
). This strongly suppresses Ca uptake leading to low leaf as well
as fruit Ca. Pre-planting liming is essential to overcome this and liming to a
level greater than that needed to eliminate Al effects is necessary to optimize
Ca concentrations in the plant and control bitter pit. Annual liming is then
neededtomaintainsoilpHandCasupply.Withoutsuchtreatmentslossesfrom
calcium deficiency disorders, especially bitter pit, are very severe (Terblanche
et al.
,
).
Calcitic or dolomitic lime, or gypsum, are used for soil rectification depending
onacidityandMgstatus.Thebalanceofnutrientsinthesoilstronglyinfluences
Ca uptake. This is depressed by non-specific cation competition by K, Mg and
NH
and enhanced by anions such as NO
and PO
(Bangerth,
; Kotze and du Preez,
; Wilms and Basso,
; Kotze,
). Post-
planting applications of K to K-rich soils can reduce fruit Ca levels (Terblanche
et al.
,
). Supply of N as ammonium can greatly depress the Ca uptake by
apple seedlings relative to that when N is supplied as NO
(Kotze,
) and
can give lower Ca content and a higher bitter pit incidence in 'Cox' apples
(Ludders,
). It also depresses the Ca content of 'Fuji' apples (Motosugi
et al.
,
).
Uptake by the roots
The first step in Ca uptake from the soil solution is penetration of the apoplast
or free space which occupies the cell wall network of the root cortexexternal
to the cell membranes. This is very rapid in spruce (Kuhn
et al.
,
) with a
half-time for
Ca entry in solution culture experiments of only
-
minutes
and a plateau level in the cortexattained in about
minutes. This indicates
very rapid equilibration in the entire apoplast of the cortex, and is compatible
with Ca-binding to mobile and fixed binding sites being the major factor
modulating the kinetics of Ca
+
entry.
In contrast, movement into the cell walls of the stele, in spruce, has a half-
time of
-
minutes and the plateau level is not attained until about
minutes from
Ca supply. This indicates the importance of the endo-
dermis as a barrier. The endodermis typically has a suberized Casparian strip
