Agriculture Reference
In-Depth Information
breakdown required minima of
g fruit fresh
weight respectively. In South Africa the minimum fruit Ca content to ensure
freedom from bitter pit is
.
,
.
,
.
and
.
mg Ca/
.
and
.
mg/
g for unsprayed and calcium-
sprayed fruit respectively (Terblanche
et al.
,
).
Calcium nutrition
Calcium is one of the major nutrient elements taken up by apple and pear
trees, gross uptake being similar to, or greater than, that of N and K and many
(
c
.
). The amount
in the fruits, however, is very low in comparison with that of N and K and is
only a few per cent of the total uptake by the trees (see Chapter
-
) times as great as that of Mg and P (Greenham,
).
ResearchonCanutritionhasthereforeemphasizednotonlywaysofincreasing
uptake by the trees as such but also, very specifically, ways of increasing Ca
supply to the fruits. Because of this, and the important interactions between
Ca in fruits and fruit quality, Ca nutrition is discussed in this chapter instead
of in Chapter
, Table
.
.
In general the supply of mineral nutrients from the soil to leaf cells, and
by analogy fruit cells, is considered to involve two stages involving mass flow,
diffusion and ion-exchange mechanisms and two stages primarily dependent
on active processes involving metabolic energy (Bowling,
; Shear,
).
Supply by the soil
The first stage in Ca uptake involves movement of Ca to the root surface.
For this to proceed with maximum efficiency the ion concentration in the soil
water must be high enough to enable the nutrients needed by the plant to reach
the root by mass flow. Under good orchard soil management the exchange
complexof the soil is dominated by Ca, and soil solution concentrations of
Ca are usually the highest of all cations (Adams,
). The Ca
content in the soil solution in most temperate zone soils varies between
; Korcak,
.
and
mM appears adequate. Mass flow
of soil solution, dependent on the transpiration rate, should provide ample
Ca fluxto the root surface under these conditions (Bangerth,
mM whereas at the root surface
.
-
). Adequate
soil moisture is, however, a pre-requisite for this. Liming may, however, be
needed to counter excess soil acidity and increase the supply of Ca. Marks
(
) found apple leaf Ca concentrations are usually at or below optimum
levels, in contrast to those of N, P and especially K which are commonly above
the optima for satisfactory growth. In the Netherlands, on sandy soils, Van der
Boon (
H
O) soil dressings increased leaf
Ca concentration and slightly reduced bitter pit incidence in 'James Grieve'
apples. A major problem even in temperate-zone soils is that incorporation
) found that gypsum (CaSO
.
