Agriculture Reference
In-Depth Information
negative effect of excess P in the soil reduces copper, zinc, boron, iron and other mi-
cronutrient availability. Phosphate also may affect nitrogen nutrition unfavourably.
In spite of this, P seems to have some effect in reducing the acid content.
Levels of K above the yield-limiting level are of only moderate importance to
fruit production and quality. Field evidence indicates that an increase in potassium
supply increases fruit size of sweet oranges, grapefruits, limes, and lemons. High
level of K delays fruit colour development in sweet oranges, Satsuma, and 'Temple'
tangor, remaining partially green up to harvest. Potassium applications frequently
result in an increase in ascorbic acid in the juice. High K rates reduce the incidence
of
creasing
, and
splitting
can also be reduced or almost completely avoided by rais-
ing the level of K in affected trees.
High levels of K often, but not always, increase peel thickness and reduce juice
percentage in oranges and grapefruit. Increasing K strongly increases TA in juice
and hence reduces the TSS:TA ratio, thus delaying maturity. In contrast, K applica-
tions to lemon trees reduce peel thickness and increase the percentage of juice in the
fruit, and also increase TA and the concentration of ascorbic acid in juice. Increased
K rates reduce the yield of peel oil in orange and lemon and tend to decrease yellow
pigments in juice of the sweet orange.
In Citriculture, concern for calcium (Ca) is due to its indirect effect of modify-
ing soil fertility and not on its direct effects as a nutrient. However, there are some
known effects of Ca that can improve citrus fruit quality. As mentioned above cal-
cium nitrate sprays have beneficial effects on fruit
splitting
, although the response
appears to be erratic. The application of calcium salts prior to or during maturation
has been shown to be effective in reducing chilling injuries by reducing cuticular
permeability. Calcium carbonate applied just at the beginning of colouring reduces
the occurrence of puffing by reducing rind water content. Calcium also delays man-
darin peel ripening and senescence.
The low native level of Mg in soil causes magnesium deficiency, and it is partic-
ularly acute on the light sandy soil from which Mg readily leaches. The deficiency
results in substantially smaller fruit that is lower in TSS, TA and vitamin C. In or-
anges, there is a quite marked paleness of colour of both pulp and peel.
Among micronutrients, boron, copper, zinc and manganese deficiencies may be
of some importance. Boron (B) deficiency produces gum pockets and greyish to
brownish discoloration in albedo of young and mature fruits, and reduces TSS and
juice content. Copper (Cu) deficiency causes dark excrescences of gum on the rind
of fruit, which may be associated with small cracks; at maturity, fruits are often mis-
shapen, with coarse rinds, and have a low content of TA and vitamin C. Zinc (Zn)
and manganese (Mn) deficiencies influence fruit set and size of some cultivars, such
as Clementine mandarins, which are quite sensitive.
Excessive fertilizer applications to the soil or by spraying, inappropriate pH so-
lution, the presence of relatively high quantities of biuret as an impurity in urea, or
perchlorates as impurities of potassium nitrate, can produce symptoms of toxicity
in the leaves that indirectly affects fruit quality.
The timing of foliar application of certain nutrients has a great impact on the
nature of their effects on the tree and may solve certain fruit quality problems. On
