Citrus Production - Horticulture: Plants for People and Places - page 185

Albrigo ( 1994 ), Spiegel-Roy and Goldschmidt ( 1996 ), and Vacante and Calabrese
( 2009 ) for a more extensive information.
Mineral Elements
Citrus varieties respond readily to nitrogen and potassium applications, however
their effectiveness depends on the type of soil and the nutritional status of the plant.
Phosphorus has very seldom been found lacking in soils planted to citrus, with the
exception of some countries in South America. The minor elements seem to have
no detectable effects on fruit production and quality in absence of gross visible
deficiencies. Nitrogen is the most important element regarding fruit production and
quality.
Nitrogen fertilizer is generally required in greater amounts by citrus than any
other fertilizer. With an N deficiency, yield reduces markedly and fruit tend to be
smooth, smaller and of somewhat lower acid content. High N levels are particularly
detrimental to orange peel colour, thus an increase in N rate increases the percent
of green fruit. Moreover, in addition to temperature, the degreening and regreening
of citrus fruit are affected by N fertilization. Most citrus fruit degreens in response
to reduced flow of N to the fruit accompanied by increased concentrations of sugar
in the epicarp, both usually induced by cool temperatures; regreening of late-season
citrus fruit in the spring and summer can be attributed to renewed N flux and a re-
duction in sugar concentrations induced by warming temperatures. Volatile peel oil
yield increases with increased N rates in sweet oranges and lemons. In orange juice,
(1) increased N rates increase both the red and yellow pigments, and (2) varying N
levels on TSS and TA content has been small and inconsistent. A similar trend has
been observed in the concentration of ascorbic acid. Nitrogen level generally has no
effect on lemon fruit quality.
High levels of N in orange trees are associated with thicker peel, a lower juice
percentage and coarser peel texture. In Satsuma mandarins, high N fertilizations
may increase fruit size, but they produce poor quality fruit with rough peel, low
sugar and high acid content.
Nevertheless, nitrogen applications are essentially ineffective on a wide range
of N-rich soils ranging from fine sand to moderately heavy loamy clay soils. More-
over, withholding N from previously well-fertilized soils had no effect on fruit com-
position.
The timing of N application and the form in which it is applied influence fruit
quality of oranges, mandarins and grapefruits.
Regarding phosphorus, neither yield nor fruit size of oranges and lemons in-
creased with an increase in phosphorus rate. Only for grapefruit an increased yield
and fruit size paralleled P content in leaves. In solution-culture experiments, it has
been shown that fruit symptoms of P deficiency include coarse, thick but well-
coloured rind, hollow cores, and high total acidity (TA) in the juice, and that the
correction of the deficiency generally reversed these characteristics. An important