Agriculture Reference
In-Depth Information
Lack of drainage can result in accumulation of salts, which, in turn, can reduce
tree development, leaf size, which can dehydrate and cause abscission of the fruit,
resulting in reduced fruit size and yield. Citrus are very susceptible to salinity where
the conductivity of soil to a depth of 1.25 cm is higher than 3.2 dS m
−1
is considered
dangerous for the growth of citrus.
The physical characteristics of citrus soils are more important than the native
fertility. Soil analysis for diagnosing the nutritional status of a tree has serious limi-
tations. In fact, there is a lack of a general correlation between tree behaviour and
soil composition fertility. Thus, nitrogen analysis is of limited value for diagnostic
purposes. For example, less than 5 ppm nitrogen as nitrate does not necessarily
indicate a deficiency, but rather that the nitrogen level must be verified by leaf
analysis. Phosphorus analyses are of value only in the evaluation of accumulation,
movement, and redistribution of soil phosphorus, since its loss by removal with
the fruit amounts to only about 2 % of the total phosphorus in the 0-1 m soil depth.
Soil analysis for potassium is also of limited interest. Although there is a significant
correlation between leaf potassium content and exchangeable potassium in the soil,
in orchards where there is a need for potassium it is difficult to obtain an adequate
increase in leaf potassium content from soil-applied potassium.
In general, a soil of low fertility with good drainage and other physical charac-
teristics is superior to a soil of high fertility with poor physical characteristics. The
deep, well-drained sandy loam soils are considered better for citrus production.
Climatic Conditions
Probably the most important climatic variable determining fruit set and quality is
temperature. In citrus trees growing in tropical-type climates, vegetative growth
competes with fruit growth and such a competition may be reflected in the intensity
of fruitlet abscission, fruit size, carbohydrate reserves and even in fruit colour. It,
together with the relative low rates of photosynthate production, emphasizes the
limitations of carbon for citrus tree growth that may result in alternate bearing and
reduction of fruit quality.
In seeded cultivars, temperatures ranging from 15 °C to 20 °C improve pollen
germination and pollen tube growth. In parthenocarpic cultivars, temperatures be-
tween 20 to 22 °C/11to 13 °C (day/night) contribute to increased fruit set, whereas
those between 30 to 34 °C/21 to 25 °C (day/night) fruitlet abscission is promoted in
'Valencia' sweet orange.
Fruit size is generally associated with air temperature. Night temperatures rather
than day temperatures largely control fruit growth rate. Fruit growth tends to be
greater in the 20-25 °C temperature range regardless of the day/night combination,
with the final fruit size of 'Ruby Red' grapefruit positively correlated with spring
temperatures and negatively correlated with summer temperatures.
Low air temperatures, below 13°C, cause colour-break of citrus fruits, while
high air temperatures influence regreening. However, temperature effects upon fruit
