The physical and chemical characteristics of the soil are fundamental to successful citrus growing.
It is hard to define the ideal soil for citrus growing, since it is necessary to combine characteristics of very diverse nature, which sometimes move in opposite directions. Consequently, the optimum state is an intermediary one.
The soil in the Region of Valencia, zones where citrus growing is more developed, such as the zone addressed by our study, share certain characteristics. Soils are generally deep, favouring strong root development, and thus ensuring that trees are well rooted and receive good nutrition. Soil texture ranges from sandy to clayey. The type most commonly found has a low capacity for water retention and a limy consistency (Agustí M, 2000).
The chemical characteristics of the soils vary considerably, and in these contexts depend to a large extent on the fertilization programmes implemented through the soils' cultivation years. It may be said, however, that evidence of deficiencies is often present, especially in micronutrients and Mg; in a low degree of correlation between their content in mineral elements and their folate concentration; and in a close relationship between the potassium content of the soils and the vigour of the trees.
Climate is a crucial factor in plant growth. Thus, all factors influencing climate have a decisive effect on the growth and cultivation of citrus.
Citrus grows in the zone between Latitudes 40º North and 40º South. Altitude is another factor to be taken into account when seeking to compare citrus farms located in different zones, since differences have been identified in at least two characteristics - the photoperiod and seasonal temperatures - which undoubtedly affect the growth of the crop, and hence the growing methods employed by the farmer. With respect to the present study, the farms analysed are all located less than 100 metres above sea level.
The most influential variable in terms of plant growth, flowering, and size and quality of fruit, is the temperature. The optimum growing temperature is between 23 and 34 degrees centigrade, although citrus can withstand higher and lower temperatures, depending on the point in the annual cycle at which they occur. In the Region of Valencia, temperatures during the active phase of the tree range between optimum values, while in the phase in which the tree is still, temperatures are much lower (between 5 ºC and 15 ºC), although the effect on the tree is not negative, since this occurs during the tree's vegetative state.
Relative Humidity does affect the quality of the fruit, although in this regard, citrus can adapt to extreme conditions. In the zone considered by this study, the average normal levels for relative humidity range from 40 to 60 percent.
The water need of citrus has been much studied, and is considered to be between 7 500 and 12 000 m3/ha. The influence of this factor on growth is critical, since the water not provided by rain must be supplied by the farmer, and this has a direct effect on the growing system, and thus on production costs. Rainfall in the growing region in question is XXXX mm, although it is distributed unequally, with periods of torrential rains, followed by very dry periods, which means that an irrigation system must be installed.
The Papadakis Classification enables us to establish climatic analogies for regions at the same latitude, and can be used when introducing new varieties or ecotypes, and to improve classification of optimum growing zones.
The results obtained were as follows:
Coldest month: January.
Ave. of min. absolute temps. in coldest month: -0.10 ºC.
Ave. of min. temps in coldest month: 5.10 ºC.
Ave. of max. temps. in coldest month: 15.40 ºC.
Winter-type: Citrus (Ci).
Average of maximum temperatures in hottest week: 27.35 ºC.
Summer-type: Gossypium (less hot) (g).
THERMAL REGIME: Semi-hot subtropical (Su).
Annual precipitation: 525.9 mm.
Annual PET: 895.9 mm.
Annual humidity index (Hi): 0.586
Washing water (Ln): 139.5 mm. < 20% annual PET.
HUMIDITY REGIME: Mediterranean Dry (Me).
CLIMATE-TYPE: MEDITERRANEAN SUBTROPICAL (Me, Su).
Pests play a very significant role in citrus growing, both in terms of production levels and fruit quality. This can have a serious impact on the profitability of the fruit, hence the importance of studying them.
In the growing region addressed by this study, the main pests to be found are the following: