Agriculture Reference
In-Depth Information
A more accurate calculation of a crop's water needs is the crop's
consumptive water usage, which combines leaf transpiration and water
evaporated by the soil and the leaf surfaces. These losses are af ected
by temperature, relative humidity, wind speed and radiation, and their
combined value is called evapotranspiration. A type 'A' evaporimeter, which
is a circular tank 1.20 m in diameter and 25 cm deep fi lled with water, is
used in meteorology; daily measurements are made to establish how much
evaporation occurred and it is expressed in mm/day. This value multiplied by a
location factor and the crop coei cient (Kc) (ground area covered by the crop)
will give the daily evapotranspiration or consumptive water use and is used to
calculate how much and when to irrigate according to what is evaporating.
FERTILIZATION
Fertilizer needs and dosages
A number of mineral elements are required for plant growth and development
and are classifi ed, according to the amounts needed, into macronutrients and
micronutrients. Macronutrients are needed in higher amounts and include
nitrogen, phosphorus, potassium, sulfur, calcium and magnesium. The micro-
nutrients are needed by plants in lesser amounts and include iron, zinc,
boron, manganese, copper, molybdenum and cobalt. Both groups are equally
important; when plants do not have enough of any one of the macro- or
micronutrients, they are unable to complete their life cycle. In an orchard, a
shortage or defi ciency is overcome by the application of fertilizer.
The amount of fertilizer to apply can be established in many ways. In
some tropical regions the nearest university or agricultural extension oi ce
can provide a fairly good estimate as to the amount of fertilizer to apply for
specifi c crops; published information can be a useful initial orientation as to a
specifi c crop plant's needs, and some neighbouring farmers' fertilizer practices
can help as a starting point. A more technical approach is to determine the
amount needed to replace that 'exported' by the harvest. This estimate needs
to be adjusted for the amount of nutrients lost through pruning and ei ciency
of nutrient uptake and the amount unavailable to the plant roots. Another
system is to establish trials using dif erent dosages of fertilizers for a given
species or variety under your climatic and soil conditions. These trials can
take a long time, and often the answer is needed sooner than the results are
obtained. The other approach is to carry out soil and plant tissue chemical
analysis.
 
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