Although many people think that the following is rocket
science, it is probably easier than you think and it will give you a basis for deciding
for yourself, what fertilizing method is best for your citrus in containers.

90 % of the following is extracted from the excellent
document “Fertilizing Greenhouse Crops in Alabama”.

As rightly told in the introduction of this document, "many growers are confused about
what type and rate of fertilizer to
use. Many try to follow a simplified “fertilizer recipe” developed
by others without monitoring or
understanding the impact it may
have on plant growth under
their growing conditions."

FERTILIZATION FREQUENCY

Two methods are commonly used by growers today:

(1) constant liquid
fertilization, which involves applying a dilute solution of fertilizer at
each irrigation; and

(2) scheduled
fertilization, which calls for a more concentrated solution of fertilizer
that is applied at periodic intervals, usually once per week. Many growers
prefer constant liquid fertilization for the following reasons:

• Fertilizer nutrients are constantly available in the soil
solution for root uptake with little fluctuation in concentration. With
scheduled fertilization, fertilizer concentrations may be very high at the time
of application but may be very low a week later, just before the next fertilization.

• The chance of dangerously high fertilizer concentrations
in the media is low because a dilute solution is applied every time the plants
require water. In practice, many growers make one clear water application
every 3 to 4 fertilizations as insurance against fertilizer levels that are too
high.

• Inherent in this method is that nutrient application
adjusts to the needs of the plants. For example, during sunny, warm weather,
plants require more nutrients for growth. Under these conditions, the potting
media will dry faster, require watering more often, and, therefore, receive
fertilizer more often. Conversely, during cool, cloudy weather, plants need
fewer nutrients for growth. Under these conditions, the potting media will dry
more slowly, require water less frequently, and, therefore, receive fertilizer
less often.

FERTILIZATION RATE

Fertilization rates for water soluble fertilizers in the
greenhouse industry are based on parts
per million (ppm) of nitrogen in the fertilizer product. It would seem that
with all the fertilizer formulations and different types of crops the decision
of what rate to apply would be a difficult one. However, a rate of 240 to 720
ppm nitrogen applied weekly or a rate of 90 to 250 ppm nitrogen applied at
every watering works well. Concentration ranges are provided because growers
often reduce the rate (lower end of recommended range) in the winter, during
cloudy periods, or when other factors slow growth and raise the rate (higher
end of range) in the summer, during sunny periods, or when other factors speed
growth. When the category of a particular crop is known (citrus: 100 to 150 ppm
N), it is easy to assign rates to it. Growers may also increase or decrease
fertilizer rates depending on the stage of growth, the desired rate of growth,
or the leaching percentage during
irrigation.

Leaching percentage (the percent of total irrigation applied
that drains from the containers) can have a significant impact on crop
fertilizer requirements. In general, a higher leaching percentage requires a
higher fertilizer rate.

A few citrus
fertilizing facts (don’t panic!):

- EC (electroconductivity)
is proportional to the amount of dissolved salts (fertilizer), and is a way to
measure the "ease" of roots to take up water and nutrients

The highest is the EC, the harder is the uptake of water and
nutrients.

It is assumed that the optimum value for citrus is 1.7 dS/m.

Below this, you are not using all the fertilizer absorption
possibilities, and above it, you are starting to penalise the water and fertilizer
absorption efficiency.

But you won’t need an EC meter, as a reference (tested by
me), a 200 ppm N Foliage Pro
solution diluted in destilled water has a EC of (aprox.) 1.45 dS/m., below the threshold level of
1.7 dS/m. and therefore, safe. You could use it even for a constant
fertilization program.

With this value you can calculate the EC of any FP solution,
for example:

- 100 ppm N FP: 100 ppm= 200
ppm/2, so EC= 1.45/2= 0.73 dS/m

- 150 ppm N FP: 150 ppm= 200
ppm/1.33, so EC= 1.45/1.33= 1.09 dS/m

You have to sum the EC of your tap water to the above values.
A usual value of a good quality tap water is 0.1 dS/m.

- ppm of N,

As told above, for citrus is assumed that the optimum ppm N
range is 100 to 150 ppm for a constant fertilization in each watering.

How to calculate the ppm N of your solution?

N is the first value of the three numbers of fertilizer
labels (N-P-K). For example, in Foliage Pro, it is 9 (9 % N).

Well, I want to make one gallon 200 ppm N FP solution.

Determine the volume (in mililiters ml) of fertilizer to
dissolve per liter of water to achieve the desired ppm value by dividing by the
N percent (9 in this case) of the desired nutrient, then divide by 10.

In this
case, 200 ppm nitrogen is desired, then:

200 ppm / 9 / 10 = 2.22 ml of fertilizer per liter (L)

1 teaspoon=5 ml

1 tablespoon=15 ml

1 gallon=3.78 liters

So, 2.22 ml/liter X 3.78=
8.4 ml/gallon

Diluting 8.4 ml of Foliage Pro in 1 galon of water I get a
200 ppm N solution (in round numbers, ½ tablespoon per gallon).

CONCLUSION

In my opinion, the best fertilization program for citrus is:

- 100 to 150 ppm N in each watering. With Foliage Pro and
good quality tap water you could use even up to 200 ppm in each watering.

- When watering, make sure that at least a 10% of the
solution is leached.

- Make one clear water application every 4 fertilizations as
insurance against fertilizer levels that are too high (to avoid salt build up
around roots).

- With this method you won’t need any other fertilizer,
forget Osmocote!

A weekly fertilization is easier and comfortable (needless to say
monthly!), but not the best for your citrus.