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Re: citrus fruit drop

>I have a dwarf Meyer lemon that I've been growing for a couple of years in an
>18' terra cotta container.  It has been putting out lush new foliage and tons
>of blossoms, but lately some  of the fruit has started to yellow and drop off
>when reaching pea-size.  I planted it in a super-soil mix (relatively sandy)
>so I thought it needed more water, but daily dousing seemed to effect little
>change.  Any suggestions?
>
>Walter (Ted) Vorster, Berkeley CA

{I checked with the leading grower of citrus in our area, Four Winds
Nursery of Fremont,California before posting this reply.}

1.  On fruit drop, particularly in containerized stock:  the overwhelming
cause of premature fruit drop is lack of moisture at the roots.  A certain
amount of fruit naturally dehisces but noticeable dropping is either
overly-frequent watering or under-watering.  My mentor, professor Dave Cox,
taught a simple but 100% method of regulating irrigation; touch the soil
with the tip of your finger, then place said finger tip on your lower lip.
If it feels cool to the touch on your lip don't water.  If it feels warm,
water heavily.

2.  Fertilization:  'complete fertilizer' is an ambiguous term with no
official definition.  One UC publication defines it as any fertilizer
containing nitrogen(N), Phosphorus(P) and Potassium(K).  Taylor's Garden
Encyclopedia defines it as any fertilizer containing NPK + an unspecified
amount of other nutrients.  There is also the danger of trying to discern
the proper amount of too many nutrients.  Somewhere in my files I have an
ash analysis of about a hundred different commercial crops with the
respective amounts of NPK, Calcium, Magnesium, Sulfur, Iron, Zinc,
Manganese, Boron, Molybdenum and Chlorine(with no mention of Bromine which
has recently been added to the list of nutrients required for plant
growth).  Such an analysis might be relevant if you were growing citrus
hydroponically but they make no sense when the plant is grown in a soil
medium that presumably contains some ions useful to the plant.
The question to ask is what do I need to add to supplement what nature has
supplied.  A citrus grower in Southern Florida where the soil is sand on
top of coral necessarily adds copious amounts of just about every possible
nutrient.  Growers in most mediterranean areas will probably find that
their main concern is *Nitrogen*.  The quantity of nitrogen in the soil is
determined by the 'net ET', i.e. how much precipitation falls vs. how much
evaporates away.  Med climates generally are very low in N since rainfall
is low and evapotranspiration is reasonably high.  Low rainfall means
sparse vegetation, means low amounts of organic matter in the soil(and what
*is* in the soil is quickly consumed by microorganisms that feed almost
year-round in the warm soils).  Low OM means low amounts of N.  It also
generally means high soil pH which makes Iron, Zinc & Manganese
availability a problem.
So for in-ground citrus in med climates you'll probably need to add lots of
nitrogen and lots of acidifying materials(granular sulfur, OM, phosphoric
acid, et.al.) to deal with the iron/zinc/managanese problems.
In containers its generally cost effective to simply buy a fertilizer with
every nutrient in it.  Plastic coated ureas like Osmocote have become the
universal solution to container growing of just about everything in the
States.  Since citrus requires a low pH it's wise to watch the new growth
for yellowing, especially yellowing of the intra-veinal areas(the veins
remain deep green).  That's generally a give-away that the soil is too
alkaline.  Yellowing of *older* leaves signifies a nitrogen deficiency.
Since approx. 15% of any citrus plant is N it is easy for containerized
stock to drain the N reserves of the soil medium.  Adding N should quickly
fix the problem.

BTW, what is the soil texture around Perth?  Sandy loam?  Clay loam?  Sand?
Jerry Heverly, Oakland, CA
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