Olives will grow and fruit in a wide
range of soils, even under arid conditions. Tree spacing is less in modern
groves than it was historically, when trees were frequently planted around the
Mediterranean at 12 x 12 metres spacing. Ignoring super high-density groves
where the trees are close spaced for “over the top” harvesting with machines,
it is currently usual to plant at distances between 5 x 5 to 7 x 7 metres.
Temperature
constraints are that damage will occur below minus 6ºC. Severe damage, and perhaps trees killed,
occurs very little below that, but, like many species of fruit and nuts, olives
have a chilling requirement – i.e so
many hours of temperatures below 7ºC,
(45ºF for those who use Fahrenheit) before they will flower the
following spring. How many hours each cultivar needs is variable, and it seems
extremely little research has been done. No doubt cultivars evolved to suit the
environment in which they are found naturally, and a failure to reach the
chilling requirement of any cultivar means that flowers will not bloom. A mean
daily temperature below 12ºC for several weeks seems sufficient to meet the
chilling requirement of all cultivars. How many weeks is open to debate.
As with all
plants, better quality soil, adequate nutrients and adequate water lead to much
more production. Soil is usually a “given” for most people, and, within the
temperature ranges required, almost any soil can be made to grow olives.
Millions of trees have been planted in individually terraced planting spaces on
very steep hillsides. If looking to buy a new property specifically for
planting olives, then somewhere with a good depth of soil, say at least a metre
and not solid clay, and plenty of available water for irrigation are the main
requirements.
With limited
knowledge of olives, and a desire to begin a non-animal enterprise when we
first bought our present property, I was led to plant a grove at 6 x 6 metres
spacing. I am quite happy with that and have planted more since at the same
spacing. This works out at 277 trees/ha of planted area. Over the last 10 years
I have read an enormous number of articles, research papers and advice from a
huge number of people and organisations involved in olive production in
probably every country in the world where olives are grown, as well as spoken
with several knowledgeable people. I have made decisions, based on all this
information, that I use to provide food and water for my trees. Whilst at least
one expert agrees with each individual decision - because I have chosen to
follow at least one expert’s advice in everything I have decided, some will
disagree with my overall management, but then they disagree with each other,
ranging from extremely little fertiliser to extremely high levels, and from
allowing quite severe moisture deficiencies (at times) to maintaining full
moisture profile of the soil.
Note that being
in the northern hemisphere the calendar year begins a few days after
mid-winter, so southern hemisphere information that refers to dates is in the
reverse production cycle position. Based on soil analyses, fertilisers used and
crops grown in the groves since the previous analysis, I apply granular
fertiliser as soon as practicable in the year. I top up phosphorus(P) and
potassium(K) if necessary. I apply Nitrogen(N) according to a personal
assessment of how much to apply. I may have grown a winter leguminous crop for
harvesting or for incorporation as a green manure. Alternatively, a crop of
maize may have been grown the previous summer and the stover left on the
surface as an overwintering mulch; or a summer leguminous crop may have been
sown and either harvested or ploughed in. I am also conscious of the
requirement for adequate trace elements without fertilising to excess. I have used foliar feeds, sometimes called
foliar fertilisers, this year and propose to make them an integral part of my
fertiliser programme. I will use high N early in the year and low N as the
fruit matures. I may apply a second dose of N granular fertiliser in the spring
too – again depending on a personal assessment of need. I prefer to split
fertiliser into smaller doses rather than apply a full season’s requirements at
once, but this is not always practicable. I will aim to have a pH of 7 or
slightly above. Somewhere in the range of 6.5 to 8 seems to be preferred, but
as I said, the experts do not always agree on the ideals
The groves are
irrigated, either through drip irrigation or overhead sprays. The drip system
is a single line with four litres/hour drippers at one metre spacing and laid
on the ground. The different irrigation methods determine what I can and cannot
grow in the summer. With overhead I can grow crops between the rows. With
drippers I am limited in summer, but not winter. I still intend to grow comfrey
on these lines (to be cut and used as a mulch for the trees) but this might be
delayed for another couple of years because the weed seed bank in the soil is
still too high.
There is nothing I can do about the
texture of the soil I have except to increase the Organic Matter(OM) content.
Higher OM means a soil that is more capable of retaining both nutrients and
water. Many semi-arid and arid soils have an OM content of 1% or even less but,
from anecdotal information only, it seems that the highest yields come from
trees grown on land with OM of several percentage points. I will also avoid
bare soil as much as possible, endeavouring to maintain some ground cover to
stop the possibility of water erosion in winter despite the fact that I have not
experienced this happening, and to ameliorate high soil temperatures in
summer. This means cultivating as
infrequently as possible – anathema to some growers who insist on permanent
bare land cultivated regularly throughout the year.
The sole reason behind my decisions is
the way in which the olive tree grows and produces its fruit. The following
three paragraphs are a combination of information in a University of Cordoba,
Spain publication and a paper by Razouk et al, with the long title of
“Optimal time of supplemental irrigation during fruit development of rainfed
olive trees in Morocco” with some additional comments based on other
information I have sourced.
Yield of
olives is the result of three main developmental processes that occur from
flowering to harvest: fruit set, fruit
growth and oil accumulation in the fruit pulp. Vegetative growth is critical in
terms of olive fruit production, because flowers are borne in inflorescences at
the axil of leaves of one-year old wood. This means that flowering and fruit
set originate in the axillary buds of last year’s
growth. So, the big problem is ensuring that this year’s fruit crop has
sufficient nutrients to yield a heavy crop of large (for the variety) olives,
but at the same time producing sufficient growth of new wood for next year’s
crop. In less than optimum conditions one or the other, or both, will suffer.
This is one of the reasons why there is so much biennial bearing in olives. New
growth is restricted by the demands of a heavy crop on the tree, but flourishes
when the previous year’s poor growth carries a small or nil crop, and the tree
produces another good crop in the following year. A vicious circle.
The reproductive cycle from flower bud
initiation to fruit ripening takes 15-18 months depending on cultivar and
growing conditions, as initiation starts in one summer and fruits from this
initiation ripen in the autumn or winter of the following year. The terminal
bud of a shoot is almost always vegetative and shoot growth starts with this
bud break in spring, when temperatures rise above 12ºC. Growth continues as
long as temperatures are below 35ºC, and there are no soil water deficit or
other environmental stresses. Growth begins pre-flowering, but natural soil
water supplies are almost always sufficient to avoid the need for irrigation at
this time. Critical stages for the tree are flowering in April/May; a time of
rapid growth about mid June; pit hardening about the end of July or early
August, and a second rapid growth about mid September. The first rapid growth
stage occurs just as most Mediterranean soils are becoming water deficient for
the summer. Pit hardening and the second rapid growth stage are well and truly
in the dry summer and early autumn before the winter rains begin. Some fortunate
areas receive some summer rain. Pit hardening is about the time of flower
initiation for next year’s crop too, so a lot of stress on the tree at this
stage. The growth stages and fruit development were the factors considered by
Razouk and his associates.
So, back to why
I do what I do. I rely on soil analyses in preference to leaf analyses. The
reason for this is simple – the chemical and nutritional composition of any
particular leaf changes throughout the year, and younger leaves have a
different composition to older leaves. Because of these changes I believe that
leaf analysis is of less value than soil analysis for long term planning. We
could obviously become super technical and do all sorts of tests, but that is
for research scientists, not your average grower. Some sources of information,
particularly those selling foliar feeds, insist on using leaf analysis (taken
in July each year) as the decision maker in fertiliser requirements. Following
the work of some researchers, maintaining the soil at adequate levels of all
plant nutrients is recommended – bearing in mind that an excess of some can
prevent an uptake of others, even although they are available. I take an
approach that if levels of the macro nutrients are considered adequate by most
authorities I am probably offering my plants the best that can be reasonably
done by any farmer.
I agree with
those who say that applying foliar feeds shows an almost immediate response,
whereas granular (or liquid) fertiliser applied to the soil may not. At the
same time it is not a lasting effect – similar to the application of prilled
lime, which is a very finely ground limestone that is formed into prills the
same as granular fertilisers and break down rapidly after application to the
land. This gives a quick fix to a low pH but does not have the ongoing pH
correcting ability of coarser ground limestone. The same applies to the foliar
feeds, they raise nutrient levels within the aerial part of the tree for a
short period of time. I do, though, go along with the school of thought that
foliar feeds assist the plants to assimilate nutrients from the soil. P in
particular can be a problem for plants to take up because it is fairly immobile
in the soil, and it appears that applying foliar N can assist with that problem,
although it also seems sensible to have some P in the feed. Foliar feeds do not
replace the soil nutrients, they complement the soil supply. You cannot feed a
tree just through its leaves.
I apply N
fertilisers to the soil so that the tree has the nutrients to produce fresh
young growth in early spring, and will apply a high N foliar feed, containing
P, K and trace elements, as a booster pre-flowering, possibly a second high N
foliar feed too. Some authorities believe that Boron(B) should be applied
pre-flowering. Both compound fertilisers and foliar feeds that I use contain
some B and I am not inclined to add more as a special feed unless I apply only
N to the soil when I might mix some additional B with the foliar feed. B is
toxic to most living things at fairly low levels – it is used by people who do
not like “poisons” in order to kill ants. It certainly is a poison (as are most
things) above certain levels. If I think the pH level needs raised, I will
include lime when filling the fertiliser spreader with the granular N or
compound fertiliser. Lime spreads easier when mixed with granular fertiliser.
Provided the
trees are growing well there will be no further fertilisers applied
specifically for the olives until late summer, but any crops grown between the
rows of trees will be fertilised independently of the needs of the olives, and
some of this is available to the trees. After pit hardening I will apply at
least one and probably two foliar feeds with lower N and higher levels of P and
K, plus trace minerals. It is convenient to apply these foliar feeds with
sprays against olive fruit fly – a necessity where I live. There are so many
untended groves that all olive pests and diseases are able to proliferate and a
failure to treat against them results in enormous crop losses, sometimes total
failure. It is also necessary to spray against a fungus that can cause total
crop loss too and this is done nearer harvest. Again it is convenient to add a
foliar feed with these sprays if desired.
I use the overhead
spray equipment for irrigation primarily because I have it and it suits me to
use it in a grove which I can summer crop. The drippers were installed to meet
the needs of a new grove, but using existing pumps and underground main lines.
I have the dripper lines on the surface because I prefer them where I can see
them and I make an inspection as soon as I begin an irrigation. That way I can
spot any problems along the lines and fix them immediately without switching
off the pumps. An underground line that suffers damage will not show up until
the water begins to puddle on the surface, which undoubtedly means some trees
have missed out on their water, and it is necessary to dig up the line to mend
it. There is also the problem of where to bury the line. In the early years the
trees occupy only a small area of the soil so the drippers need to be
relatively close. As they grow, the lines are moved back from the trees and the
trees make use of additional drippers as they spread their root zone.
Not all the
water I currently pump is utilised by the trees, but it will be in time, and
until then I make use of drippers outside the root zone by sowing pumpkins and
squashes as feed for the goats, as well
as some household use and also tomatoes, cucumbers, melons, etc. immediately
adjacent to a dripper. In previous years I have also sown the drought hardy
black eyed peas between the rows and they have benefitted from the water from
the drippers, particularly those peas close to the drip line. The site is a
very slight slope so the water naturally travels downhill and the drip line is
uphill of the row of trees it supplies. Both types of irrigation are left in
their permanent positions. Hand harvesting means there is no need to move the
lines off the fields.
Irrigation
commences around mid-June every year for the whole property, which makes it
convenient for the first irrigation requirement of the olives – unless there
has been a particularly dry spring when an earlier start to the irrigation
season may be required. There is always plenty of water available in the early
summer, and this first irrigation has been found to be the most beneficial from
an economic perspective. It assists both the current crop and the ability of
the tree to produce growth for next year. It is more valuable than the water
needed at pit hardening and the second rapid growth stage. This is a most
fortuitous situation, because those relying on natural water supplies, as I do,
may run out of water if the winter and spring rains were low or ceased early.
My strategy then
is to maintain a good soil moisture level as long as possible, and of course
other summer crops grown in the groves need this water. My theory behind this
is that if I run out of irrigation water, then the more there is in the soil
the longer it will be before moisture stress affects the crop. I know from my
2012 maize crop that it was successful without irrigation from silking onwards.
The olives did not receive their pit hardening and mid-September water that
year. Provided I have the water I will continue to irrigate past the second
flush of growth, but in 2013 the last irrigation was just after pit hardening.
Reducing irrigation earlier in the season does not prolong my season, being
dependent upon a river continuing to flow.
For those with
water storage, Razouk showed that a large quantity applied at the first rapid
growth stage (he used 500 litres per tree, on mature trees) can be followed by
the next at pit hardening, and preferably a third at mid-September. If there is
storage for only one irrigation, or natural supplies only into the early
summer, then apply water in June. Whether water is available or not I will
cease irrigation at the end of September. This allows time for the fruit to
mature without containing too much water - which would result in a lower oil
content, giving a poor yield of oil for the fruit weight. This is extremely
important for anyone paying a contractor crusher to produce their oil and
charging, as apparently many do, on the weight of fresh fruit rather than yield
of oil.