Fertilizers and Amendments
Diagnosing Nutrients and Soil Problems
Correcting Nutrient Problems
Correcting Soil Problems
Sources
Following are some basic concepts about fertilizing olive trees. For more in-depth
information, we recommend the books and articles from Paul Vossen, as well as the book
from Louise Ferguson and G. Steven Sibbett.
When it comes to fertilizing olive trees, more is not always better. In fact, more fertilizing
often creates negative results. “Olive trees are not big feeders”, says Paul Vossen. “They
are semi-wild, hardy, tough plants that will tolerate poor growing conditions, especially low
fertility, better than most plants. They also tolerate a very wide range of soil pH.” In fact,
olive trees tend to grow too vigorously in very fertile soils. They grow too tall and produce
little fruit. Often, when pruned to limit their growth, they respond by sending long and
vigorous non-fruiting shoots. The oil quality may be poorer in very fertile soils.
Paul Vossen also observes that a common mistake of new growers is to over-fertilize when
their trees are not growing adequately, rather than make sure that irrigation and weed
control are adequate. Olive trees respond much better to correct irrigation (enough but not
too much) and weed control than to fertilization. There are many myths and exaggerated
claims (sometimes encouraged by fertilizer manufacturers) that fertilizing will magically
cure or prevent all sorts of problems and result in better quality olives and olive oil. It is
often the opposite, however. In order to know when fertilizer is needed, observation and
analysis are vital.
DIAGNOSING NUTRIENT AND SOIL PROBLEMS
Three general categories of observations can be used: visual symptoms, plant or tissue
analysis, and soil and water analysis.
Visual Symptoms
Diagnosis from visual symptoms alone can be difficult. A tree may have already suffered in
growth or in yield by the time visual symptoms appear. Problems other than nutrition may
cause similar problems. Borderline deficiencies or deficiencies involving more than one
element may be difficult to diagnose. It is best used in conjunction with plant or soil
analysis.
Plant or Tissue Analysis
Olive leaves are sampled and chemically analyzed for mineral deficiencies and toxicities. A
leaf’s mineral composition depends on its maturity, current climactic conditions, availability
of mineral elements in the soil, cultural practices, and other factors. The mineral nutrient
level in the leaf integrates all these factors and thus reflects the nutrient status of the tree.
Although there are exceptions, the general rule is that nutrient deficiencies are best
measured from late June through early August. The concentration of mineral nutrients in
leaves changes as the leaves first emerge and then expand to full size. For many
elements, the smallest change in concentration occurs during that period. Olive leaf
samples should be taken then because critical levels have been established for that time
period.
Leaf samples should be taken from the middle of non-bearing, current season shoots. A
sample of 80 to 100 is sufficient. Ideally, a sample should be taken from similar trees.
Samples from different varieties, or from parts of the orchard with different soils,
microclimates, or irrigation systems should be sampled separately. Samples should consist
of a few leaves of as many similar trees as possible, selected at random throughout the
orchard. Avoid any leaves that are abnormal in appearance or from abnormal trees unless
this is the specific problem to be solved. In this case, the abnormal leaves or trees should
become a separate sample.
Soil Analysis
Olive trees, like most tree crops, have extensive root systems that occupy a larger volume
of soil than those of most annual crops. Soil can vary widely within such an area. Thus it
may be difficult to take a soil sample that accurately represents the root area and nutrient
levels that the root extracts. In general, soil analysis is not accurate enough to diagnose
fertility in olive trees.
It is a good idea, however, to do soil and water analyses before planting a new orchard as
discussed in our Soil and Water Analysis page. It is also useful in mature orchards if a soil
problem is suspected. It can help diagnose toxicities from excessive concentration of
sodium, chlorine, and boron, as well as other problems, which helps in planning corrective
procedures.
In general, soil samples can be taken at any time, as soil characteristics and nutrient
levels are relatively stable. There are a few exceptions. Nitrogen levels can be depleted
after a long wet season. In many soils, nitrates, chlorides, and boron can be leached by
winter rains and irrigation. The sampling procedures depend on the problem and area
involved. Is one tree affected or many? Is there a visible pattern? Ideally, 3 to 10 spots in
a site should be sampled. Because soils differ in composition at different depths, the top 6
to 12” (15 to 30cm) should be a separate sample, as well as each subsequent foot
downward. Samples taken from different distances from the trunk may be combined, but
different soil depths should be separate. Samples should represent the effective rooting
zone. A soil auger may be used to obtain samples. Generally, about a quart (1L) of soil per
sample is adequate. The testing lab will often provide an interpretation of the results as
well as suggestions for corrective action.
CORRECTING NUTRIENT PROBLEMS
A lack of nitrogen (N) is the only common deficiency in olives. Potassium (K) and boron (B)
deficiencies are quite uncommon. Deficiencies of other nutrients are rare.
Nitrogen
Nitrogen levels directly affect fruit set, yield, and shoot growth. It is important to
remember, however, that both nitrogen deficiencies and excesses can hurt yields. The goal
is to maintain leaf nitrogen levels of 1.5 to 2.0%. Deficiency level is less than 1.4%. In
deficient trees, the leaves are small and yellowish. Shoot growth is less than 8”. These
symptoms often occur in soils that are cold and wet during the winter when nitrogen is not
as readily available, but disappear in the early summer. Nitrogen can be applied either
with organic or with conventional fertilizers.
Organic applications can be done either with certified organic materials such as feather
meal, blood meal, or compost, or with a leguminous cover crop. Rates of applications
should be approximately 40 to 50 pounds of actual nitrogen per acre, per year, in a
mature orchard, taking into account the rate of nitrogen in the fertilizer. Organic fertilizers
such as compost can take quite a while to decompose and release the nitrogen in a form
that the plants can absorb. They decompose over a period of about 15 years with the most
intensive release during the first year or two. Annual applications over many years can
build up high nitrogen levels.
Conventional nitrogen fertilizers such as urea, ammonium nitrate, ammonium sulfate,
potassium nitrate, or calcium nitrate also work. The important thing about nitrogen is to
make periodic applications in accordance with leaf analysis and adequate shoot growth of
between 8 and 20” (203 to 508mm). It is also important to note that nitrogen can carry
over in the soil for several years, especially in heavy clay. In sandy soils, it tends to leach
and can cause ground water and run-off pollution, so make sure not to apply too much. It
may not need to be applied each year.
For dry farmed trees, it should be put on just before a rain in mid to late winter. Under
irrigated conditions, it can be applied periodically throughout the growing season and
watered in. For drip irrigation, it can be placed right under or through emitters.
Note that olive trees cannot tell if the nitrogen comes from an organic source or from a
conventional fertilizer. From a chemical point of view, it is all the same to the tree. There
are different reasons to choose one or the other, however. Organic materials have the
characteristic (which can be desirable or not) of being released slowly. They are less likely
to leach into ground or surface water. Conventional sources are generally cheaper and
easy to handle. But the differences between the two are getting more subtle with the
availability of slow release conventional fertilizers and high analysis organic fertilizers
which are easy to apply. Costs do vary considerably, however.
Potassium
Low levels of potassium in olive leaves and deficiency symptoms have been observed only
in rare cases. Note that soil levels of potassium do not correlate well with leaf nutrient
levels or deficiency symptoms. Potassium levels below the critical leaf level can occur long
before any symptom appears. Adequate concentration is over 0.8%. There is a deficiency
at less than 0.4%. Deficient leaves are light green, with tip burn, and there may be dead
areas in the trees. Similar symptoms sometimes appear because of poor soil drainage.
Most potassium fertilizers are mined form natural sources and classified as organic. The
rate of application to correct a deficiency is generally 10 to 20 lbs of potassium per tree at
the drip line. It can be dug in to be more effective or applied right under the drip emitters
or through the drip system in soluble forms. Composts and other organic fertilizers all
contain some potassium, so regular application would most likely never allow a deficiency
to arise.
Boron
Boron deficiencies are rare as well. In California, they have been observed in Butte County.
Olive trees are much less sensitive to high boron levels than most other crops. Adequate
boron concentration is 19-150ppm. The tree is deficient below 14ppm and there is an
excess concentration above 185ppm. In deficient trees, the fruit is misshapen (“monkey
face”). The growth is short and branched. There is limb dieback, rough bark, and small
leaves with top dieback.
Deficiencies can be corrected by applying a pound of borax to each tree, which will last
many years. It is important not to apply too much because of the risk of toxicity.
Phosphorus
Phosphorus deficiencies have been observed in Europe. Adequate concentration is 0.10.3%.
A Note about Foliar Sprays
Beware of foliar sprays. All the necessary nutrients for olive trees are available via ground
application, and the effect is much more lasting. Research trials have proven that all
nutrients can be taken up by olive roots. Foliar sprays of nitrogen, boron, and potassium
will be rapidly taken up by the tree. Nutrient levels will increase dramatically in the leaves,
but only for a short while, usually just a few weeks. There is no measurable effect on shot
berries, fruit set, shoot growth, number of flowers, number of perfect flowers, fruit yield,
or fruit size. All of this has been measured in University of California trials. Foliar sprays
also have had no effect on alternate bearing in various experiments around the world.
Foliar applied compost tea will not control or prevent peacock spot on olives, nor has it
been shown to have any other positive effects. The best plan is to have adequate nutrition
for the tree through ground application. Foliar sprays can sometimes be useful if a quick
and short-lasting effect is needed.
CORRECTING SOIL PROBLEMS
Various soil problems identified through soil analyses can be corrected or at least
diminished. Following are the most common soil problems and corrective measures with
amendments.
Correcting Soil pH
As noted above, olive trees are quite tolerant when it comes to soil pH. When soils are
overly acidic, lime is commonly used to correct the pH. The amount required varies with
soil texture. The approximate amount of finely ground limestone needed to raise the pH of
a 7-inch (18.cm) layer of soil by one pH unit from an initial pH of 4.5 or 5.5 ranges from
about ½ ton per acre for sandy soil to about 2 tons for a clay loam. Usually, only the
surface becomes acidic enough to require liming.
Compensating for Low Cation Exchange Capacity
The cation exchange capacity (CEC) indicates the ability of the soil to hold many mineral
nutrients against leaching. These nutrients are absorbed by tree roots from the soil
solution. Soils with a higher CEC are typically more fertile, as they have a greater capacity
for mineral nutrients. The CEC depends directly on a soil’s clay and humus content. At one
end of the spectrum, a sandy or loamy sand soil has a CEC of 2 to 7 milliequivalents
(meq)/100g. Clay or peat soils have a CEC greater than 40 meq/100g. As a rule of thumb,
fertilizers should be applied more frequently and in smaller amounts to soils with lower
CEC.
Correcting Sodic (Alkali) Soils
Soils that contain excessive amounts of exchangeable sodium in proportion to calcium and
magnesium are termed sodic or alkali soils. They are characterized by a dispersion of soil
particles that reduces the soil permeability to water and air. By definition, a sodic soil has
an exchangeable sodium percentage (ESP) of greater than 15. This means that 15% of the
soil exchange capacity is associated with sodium, and the rest with calcium, magnesium,
and other cations. Olive trees are affected when ESP levels reach 20 to 40. Soils with this
problem are generally planted with annual crops.
This can be corrected by the application of gypsum. How much can be determined by a lab
analysis. After the gypsum is applied, the displaced sodium must be leached below the
root zone. Organic materials such as manure, cover crop, or crop residues may help
improve the soil structure for leaching. In established orchards, heavy irrigation during the
dormant period minimizes the damage to tree roots from lack of aeration.
There is a close association between the composition and concentration of soil salts and
salts in irrigation water. When used for irrigation, water with high sodium relative to
calcium and magnesium is likely to result in a sodic soil. This is something to watch for.
Correcting Water Penetration Problems
Irrigating with water that is very pure (low salt) may slow infiltration into sandy loam or
fine-textured soils. It can drop to less than 0.1” (2.5mm) per hour, making it difficult to
satisfy the orchard’s water requirements. Gypsum application can help correct this water
penetration problem. The infiltration rate can be increased by as much as fivefold by
applying 1 to 2 tons of gypsum per acre in late spring or early summer, just before peak
evapotranspiration. Gypsum is generally beneficial for three to five irrigations. Infiltration
can also be improved by mixing gypsum directly into the irrigation water with specially
designed equipment. Manure applied to the wetted areas can add soluble salts to the
irrigation water.