Foliar sprays are useful for
delivering fertilizers, fungicides, pesticides and PGR’s to plants.
can be broadly categorized as either “non-systemic” or “systemic”.
“Non-systemic” sprays are used to treat problems located on the surface of
the leaf. Most pesticides are non-systemic. Fungicides used to treat certain
types of “surface” fungi such as mildews are also usually non-systemic.
“Systemic” spays are absorbed into the plant via ‘stomata’ and then
transported via the vascular system to where they are needed (Fig 16.1).
Foliar fertilizers, PGR’s and many fungicides are common examples of
wetting agent (or ‘wetter’) should
always be added to foliar spray solutions. Wetters are a type of surfactant
used to improve a spray’s capacity to “wet” foliage. They achieve this by
lowering the surface tension of the foliar spray solution. With a lower
surface tension, the spray droplets can collapse and spread evenly across
the surface so that greater coverage occurs (Fig 16.3). Where systemic
sprays are being used, the spray is able to cover more stomata which
increases the opportunity for absorption (Fig 16.1).
the absence of a wetter, large droplets will form on the leaves (Fig 16.2).
These will either roll off the leaf quickly and be wasted, or may cause
burning when intense light is present. Leaf burn is more common with
relatively concentrated solutions.
leaves are harder to wet than others and therefore require a higher dose of
wetter. For some ‘waxy’ leaves it is not possible to achieve complete
wetting. In this case, be careful not to exceed the label’s recommended dose
rate. There is a threshold concentration beyond which the wetting capacity
does not increase. It is important to recognize this because excessive
wetter concentrations can cause leaf burn.
Wetters also usually possess “hygroscopic” properties. This helps extend the
duration that the foliar spray remains wet on the leaf. This is important
because absorption of the active component only occurs while the spray
Unlike roots, foliage is not adapted to absorb large amounts of fertilizer
(nutrients). However, foliar spraying is able to take advantage of the
significant combined 'surface area' of leaves and stems on a plant. This is
defined in the following report by DR. H. B. Tukey (Dept of Horticulture,
Michigan State College):
“The leaf surface of a
12-year-old apple tree in Washington State is equivalent to one-tenth of an
acre, even though that tree only occupies about one-hundredth of an acre. So
there is a large feeding area”.
Consequently, when foliage is sprayed with a
fertilizer formulation that can be
easily absorbed, there is a large opportunity for nutrient input.
Sprays are mainly used for supplying nitrogen, iron and zinc. However,
potassium and other trace elements can be absorbed through foliage. Radio
active tests show that micro-nutrients, once sprayed, are in the sap stream
within 1 hour. This means foliar sprays can be effective for quickly
correcting certain nutrient deficiencies. They are also a useful supplement
to root feeding
when up-take is restricted because roots are diseased, damaged, or simply
Specific foliar fertilizer formulations
can be used to influence plant characteristics such as fruit set, fruit size
and pest and disease resistance.
Best practice for foliar spraying
Without proper practice, foliar sprays can be wasted or cause problems such
as leaf burn and mould. The following guidelines will help prevent the above
mentioned problems from occurring:
The best time to spray is usually early morning, about 1 hour before
daylight. This gives stomata sufficient time to absorb before light
recommences. Spraying earlier than this risks mould growth because the
foliage will be damp for a longer period of time.
the early morning, humidity is also relatively high, temperature is
relatively low and there is minimal light. Under these conditions the spray
remains wet for longer so that maximum absorption can occur. In hot, dry
conditions, the spray can dry too quickly and cause leaf burn and staining.
2. For systemic sprays especially, never spray when the air
temperature is above ~25°C (77°F). Apart from the concerns mentioned above,
absorption at these temperatures is poor because in many species the stomata
are generally closed.
3. The spray device should be fitted with nozzles that produce a
fine mist. This helps maximize surface coverage, especially on the underside
of leaves where the majority of stomata are located. This is also important
for non-systemic sprays such as pesticides because insects tend to harbour
on the underside of foliage.
4. Spray when wind is minimal. This is especially important with
finely atomized sprays because they drift readily. If growing indoors ensure
oscillating fans and ventilation units are switched off.
5. Only lightly spray the leaves and stems with a thin film of
moisture. There should be little or no run-off. Drenching the surface of the
foliage is wasteful and can restrict the stomata’s ability to absorb.
6. Use low salinity/ soft water. This will reduce the risk of leaf
staining and burning.
7. Where growing outdoors, delay spraying if rain is imminent. If
rainfall occurs within 1 hour of spraying, re-spray within the next 1-2
8. Test compatibility before spraying the entire crop i.e.
Test-spray a small patch of leaves and observe for at least 2 weeks.