Progressive Crop Consultant July/August 2019

Iron Deficiency
in Fruit and Nut Crops in California
BY MOHAMMAD YAGHMOUR, | Area Orchard Systems Advisor, Kern County
PHOEBE GORDON, | Area Orchard Systems Advisor, Merced County
Micronutrients play a very
important role in fruit and
nut tree growth and development.
Iron (Fe), which is an immobile
micronutrient in the plant, is associated
with chloroplasts and plays a role
in chlorophyll synthesis. While Fe is
considered the fourth most abundant
element in the Earth’s crust, approximately
five percent by weight, iron
deficiency is a worldwide problem, and
a common micronutrient deficiency in
fruit and nut crops (Figure. 1) though it
is uncommon in California.
Many orchards in the Central Valley
are on semi-arid soils in areas where
the evapotranspiration exceeds
precipitation. Arid and semi-arid soils
can also be found in the southwestern
USA and the Mediterranean areas. In
this article we will be focusing more
on calcareous soils with free calcium
carbonate (CaCO3) and soil solution pH
in the alkaline range (i.e. above 7.5).
Before we get into the specifics of iron
in the soil solution, we’ll give a brief
Photo courtsey of Mohammad Yaghmour.
Figure 1. Advanced iron deficiency on almond tree in Kern County. Interveinal
chlorosis are a typical symptoms of iron deficiency.
26 Progressive Crop Consultant July /August 2019
description of pH. It indicates the
concentration of H+ ions (protons)
in a solution. Soils with low pH have
more H+ ions than soils with a high
pH. Because the equation is actually a
logarithm (Equation 1), the amount of
H+ ions does not increase linearly as
pH decreases, it increases by a factor of
10. Thus, water with a pH of 5 has 10
times the amount of H+ protons than
water with a pH of 6. Therefore, it is
progressively harder to correct soil pH
the farther it is from 7.
pH = -log[H+]
Equation 1: equation for conversion of
the concentration of H+ ions in solution
to pH. Since the equation is logarithmic,
there is a 10x difference between
consecutive values.
Soil pH is important as the different
soil minerals that contain and release
iron (Fe) into the soil-water solution
decrease in solubility as pH increases,
which results in only a tiny fraction
of the total Fe that is in the soil to
be available. In general, iron is more
soluble and more available as pH
decreases (acidic
soils). Plants
absorb some iron
by diffusion at the
root tips from the
soil solution, and
iron deficiency
in California is
mainly due to
plants’ inability
to take up iron
due to soil factors
such as poor soil
aeriation and/or
high concentration
of HCO3- in the
soil.
Under low iron availability in the
soil, the ability of trees and plants to
mobilize iron immediately around
the root is due to differences in genes
between species. Scientists have
categorized plants either as “Strategy
I’ or ‘Strategy II’ based on their ability
to mobilize Fe in the soil and make it
available for uptake. Strategy I plants
include all plants except grasses and
include fruit and nut trees, while
Strategy II plants comprise grasses such
as wheat and corn. Under Fe deficient
soil conditions, Strategy I plants excrete
H + into the soil, which acidifies it and
makes iron more available for uptake.
In poorly aerated calcareous or
saturated soils, carbon dioxide will
become trapped in the soil due to poor
gas exchange with the atmosphere.
This will cause the production and
accumulation of bicarbonates as a
result of the interaction between CO2
and calcium carbonates in the soil.
Bicarbonates react with the H+ released
by roots and interfere with their ability
to increase iron availability.
Symptoms of Iron Deficiency
The development of Fe deficiency
symptoms is most prominent on
young, newly developing leaves
(Figure 2, see page 28) because this
element is immobile in the plant.
The symptoms are characterized by
interveinal chlorosis, (Figure 3, see
page 28). Under severe conditions,
leaves have a white coloration due to
the disappearance of chlorophyll, and
leaves can turn necrotic and abscise.
Leaf chlorosis due to iron deficiency
reduces photosynthesis and will result
in reduced fruit yields and fruit quality.
These attributes are only for iron
deficient plants; overfertilizing with iron
will not increase these functions in the
Continued on Page 28