Advances in agronomy volume 128

ADVANCES IN AGRONOMY
Advisory Board

PAUL M. BERTSCH

RONALD L. PHILLIPS

University of Kentucky

University of Minnesota

KATE M. SCOW

LARRY P. WILDING

University of California, Davis

Texas A&M University

Emeritus Advisory Board Members

JOHN S. BOYER

KENNETH J. FREY

University of Delaware

Iowa State University

EUGENE J. KAMPRATH

MARTIN ALEXANDER

North Carolina State
University

Cornell University

Prepared in cooperation with the
American Society of Agronomy, Crop Science Society of America, and Soil
Science Society of America Book and Multimedia Publishing Committee
DAVID D. BALTENSPERGER, CHAIR
LISA K. AL-AMOODI

CRAIG A. ROBERTS

WARREN A. DICK

MARY C. SAVIN

HARI B. KRISHNAN

APRIL L. ULERY

SALLY D. LOGSDON


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CONTRIBUTORS
David Bonnett
International Center for Maize and Wheat Improvement (CIMMYT), Texcoco, Mexico
Hadi Bux
Institute of Plant Sciences, University of Sindh, Jamshoro, Pakistan
Peidu Chen
Nanjing Agriculture University, Nanjing, China
Ian Dundas
School of Agriculture, Food and Wine, University of Adelaide, Adelaide, South Australia,
Australia
Emin Bulent Erenoglu
Soil Science and Plant Nutrition Department, Cukurova University, Adana, Turkey
Sumaira Farrakh
Department of Biosciences, COMSATS Institute of Information Technology, Islamabad,

Pakistan
Shmulik P. Friedman
Institute of Soil, Water, and Environmental Sciences, Agricultural Research Organization,
The Volcani Center, Bet Dagan, Israel
Hayriye Ibrikci
Soil Science and Plant Nutrition Department, Cukurova University, Adana, Turkey
Alvina Gul Kazi
Atta-ur-Rahman School of Applied Biosciences, National University of Sciences and
Technology (NUST), Islamabad, Pakistan
Masahiro Kishii
International Center for Maize and Wheat Improvement (CIMMYT), Texcoco, Mexico
Cheng-Bao Li
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,
Chinese Academy of Sciences, Nanjing, China
Tariq Mahmood
Department of Plant Sciences, Quaid-i-Azam University, Islamabad, Pakistan
Aime´ J. Messiga
Agriculture and Agri-Food Canada, Quebec, Canada
Christian Morel
INRA, UMR 1220, TCEM (INRA-ENITAB), Villenave d’Ornon, France
A. Mujeeb-Kazi
National Institute of Biotechnology and Genetic Engineering (NIBGE) Faisalabad, Pakistan

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Contributors

Francis Ogbonnaya
Grain Research and Development Corporation (GRDC), Barton ACT 2600, Australia
Awais Rasheed
Department of Plant Sciences, Quaid-i-Azam University, Islamabad, Pakistan
Abdul Rashid
Pakistan Academy of Sciences, Islamabad, Pakistan
John Ryan
International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria
Rolf Sommer
International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria
Jose´ Torrent
Departamento de Agronomı´a, Universidad de Co´rdoba, Co´rdoba, Spain
Richard R.-C. Wang
USDA-ARS, Forage and Range Research Laboratory, Logan, Utah, USA
Yu-Jun Wang
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,
Chinese Academy of Sciences, Nanjing, China
Joann K. Whalen
Department of Natural Resource Sciences, Macdonald Campus of McGill University,
Quebec, Canada
Steven Xu
USDA-ARS, Northern Crop Science Laboratory, Fargo, North Dakota, USA
Sui Kwong Yau
Faculty of Agricultural and Food Sciences, American University of Beirut, Beirut, Lebanon
Dong-Mei Zhou
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science,
Chinese Academy of Sciences, Nanjing, China
Noura Ziadi
Agriculture and Agri-Food Canada, Quebec, Canada

PREFACE
Volume 122 of Advances in Agronomy contains four excellent reviews dealing
with crop and soil sciences. Chapter 1 is a comprehensive review of micronutrient constraints on crop production in the Middle East–West Asia
region. Topics that are covered include climate and soils of the region, soil
factors and micronutrient behavior, diagnostic approaches for determining
micronutrient problems, micronutrient research dealing with soil behavior
and crop responses, and managing micronutrient deficiencies. Chapter 2
deals with assessment and modeling of soil available phosphorus in sustainable cropping systems. Detailed discussions are included on phosphorus in
agricultural soils and measurements to assess soil available phosphorus.
Chapter 3 discusses the Wien effect in suspensions and its application in soil
science. Topics that are covered include fundamentals of the Wien effect and
measurement methodologies. Chapter 4 deals with genetic diversity to
improve wheat production and impacts on food security. Topics covered
include plant breeding and genetic strategies.
I appreciate the excellent reviews of the authors.
DONALD L. SPARKS
Newark, Delaware, USA

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CHAPTER ONE

Micronutrient Constraints to Crop
Production in the Middle
East–West Asia Region:
Significance, Research,
and Management
John Ryan*, Abdul Rashid†, José Torrent{, Sui Kwong Yau},

Hayriye Ibrikci}, Rolf Sommer*, Emin Bulent Erenoglu}
*International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria
†
Pakistan Academy of Sciences, Islamabad, Pakistan
{
Departamento de Agronomı´a, Universidad de Co´rdoba, Co´rdoba, Spain
}
Faculty of Agricultural and Food Sciences, American University of Beirut, Beirut, Lebanon
}
Soil Science and Plant Nutrition Department, Cukurova University, Adana, Turkey

Contents
1. Introduction
1.1 Awareness of micronutrients in the Middle East–West Asia region
1.2 Milestones in micronutrient research
2. Middle East–West Asia: An Overview
2.1 Climate: Rainfall and temperature
2.2 Land features and soils
2.3 Farming systems and crops
3. Soil Factors and Micronutrient Behavior
3.1 Iron in soils and cropping implications
3.2 Zinc, copper, manganese, and boron
4. Micronutrient Disorders: Diagnosis Approaches
4.1 Crop sensitivity to micronutrient deficiencies
4.2 Deficiency symptoms in common Middle East–West Asia crops
4.3 Soil testing in the Middle East–West Asia region
4.4 Plant analysis, a complement to soil testing
4.5 Crop responses to micronutrients
5. Micronutrient Research: Significance, Soil Behavior, and Crop Responses
5.1 The intractable problem of iron

5.2 Zinc, a serious regional concern
5.3 Boron, too little or too much?
5.4 Manganese and copper: Minor concerns
6. Managing Micronutrient Deficiencies
6.1 Conventional approaches
Advances in Agronomy, Volume 122
ISSN 0065-2113
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2013 Elsevier Inc.
All rights reserved.

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6.2 Biofortification, an emerging concept
6.3 Fertilizer-use efficiency and residual effects
6.4 Soil micronutrient budgets and balances
6.5 Micronutrient content of crop seeds
7. Future Research Needs
8. Conclusions
Acknowledgments
References

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Abstract
In addition to nine major nutrients, eight micronutrients [i.e., boron (B), chlorine (Cl), copper
(Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn)] are also essential for healthy growth and reproduction of higher plants. Globally, crop production is
largely dependent on chemical fertilizer use, especially in developed countries. While fertilizer use, particularly nitrogen (N) and phosphorus (P), has increased substantially in the
past four decades in developing countries, such as Pakistan and India, fertilizer use is limited
in many areas of the world where agriculture is constrained by harsh climatic conditions,
especially low rainfall. The disparity between developed and developing countries is particularly acute with respect to micronutrient awareness and use.
One area of the world that is characterized by major climatic and soil constraints,
often exacerbated by unfavorable socioeconomic conditions, is the Middle East–West
Asia region. This review provides a current perspective on that region of the world
where crop yields are invariably low due to drought, with limited inputs and inherent
soil nutrient deficiencies. With a high population, there is an urgent need to sustainably
expand output. However, there is generally limited awareness of the potential significance of micronutrients in agriculture as factors in crop production, as well as limited
research on micronutrients in most countries of the region. The long history of cultivated agriculture in the Middle East–West Asia region and the peculiar characteristics
of its soils and climate predispose it toward problems of micronutrient deficiencies.
Over three decades ago, a global study on micronutrients indicated widespread
deficiencies of iron (Fe) and zinc (Zn), in contrast to copper (Cu) and manganese
(Mn), but suggested the likelihood of excess levels of boron (B) in some countries of
the region. This overview primarily addresses three focal points in the region, Pakistan
in the east, Syria/Lebanon/Turkey in the center, and Spain on the western fringes,
reflecting the zones of activity of the respective authors; the latter focal point is a developed region, where, because of soil and climatic similarities, the research is relevant to
the whole Middle East–West Asia region.
While providing some international context, this article brings together and summarizes published work in the areas of crop and soil micronutrient availability, their
behavior in soils in relation to crop growth, and strategies to deal with either deficiency
or toxicity, including crop selection for tolerance and subsequent genetic manipulation.
Considerable strides have been made in elucidating the significance of both Zn and Fe
in the region's mainly calcareous soils, through soil and plant analysis, with the resulting
knowledge providing a sound basis for management interventions through validated

Micronutrient Constraints to Crop Production

3

field research. While B deficiency is common in some countries such as Pakistan, the
problem of B toxicity (BT), where it exists, is only handled by crop adaptation.
The review also highlights the implications of micronutrient constraints in the soil–
plant–human–animal continuum. Intensification of agricultural production as a result of
overall macronutrient use, expansion of irrigation, and introduction of new or “niche” crops
is likely to accentuate micronutrient deficiencies in the region, but developments such as
conservation agriculture may counteract this trend. As the trend for land-use intensification increases because of higher yields due to fertilizer use and irrigation and the introduction of new crops, and as other nutrient constraints are eliminated, micronutrients will
inevitably assume greater significance in the future agriculture of the Middle East–West
Asia region together with improvements in plant breeding and crop management.

1. INTRODUCTION
The urgency of addressing the issue of a fast expanding world population, in particular by eliminating hunger and malnutrition in lessdeveloped countries, underlines the need for policies that ensure sustainable
agricultural productivity while preserving the environment and the natural
resource base. Nobel Laureate and father of the Green Revolution, Norman
Borlaug highlighted the central role of soil fertility and mineral nutrition,
along with improved crop varieties and water availability, in ensuring nutrition (Borlaug, 2003) and addressing the enormous challenges facing mankind in dealing with it (Borlaug, 2007). The task of ridding the world of
hunger and the continued provision of adequate food for future world population calls for exceptional response from the global scientific community
(Godfray et al., 2010). No wonder, never before has the issue of food security so impacted the public through the media (Cribb, 2010).
Much of the world’s food supply today is attributed to the use of chemical fertilizers (Stewart et al., 2005), in addition to improved crop varieties
and better crop management; future increases will be even more dependent
on fertilizer inputs, particularly nitrogen (N), phosphorus (P), and, to a lesser
extent, potassium (K). Soil deficiencies of these major nutrients are now well
understood and largely eliminated in modern commercial agriculture
through the routine use of fertilizers.
However, in many developing countries, chemical nutrient infertility

still poses a major limit on crop productivity (Loneragan, 1997). In addition
to nine major nutrients, eight micronutrients essential for healthy growth
and reproduction of higher plants are boron (B), chlorine (Cl), copper


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John Ryan et al.

(Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc
(Zn) (Alloway, 2008b). In general, while the use of fertilizers N, P, and K has
increased in recent decades in developing countries (IFA, 2011), the use of
micronutrients is very limited, and even nonexistent in many cases, especially in dryland agriculture (Tow et al., 2011). However, in many situations, the amounts of fertilizer of any description available are inadequate
for optimum economic yields; agriculture in such situations is constrained
by adverse biophysical and socioeconomic circumstances. Only in relatively
recent times has there been a focus on the issues of dryland ecosystems of the
world and their potential for sustainable cropping (Stewart and Robinson,
1997). In the context of the Middle East, Lal (2002) pointed to the potential
of dry areas to produce crops as well as sequester carbon if properly managed.

1.1. Awareness of micronutrients in the Middle East–West
Asia region
Despite being the center-of-origin of settled agriculture and of western civilization, as well as the location where many of the world’s major crops,
especially cereals, pulses, and nuts, have evolved (Damania et al., 1998;
Harlan, 1992), the Middle East–West Asia region is still largely a food-deficit
region, with the exception of a few countries, such as Turkey, approaching
self-sufficiency in staple foods. The underlining factors, both biophysical and
socioeconomic, contributing to the food insecurity were recently stressed by
Khuri et al. (2011) with respect to the Arab countries of the Middle East.
The vast swathe of the globe, from Morocco to Pakistan, is characterized

by a Mediterranean-type of climate merging into a continental one
(Kassam, 1981), and an agricultural system that is largely traditional and more
of a subsistence character (Gibbon, 1981). As the dominant climatic feature
is low and erratic rainfall, the dominant system of rainfed cropping, involving cereals and legumes and associated livestock production, is invariably
restricted by drought (Cooper et al., 1987).
Despite the perception of its traditional agriculture of cropping and
pastural systems, change is taking place, either from nature or from man.
Given the current debate on climate change, drought is likely to be exacerbated in some areas of the world—the Middle East–West Asia region,
already under climate pressure, is likely to be one such region (IPCC,
2008). Due to increased land-use pressure, driven by high population growth
rates in the past few decades, there has been increasing emphasis on irrigation
where water is available, either from rivers or from groundwater, mechanization, and the use of chemical fertilizers, mainly N and P (Ryan, 2002).


Micronutrient Constraints to Crop Production

5

In essence, the developments that have occurred earlier in the West are now
emerging in the Middle East–West Asia region.
However, the state of awareness on micronutrients in agriculture of the
Middle East–West Asia region has lagged behind that of the major nutrients.
For example, in two workshops of the soil test-calibration program involving soil fertility and crop scientists from the West Asia–North Africa
(WANA) region, including the authors of this review (Ryan and Matar,
1990, 1992), no mention was made of micronutrients, as the general perception was that the only nutrients of importance were N and P. In a subsequent, more comprehensive international workshop on soil fertility, there
were only a few reports on micronutrients, that is, from Turkey, Iraq,
and Syria (Ryan, 1997). Similarly, a recent major review on dryland agriculture (Rao and Ryan, 2004), including several contributions from the Middle
East–West Asia, contained only one paper that referred to micronutrients.
However, some earlier internal publications at the American University
of Beirut (Ryan et al., 1981b), Pakistan (Anonymous, 1998), and Turkey

(Cakmak, 1998) did bring together various in-country publications dealing
with micronutrients, which were not typical of the Middle East–West Asia
region as a whole.
The first publication that ever indicated any potential problems with
micronutrients, especially iron (Fe), zinc (Zn), and boron (B), emanated
from a Food and Agriculture Organization (FAO)-sponsored study on
micronutrient status in selected countries around the world, led by
Sillanpa¨a¨ (1982), which formed the basis of subsequent reports (Katyal
and Vlek, 1985; Sillanpa¨a¨, 1990). That study involved sampling soils from
around the world and conducting pot experiments in the greenhouse with
various micronutrients added to each soil batch. Subsequently, recent
reports by Rashid and Ryan (2004, 2008) were further attempts to develop
a coherent picture of micronutrient research in soils and crops of the Mediterranean climatic region.
This chapter elaborates on micronutrient research, highlighting recent
developments, particularly in relation to Fe, Zn, and B, and stressing the role
of plant adaptation to micronutrient deficiencies and toxicities, as well as the
implications of these problems for crop productivity and human health.
Indeed, except for southern Australia, the issue of toxicity has only been
highlighted in Turkey, Syria, and Lebanon (Yau and Ryan, 2008). The
authors represent focal areas of micronutrient research across the broad Middle East–West Asia region, that is, from Pakistan in the east to Lebanon,
Turkey, and Syria in West Asia, to Spain in the west. The effort was


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John Ryan et al.

catalyzed by the International Center for Agricultural Research in the Dry
Areas in Aleppo, Syria, one of the worldwide agricultural research centers
(Deane et al., 2010).

As a background to the discussion on micronutrients in agriculture in the
Middle East–West Asia region, it is relevant to present a context for such
consideration, with a brief reference to the dominant influence of rainfall
on agroecosystems as they impinge on micronutrient use directly or indirectly. The potential significance of micronutrients in the region can be
appreciated only by considering the specific properties of the soils and the
crops they support.

1.2. Milestones in micronutrient research
Diagnosis of deficiency and toxicity of nutrients, especially micronutrients,
can contribute to better nutrition of crops and greater productivity. While
the state of knowledge on micronutrients was brought together by Mortvedt
et al. (1972) in the first-ever monograph Micronutrients in Agriculture published by the Soil Science Society of America and later updated (Mortvedt
et al., 1991) to include new developments in micronutrient research, most
of the contributions came from the developed world. Extensive reviews are
available on the geographic distribution of micronutrient problems and their
correction in many parts of the world, for example, the USA (Fageria et al.,
2003; Mortvedt et al., 1991), Australia (Robson, 1993), and tropical countries (Katyal and Vlek, 1985; Vlek, 1985). However, the recent review of
micronutrient deficiencies in global crop production (Alloway, 2008a),
based on the Special Symposium on “Micronutrient Deficiencies in Global
Crop Production” held in May 2005 at the 8th International Conference
on the Biogeochemistry of Trace Elements (ICOBTE) in Adelaide, South
Australia, did much to broaden the interest in micronutrients by documenting global research achievements.
Nevertheless, despite contributions by Rashid and Ryan (2004, 2008)
and Cakmak (1998), the state of knowledge on micronutrients in lessdeveloped areas of the world, such as the Middle East–West Asia, is poorly
described and sketchy at best. This publication is a modest effort to redress
the knowledge gap with respect to micronutrients and thus add to a growing
body of knowledge related to crop production after similar reviews on longterm crop rotations (Ryan et al., 2008), N (Ryan et al., 2009) and P (Ryan
et al., 2012). While the review does not purport to be exhaustive or cover all
countries of the region where micronutrient research has been conducted, it

Micronutrient Constraints to Crop Production

7

is hoped that the research reported from the three nodes within the region is
germane to the region as a whole and will serve as a catalyst for wider and
more embracive research on the soil, plant, and nutritional aspects of micronutrients in the Middle East–West Asia.

2. MIDDLE EAST–WEST ASIA: AN OVERVIEW
The area of the world focused on in this review is the Middle East–
West Asia region, often referred to as “Mediterranean” (Fig. 1.1). Various
terms have been used to describe the region, with no universal agreement or
accepted definitions; in many ways, some of the terms used are synonymous
or overlapping. The early term, Near East, is of historical vintage. To be specific, according to the Food and Agriculture Organization of the United
Nations, the Near East region comprises 31 countries, that is, Afghanistan,
Algeria, Azerbaijan, Bahrain, Cyprus, Djibouti, Egypt, Iran, Iraq, Israel,
Jordan, Kuwait, Kyrgyz Republic, Lebanon, Libya, Malta, Mauritania,
Morocco, Oman, Pakistan, Qatar, Saudi Arabia, Somalia, Sudan, Syria,

Figure 1.1 Sketch of the Middle East and West Asia region (except for Spain, lighter
color indicates the mandate area countries of the International Center for Agricultural
Research in the Dry Areas).


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John Ryan et al.

Tajikistan, Tunisia, Turkey, Turkmenistan, United Arab Emirates, and

Yemen. The term Middle East–West Asia is less rigidly defined, being used
to describe the area around the Mediterranean, primarily the Arab region.
The term WANA is widely used in the Consultative Group on International
Agricultural Research (CGIAR) system (Deane et al., 2010). Despite the
lack of specificity, the term Middle East–West Asia broadly covers the area
of our current concern, hereafter referred to as the “Middle East.”
Any agricultural system, regardless of how well it is developed or how
efficient it may be, is the outcome of a combination of physical and
human-related factors in order to effectively improve agricultural output,
especially in developing countries; therefore, one has to understand the
cropping systems and the influence of factors such as climate and soil. While
these factors have been described in detail by Kassam (1981), and in lesser
detail subsequently by Ryan (2011) and Ryan et al. (2006a, 2008), some
brief allusion to these essential factors is pertinent for this overview
of micronutrients.

2.1. Climate: Rainfall and temperature
Climate has historically dictated the fortunes of Middle East–West Asia agriculture and is even more critical now, given the rapid population increase
that has characterized the region, and therefore dictated food demand. Climate and cropping and pastoral systems are inexorably linked, and they, in
turn, are influenced by soils as well as influencing soils (Cooper et al., 1987).
As a result of the wide variability in land forms, that is, elevation, nearness to
sea, etc., there is wide variation in climatic factors (rainfall, temperature)
across the region. However, despite such variation, some generalization
can be made. In essence, the climate is mainly Mediterranean in the environs
of the Mediterranean Sea, and it tends toward continental in higher plateaus
and inland, with more extremes of heat and cold (Kassam, 1981).
Thus, the Middle East–West Asia climate is generally characterized by
cool-to-cold wet winters and warm-to-hot dry summers. These general
characteristics are modified locally by maritime (in North Africa) and continental (in West Asia) influences. A major feature is the variability in mean
annual precipitation (including snow in the highlands) as well as high

within-season rainfall distribution; seasonal variability tends to be greater
as mean annual rainfall decreases. With such variability, and despite advances
in modeling capability, prediction of rain is beset with difficulties. Generally,
highest rainfall occurs in coastal areas, decreasing with distance inland.


Micronutrient Constraints to Crop Production

9

Typically, rain commences in autumn (October–November), reaching a
peak in January/February, and then tapers off in April/May. That rainfall
season, however tenuous, is the “window of opportunity” for rainfed or
dryland cropping. The peak of rainfall is later in West Asia than in North
Africa, that is, April–May, with some rain following in June/July. Frequently, the first rains of the season may be delayed 2–3 months, with similar
uncertainty at the end of the normal “rainy” season. Generally, the amount
of precipitation ranges from less than 100 mm in dry desert areas (North
Africa toward the Sahara and in the Syrian/Iraqi deserts of West Asia) to over
a 1000 mm in mountainous areas, but the normal range for rainfed agriculture is within the 200–600 mm range.
With respect to cropping systems, rainfall has to be considered along
with temperature as both factors dictate evaporation from the soil and
transpiration from the crop (Cooper et al., 1987). Winter temperatures
are relatively mild in North African lowlands, but severe in the highlands;
winter temperatures are generally lower in West Asia and are especially
severe in the plateaus of Turkey and Iran, which are invariably snow
covered. Conversely, summer temperatures are hottest in West Asian
lowlands, more moderate in North Africa and less extreme in the
highland areas.
The inverse patterns between seasonal rainfall and ambient temperatures
dictate the typical Mediterranean rainfed cropping pattern, that is, late

autumn to early summer. The early rains in autumn and decreasing temperatures allow for sowing and crop establishment, followed by minimal
growth in December/January, when rainfall exceeds evapotranspiration
(ET). Subsequently, with increasing temperatures in February/March and
rapid crop growth, ET exceeds precipitation. Crops subsequently depend
on stored soil moisture to complete their life cycle in May/June; most of
the longer growing crops such as wheat invariably experience a degree of
terminal drought in the later stages of growth (Smith and Harris, 1981).
Numerous field studies from the Middle East–West Asia region have documented the relationship between mean crop yields and seasonal rainfall
within the range of rainfed cropping (Keatinge et al., 1985). While there
is a general relationship between crop growth and seasonal and withinseason rainfall, “effective” rainfall is conditioned by rainfall intensity. For
instance, Harris (1995) argued that light rain showers <5 mm are lost to
evaporation and have little or no influence on soil moisture or crop growth;
a significant proportion of Mediterranean rainfall occurs in such lowintensity showers.


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John Ryan et al.

2.2. Land features and soils
The soils of the Middle East–West Asia region are as varied as any part of the
world due to differences in factors that influence soil formation and development, that is, parent material, topography, biological factors (limited
by moisture), time, and particularly the influence of man (Ryan et al.,
2006a), given the millennia that the region has been in settled agriculture.
Most soils are derived from limestone residues and are calcareous
(Kassam, 1981), a factor that influences crop nutrient availability, particularly
phosphorus (P) (Matar et al., 1992), as well as micronutrients (Rashid and
Ryan, 2004, 2008). Several groups of the World Reference Base for Soil
Resources (IUSS Working Group WRB, 2007) are represented in the
Middle East–West Asia region: Calcisols, Cambisols, Kastanozems, Luvisols,

Regosols, Vertisols, Arenosols, and Solonchaks, all of which occur to varying extents in semi-arid rainfed cropping areas. Extensive areas of arid soils,
or arid-region soils, which by definition are too dry for rainfed cropping and
are characterized by rangelands, where water sources are available, are irrigated. Similarly, there is a wide range in soil texture, from sands to clays, and
in depth, from shallow to deep; both properties have a determining influence on the capacity of the soil to mediate soil moisture relations, influence
nutrient dynamics and availability in soils, and be cropped sustainably.
Characteristic landscape attributes of the Mediterranean region are a high
proportion of mountains and steep slopes, with a pervasive influence of man
on the landscape (Yaalon, 1997). With cultivation by man for millennia,
much of the region’s landscape is now degraded. A key feature with respect
to nutrient behavior is soil organic matter (SOM). As in arid and semiarid
soils, levels of SOM are low, that is, 1.0–1.5%, and often <1% but can reach
3% (Matar et al., 1992; Ryan et al., 2011). Factors that dictate low SOM are
the ambient temperatures that favor mineralization, low crop yields, and residue removal from the fields.
As in other areas of the world, the soils of the Middle East–West Asia are
varied, being the outcome of the factors of soil formation, with the arid to
semiarid climate having a disproportionate influence on soil properties
(Yaalon, 1997). As the limited rainfall allows for only minimal leaching of
cations, most soils in the Middle East–West Asia are calcareous (mainly
Calcisols) with a high pH. The soil and environmental conditions of the
region dictate that N and P are deficient in unfertilized soils and are required
to be added for cropping, but K is generally adequate due to the weak
weathering environment and K-rich parent materials from which the soils


Micronutrient Constraints to Crop Production

11

were derived. However, given the fundamental relationship between soil
properties, especially the free calcium carbonate (CaCO3), high pH, and

low reserves of organic matter, and micronutrient chemistry, one could
anticipate deficiencies of some micronutrients, especially Zn and Fe, as
the global study of Sillanpa¨a¨ (1982) had shown, and as subsequent investigations, however limited, have confirmed (Rashid and Ryan, 2008).

2.3. Farming systems and crops
The Middle East–West Asia or Mediterranean-type farming systems have a
range of common elements: cereals, small ruminant livestock, that is, sheep
(Ovis aries) and goats (Capra hircis), olives (Olea europaea), vines (Vitis spp.),
fruit trees, and vegetables (Gibbon, 1981). Cereals have historically dominated the cropping landscape of the Mediterranean region (Harlan, 1992).
Cereal production involves wheat, either bread wheat (Triticum aestivum
L.) or durum wheat (Triticum turgidum var. durum), and barley (Hordeum
vulgare L.) largely integrated with production of small ruminants that feed
on the crop residues, stubble or harvested straw (Cooper et al., 1987). Wheat
tends to be grown in the more favorable rainfall areas (>350 mm), where it is
cultivated in rotation with fallow, or a range of food legumes, such as chickpea (Cicer arietinum L.), lentil (Lens culinaris L.), faba bean (Vicia faba L.), and
forage legumes, such as vetch (Vicia sativa L.), peas (Pisum sativum L.), or
medic (Medicago spp.), as winter-sown crops (Harris, 1995). There is increasing interest in crops such as canola (Brassica napus L.), safflower (Carthamus
tinctorius L.), and sesame (Sesamum indicum L.).
With increasing land-use pressure, fallow is gradually being replaced by
continuous cropping. Increased pressure on land use has renewed emphasis
on rotations and cropping options to cope with such changes (Ryan et al.,
2008), as well as on maintaining soil quality. In West Asia, fallow is cleantilled during the noncropped year to control weeds, and thus conserve moisture to improve the fallow efficiency for the subsequent crop, while fallows
in North Africa are left to grow weeds as a source of spring food for animals;
this reduces the fallow efficiency. Regardless of whether the individual crop
is concerned, or the overall rotational system, the key factor is enhancing the
efficiency of the limited soil moisture (Pala et al., 2007). Barley is widely
grown, mainly in the relatively dry areas (<350 mm) and used for animal
feed, where livestock production is important, and is more associated
with fallow.

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John Ryan et al.

In addition to the main cereal and forage crops, a range of crops are
grown in late spring/early summer on residual soil moisture. These include
melon (Citrullus vulgaris L.), sunflower (Helianthus annuus L.), maize (Zea
mays), sorghum (Sorghum bicolor L.), sesame, and a variety of vegetables.
In recent years, there has been an increase in “niche” crops such as cumin
(Cumin sativum L.), coriander (Coriander sativum L.), and camilina or false flax
(Camilina sativa L. Crantz).
Regardless of how well crops are adapted to the Mediterranean or Middle East–West Asia environments, their yielding potential is always determined by the extent of winter rainfall, the effect of which is modified by
radiation and temperature (Smith and Harris, 1981). Low temperatures,
along with frost and snow, limit growth in high-elevation areas. Similarly,
high temperatures during grain filling are a major concern, often with low
yields and shriveled grains. Notwithstanding the difficulties of categorizing
agroecosystems, the early description of the Middle East by Gibbon (1981) is
generally valid today, that is, steppe-based, nomadic or semi-pastural systems, rainfed cereal production, rainfed mixed tree and arable crops, and
irrigated farming.
A schematic presentation of the various representative farming systems is
illustrated in Fig. 1.2. The dryland farming zone mainly occupies the 200- to
600-mm rainfall range, with barley/livestock in the less favorable part of that
range and wheat-based cropping in the more favorable rainfall areas. Outside

Figure 1.2 Schematic diagram of representative agroecological conditions in the Mediterranean region.


Micronutrient Constraints to Crop Production

13

the dryland range in the arid (<200 mm) regions are deserts, rangelands, and
irrigated areas. Above 600 mm, a greater diversity of crops, including horticultural ones, are grown due to dependable rainfall. The arrows in the diagram indicate the current trends, that is, increasing irrigation, especially
deficit irrigation from groundwater sources, into previously rainfed areas,
extension of barley to drier areas, and extension of fruit trees in the zones
with relatively favorable rainfall.

3. SOIL FACTORS AND MICRONUTRIENT BEHAVIOR
As with all nutrients, regardless of whether they are needed in large
amounts by crops or in small amounts, the key concern is availability to
the crop. In that context, there are two issues: the total reserves of the nutrients and the fraction of the total amount that is available for crop uptake.
While the reserves of micronutrients are largely a function of pedogenesis,
soil factors such as pH, which in turn is conditioned by soil carbonates, control their dynamics and dictate plant availability. In that context, SOM has a
major influence, not only as a reserve of nutrients, but also by influencing the
equilibria between various solubility phases in the soil.

3.1. Iron in soils and cropping implications
Among the micronutrients of interest in this review, Fe is unique with
regard to the extent to which its solubility in soils and its availability to crops
are governed by soil pH. It also has a wide disparity between the total
amount (mainly chemically inert) and the fraction in soil solution, the very
limited pool from which the plant roots withdraw Fe. While a disproportionate amount of the work on Fe reported in the following section emanated from Spain, the laws of soil chemistry make the research applicable
to the entire Middle East–West Asia region, which is characterized by
calcareous soils.
3.1.1 Influence of carbonate chemistry
Iron deficiency, which can be easily recognized by the interveinal yellowing
of young leaves (Fe “chlorosis”), is generally accompanied by yield reduction and other negative effects in many fruit tree and herbaceous species
growing on calcareous soils. Carbonates are abundant in most soils of the
Mediterranean region, particularly in the Calcisol, Cambisol, Kastanozem,

Regosol, and Vertisol groups of the World Reference Base for Soil
Resources (IUSS Working Group WRB, 2007). The carbonates are mainly


14

John Ryan et al.

represented by calcite (CaCO3) and dolomite [CaMg(CO3)]; calcite is usually dominant and can be lithogenic (primary) or pedogenic (secondary)
whereas dolomite is always lithogenic.
The total carbonate content of the soil, usually expressed as the calcium
carbonate equivalent (CCE), ranges widely and can reach values above
700 g kgÀ1 in some cultivated soils (Sa´nchez-Alcala´ et al., 2012a). The xeric
moisture regime of the Mediterranean regions has often resulted in incomplete leaching of the carbonates present in the parent material—particularly,
in young soils—or the presence of calcic horizons at a shallow depth. The
latter horizons have often been exposed to the surface by erosion or plowing,
which adds complexity to the spatial pattern of carbonate depth distribution,
and hence the incidence of Fe chlorosis.
Calcium carbonate has a dominating influence on the chemistry of the
soil solution because of its high buffering capacity, basicity, and solubility, as
deduced from the carbonate and calcite chemical equilibria (Lindsay, 1979).
In particular, the content and reactivity of carbonate affect not only the
chemistry of soil Fe but also the concentration of the bicarbonate ion
ðHCO3 À Þ, which is a major factor inducing Fe chlorosis (Alca´ntara et al.,
2000; Lucena et al., 2007; Mengel et al., 1984; Romera et al., 1992); among
other effects, bicarbonate neutralizes the protons released by the root HþATPase and inhibits the root FeIII-reductase activity (Romera et al., 1992).
The CCE does not provide a measure of the reactivity of carbonate,
which is governed by the particle-size distribution (PSD)—and thus the surface area of soil carbonate. Typically, pedogenic carbonate occurs as individual particles or aggregates in the clay, silt, and fine sand fractions and exhibits
a surface area substantially larger than that of lithogenic carbonate. A simple
measure of the reactivity of carbonate is obtained by reacting soil with an

ammonium oxalate solution, on the assumption that the reacting carbonate
fraction is directly related to the rate of formation of calcium oxalate. This is
the basis of the “active lime” (or active calcium carbonate equivalent,
ACCE) method of Drouineau (1942), which has been used—though with
variable degrees of success—as a predictor of the incidence of Fe chlorosis
(Benı´tez et al., 2002; Clemens, 1990; de la Torre et al., 2010; del
Campillo and Torrent, 1992a; Yaalon, 1957).
The surface area of soil carbonate has been estimated via (i) pH-state
acid-dissolution combined with a deterministic PSD model (Moore et al.,
1990), (ii) measuring the difference in isotopic exchange of 45Ca between
soil and decalcified samples (Talibudeen and de Arambarri, 1964), and
(iii) measuring the difference in N2 adsorption between soil and decalcified


Micronutrient Constraints to Crop Production

15

samples (Borrero et al., 1988). Interestingly, ACCE was well correlated with
the surface area values provided by the latter methods (del Campillo et al.,
1992). Thus, in view of the cost and complexity of the latter methods, the
easy-to-use, cheap, and fast ACCE method should be generally used to estimate the reactivity of soil carbonate. It should be noted that ACCE was
highly correlated with the content of carbonate in the <20-mm size fraction,
which suggests that ammonium oxalate dissolves the clay- and fine silt-sized
particles when the extracting procedure of Drouineau (1942) is used.
An inverse relationship between the surface area of carbonate and CCE
has generally been observed in soil horizons containing secondary carbonate
(Abedi and Talibudeen, 1974; del Campillo et al., 1992), probably because
carbonate accumulating in the course of pedogenesis tends to precipitate on
the surface of preexisting carbonate crystals or clog pores inside crystal aggregates. This seems to impose a maximum ACCE value, which seems to be

$350 g kgÀ1 soil in soils of the Mediterranean region (Benı´tez et al.,
2002; de la Torre et al., 2010).
The concentration of HCO3 À in the soil solution is influenced by a
number of factors. One is supersaturation with respect to calcite, which
can in turn be influenced by the presence of dissolved organic carbon, Ca
silicates, and poorly crystalline carbonate phases (de la Torre et al., 2010;
Inskeep and Bloom, 1986a; Reddy et al., 1990; Suarez and Rhoades,
1982; Suarez et al., 1992). Under field conditions, the CO2 produced by
root and microbial respiration causes an increase in the concentration of
HCO3 À in solution, particularly when high moisture conditions and poor
structure hinder escape of CO2 from the soil (Inskeep and Bloom, 1986b).
Thus, Fe chlorosis is aggravated when the spring is excessively rainy or the
soil is irrigated with water rich in HCO3 À . For these reasons ACCE per se is a
poor predictor of the concentration of HCO3 À ; for instance, ACCE
accounted for only 20% of the variance in the concentration of HCO3 À
in solution in the 30 vineyard soils studied by de la Torre et al. (2010).
3.1.2 Mineralogy and iron forms
The most soluble Fe-bearing mineral phases in well-drained soils are the
Fe oxides (a term used here to designate all Fe(III) oxides, hydroxides,
and oxihydroxides). Among the pedogenic crystalline Fe oxides, goethite
(a-FeOOH) and hematite (a-Fe2O3) predominate in soils of the Mediterranean regions, whereas maghemite (g-Fe2O3) and lepidocrocite
(g-FeOOH) are occasionally present. Ferrihydrite ($Fe5HO8Á 4H2O), a
poorly crystalline Fe oxide occurs is small amounts. The total concentration


16

John Ryan et al.

of Fe oxides in soils of regions with Mediterranean-type climates, which is

generally measured by extraction with a citrate/bicarbonate/dithionite solution (Mehra and Jackson, 1960), rarely exceeds 40 g kgÀ1 (Torrent, 1994).
Higher values occur for soils with redoximorphic features or developed on
Fe oxide-rich parent materials. Because of their nanometric size, the pedogenic Fe oxides are mostly concentrated in the clay fraction, where they
occur in a proportion that rarely exceeds 10%.
The xeric moisture regime and the low organic matter content of the
soils of the Mediterranean regions favor rapid transformation of
ferrihydrite—the initial Fe oxide resulting from the weathering of
Fe-bearing primary minerals—into goethite or a mixture of goethite and
hematite. For this reason, the content of these soils in acid NH4 oxalateextractable Fe (Feox) (Schwertmann, 1964), which measures the content
in the poorly crystalline, most soluble Fe oxides (basically ferrihydrite), represents only a small fraction of the citrate/bicarbonate/dithioniteextractable Fe (Fed). The logarithm of solubility product (log Kso) is used
to describe the solubility of the Fe oxides and can be expressed as
À
Á
log Kso ¼ log Fe3þ þ 3 logðOHÀ Þ
where the parentheses are used to indicate chemical activities. Solubility
generally decreases in the order ferrihydrite > maghemite > lepidocrocite >
hematite > goethite with solubility products ranging from $37 to
$44 (Schwertmann and Taylor, 1989). The log Kso of the “soil Fe
oxide,” that is, the phase controlling Fe solubility, is $39 (Norvell and
Lindsay, 1982), a value close to that of the freshly precipitated ferrihydrite
($38). According to these figures, the total dissolved Fe at the pH values
typical for calcareous soils (7.5–8.5) is of the order of 10À10 M, a concentration well below that required for optimal plant growth. To solve this difficulty, plants have developed different Fe uptake strategies.
Calcareous soils are generally poor in Fe oxides, with Fed values rarely
exceeding 10 g kgÀ1 in highly calcareous ones (Afif et al., 1993; de la
Torre et al., 2010; Sa´nchez-Alcala´ et al., 2011). This can be attributed,
on the one hand, to the diluting effect of carbonate and, on the other, to
limited weathering of Fe-bearing minerals—and hence formation of Fe
oxides from the soil parent material—under the high pH dictated by carbonate. Detailed characterization of the Fe oxide mineralogy of many calcareous
soils is thus hampered by the low concentration in which the various oxides
occur, though some techniques, such as diffuse reflectance spectroscopy, can

Micronutrient Constraints to Crop Production

17

provide useful information in this respect (Sa´nchez-Alcala´ et al., 2011;
Vela´zquez et al., 2004).
As deduced from the Feox/Fed ratio, the poorly crystalline, more soluble
moieties represent generally from 10% to 40% of the total content of the Fe
oxides in these soils, Feox lying generally below 2 g kgÀ1 (Afif et al., 1993;
del Campillo and Torrent, 1992a; Sa´nchez-Alcala´ et al., 2011; Torrent,
1994). Feox is usually ascribed to ferrihydrite, although positive identification of this mineral (via X-ray diffraction or Mo¨ssbauer spectroscopy) is possible only for Feox values >50À100 g kgÀ1. More intriguing is the possible
presence of nanosized and thus oxalate-soluble lepidocrocite resulting from
direct reaction of calcite with Fe2þ released during the weathering of
Fe-bearing minerals, as suggested by laboratory experiments where Fe(II)
salts were reacted with calcite (Loeppert and Hossner, 1984; Rolda´n
et al., 2002).
3.1.3 Soil properties as chlorosis indicators
As indicated, ACCE (“active lime”) has often been used to predict the risk of
Fe-deficiency chlorosis on the assumption that ACCE is a measure of the
calcite reactivity, and hence directly related to the concentration of bicarbonate in the soil solution. In fact, ACCE is systematically used for sorting
rootstocks of fruit trees or grapevine (Champagnol, 1984). However, the
predictive capacity of ACCE has often been found to be limited or of no
value (Benı´tez et al., 2002; Reyes et al., 2006; Yanguas et al., 1997), or
not superior to that of CCE (del Campillo and Torrent, 1992a). Also, different carbonate-related properties (surface area, calcite ion activity product,
bicarbonate ion concentration) are often mediocre predictors of the chlorosis (de la Torre et al., 2010; del Campillo and Torrent, 1992a).
The key influence of the reactive soil Fe phases on the incidence of Fe
chlorosis is well established. Thus, Juste and Pouget (1972) used the “indice
de pouvoir chlorosant” (IPC) (chlorosing power index), which was defined

as the ratio between ACCE and neutral NH4 oxalate-extractable Fe, as a
predictor of the Fe chlorosis risk in grapevine. Lindsay and Norwell
(1978) demonstrated the ability of a test based on a complexing agent
(diethylenetriaminepentaacetic acid, DTPA) for estimating labile Fe, Cu,
Mn, and Zn in calcareous soils. In the 1980s, the concentration in poorly
crystalline Fe oxides was thought to be a better predictor than active lime
or other tests related to carbonate for the incidence of Fe chlorosis in field
crops (Loeppert and Hallmark, 1985; Vempati and Loeppert, 1986). The
superiority of tests based on the reactivity of soil Fe forms over those based


18

John Ryan et al.

on carbonate-related properties has also been demonstrated in fruit trees,
such as peach (Yanguas et al., 1997) and olive (Benı´tez et al., 2002), and
in grapevines (de la Torre et al., 2010).
Both Feox and Feca (citrate/ascorbate-extractable Fe; Reyes and Torrent,
1997), which provide a quantitative measure of the content of poorly
crystalline Fe oxides in soil, proved superior for predictive purposes to
DTPA-extractable Fe (FeDTPA)—which is only a measure of the most labile
Fe fraction (Benı´tez et al., 2002; de la Torre et al., 2010; del Campillo et al.,
1992; Reyes et al., 2006). Iron extracted by unbuffered hydroxylamine
(Feha) was found to be a better predictor of the incidence of Fe chlorosis
in lupine (Lupinus albus L.) grown in calcareous soils than were Feox and Feca
(de Santiago and Delgado, 2006). This superiority was partly ascribed to the
sensitivity of Feha to ACCE because the final pH of the extracting solution
was found to be negatively correlated with ACCE (de Santiago et al.,
2008a). In contrast, Feox and Feca are not sensitive to the effect of carbonate

on the dissolution of Fe because care is taken to keep the pH of the
extracting solution constant (del Campillo and Torrent, 1992b; Reyes
and Torrent, 1997). It should be noted in this respect that the IPC index,
which combines ACCE and extractable Fe, appears to have only a mediocre
or poor predictive value (de la Torre et al., 2010; Reyes et al., 2006).
The relationship between leaf chlorophyll concentration [either measured directly or estimated by a chlorophyll meter (SPAD units)] and a soil
Fe test can often be described by a linear-plateau model, that is, one connecting a linear regression with a flat plateau. This allows one to establish
a critical level above which the plant response to the application of Fe
fertilizers is unlikely. Appropriate soil Fe tests provide a first indication of
the likelihood of Fe chlorosis in a certain plant species/cultivar to occur.
However, many soil and environmental factors add uncertainty to the initial
prediction. Factors such as organic matter content, soil compaction, temperature and moisture regime, redoximorphic conditions, irrigation method,
and fertilizer application level have a clear influence in this respect. In particular, critical levels for soil tests need often to be reestablished upon crop
intensification. One good example is the rapid increase in the past three
decades of the olive-growing area in southern Spain that is affected by Fe
chlorosis as a response to irrigation and increased fertilizer addition.
Base on our experience in Spain, it should be noted that plant analysis,
except for measurement or estimation of leaf chlorophyll concentration (via
SPAD), is of little value in quantifying the incidence of Fe chlorosis in experiments aimed at establishing the critical levels for soil tests. In particular, Fe


Micronutrient Constraints to Crop Production

19

concentration in chlorotic plants can be greater than that in Fe-sufficient
ones, a fact referred to as the “chlorosis paradox” (Ha¨ussling et al., 1985;
Morales et al., 1998; Ro¨mheld, 2000).
3.1.4 Iron fertilizers and soil reactions
Iron fertilizers, either as solid products or suspension or dissolved in water,

have been used in a variety of ways: injected into trunks, sprayed on leaves,
applied to the surface, injected directly into the soil or incorporated in the
irrigation water (Abadı´a et al., 2011; Lucena, 2006; Rombola´ and
Tagliavini, 2006). This array of practices suggests that there is no “best”
way to correct or prevent Fe chlorosis. The decision to apply a Fe fertilizer
and the choice of the most suitable one and its application method depend
on crop and soil characteristics, available equipment and, above all, the product application costs. In practice, this excludes most field crops and also horticultural and tree crops in areas where yields are limited because of the
moisture regime, depth and physical properties of the soil, or low availability
of other nutrients. In the Mediterranean region, irrigated fruit trees (peach,
citrus, pear, apple, kiwifruit, cherry, and quince), grapevine, some horticultural species (tomato, squash, and artichoke), pistachio, and olive receive
most of the Fe fertilizers.
The nature and merits of different Fe fertilizers, strategies for application,
and new analytical methods in their characterization have been discussed by
Abadı´a et al. (2011). Synthetic Fe(III)-chelates, the most widely used Fe fertilizers (>5000 Mg yearÀ1 in the Mediterranean region), are generally
applied with the irrigation water or in solution as foliar sprays or injected
into the soil; occasionally, the solid product is spread over the soil surface.
These compounds are notably effective as Fe fertilizers because they are stable and remain soluble over the pH values typical of calcareous soils, and can
thus be readily used by Strategy I plants (those with a Fe reductionbased strategy).
There is a great variety of synthetic chelates differing in stability and
effectiveness, which is the subject of active research (Abadı´a et al., 2011;
Lucena, 2006). Unfortunately, synthetic Fe(III) chelates are expensive,
which makes them prohibitive for low-value crops, and because of their solubility are easily leached from the soil in rainy periods. Also in the debit side
is the persistence of these products in the environment and their negative
influence on the availability of other metal nutrients, for example, Zn. Complexes of Fe with a variety of natural organics, for example, humic substances, low-molecular-weight organic acids or lignosulfonates can be


20

John Ryan et al.

used as Fe fertilizers (Cerda´n et al., 2007; de Santiago et al., 2008b); although
they are cheaper, their generally low stability in calcareous systems limits
their value if applied to the soil rather than to the foliage.
Ferrous sulfate heptahydrate [Fe(SO4)Á7H2O] is the most widely used
soluble Fe salt to correct Fe chlorosis. As mentioned before, the reaction
of Fe2þ with calcite can result in the production of poorly crystalline Fe
oxides, which are considered good mineral sources of Fe for the plant. This
product is moderately effective when applied to the foliage or in some complexes prepared with humic substances (de Santiago et al., 2008b), but they
are not so effective when spread in granular form on the soil surface (as usually practiced by some fruit growers) or dissolved in the irrigation water.
However, some field experiments in olive orchards showed its effectiveness
to be higher if applied as a concentrated solution in the soil zones of high
root density (A. Sa´nchez and M.C. del Campillo, personal communication).
This salt has the advantage of generally low price, because it is generally a
by-product of the steel, titanium dioxide, and sulfuric acid industries.
The past two decades have seen efforts to design slow-release and
environmental-friendly Fe fertilizers, some of which have been tested in
the Mediterranean region. Ferrous phosphate [an analogue of the mineral
vivianite (Fe3(PO4)2Á 8H2O)] proved to be effective to correct Fe chlorosis
in different crops when mixed with or injected in the form of a concentrated
suspension into the soil (Dı´az et al., 2009; Eynard et al., 1992; Rombola´
et al., 2003; Rosado et al., 2002). In contrast to Fe(III) chelates, vivianite
had a long-term Fe-fertilizing effect. This effect is thought to be due to
its oxidation and incongruent dissolution to nanosized, poorly crystalline
lepidocrocite (g-FeOOH) crystals (Rolda´n et al., 2002), whose solubility
is likely to be higher than that of the more crystalline soil Fe oxides. Soil
injection with suspensions of ferrous carbonate [an analogue of the mineral
siderite (FeCO3)] was also shown to be effective to correct Fe chlorosis for
several years (Sa´nchez-Alcala` et al., 2012a,b). Both products are nontoxic
and can be readily prepared from commercially available raw materials
before field application. They add to the list of slow-release Fe fertilizers,

which includes various industrial by-products and even ground basalt and
tuff (Barak et al., 1983).
Assessing the efficiency of Fe fertilizers requires practices based on a
sound knowledge of the physiology and biochemistry of Fe deficiency
(El-Jendoubi et al., 2011). Thus, one needs to carefully measure the different
responses of plants to Fe deficiency in addition to the most apparent one, that
is, leaf chlorophyll concentration. This includes plant growth parameters,