Review
Setting good practices to assess the efficiency of iron fertilizers

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Abstract

The most prevalent nutritional disorder in fruit tree crops growing in calcareous soils is Fe deficiency chlorosis. Iron-deficient, chlorotic tree orchards require Fe-fertilization, since chlorosis causes decreases in tree vegetative growth as well as fruit yield and quality losses. When assessing the effectiveness of Fe-fertilizers, it is necessary to use sound practices based in the state-of-the art knowledge on the physiology and biochemistry of Fe deficiency. This review provides an overview on how to carry out the assessment of the efficiency of Fe-fertilizers, discussing common errors found in the literature, outlining adequate procedures and giving real examples of practical studies carried out in our laboratory in the past decade. The review focuses on: i) the design of Fe-fertilization experiments, discussing several issues such as the convenience of using controlled conditions or field experiments, whether fertilizer assessment experiments should mimic usual fertilization practices, as well as aspects regarding product formulations, dosages, control references and number of replicates; ii) the assessment of chlorosis recovery upon Fe-fertilization by monitoring leaf chlorophyll, and iii) the analysis of the plant responses upon Fe-fertilization, discussing the phases of leaf chlorosis recovery and the control of other leaf nutritional parameters.

Highlights

► Iron deficiency chlorosis is a major constraint in fruit crops grown in high pH soils. ► Iron fertilizers must be assessed for efficiency using sound practices. ► This review outlines adequate procedures and discusses common errors. ► The review includes real examples of practical studies.

Introduction

The most prevalent nutritional disorder in fruit tree crops growing in calcareous soils is Fe deficiency (see reviews in [1], [2]). The main symptom of Fe deficiency in plants is leaf yellowing, which is usually called leaf chlorosis; this occurs both in growth chamber and field-grown plants (e.g., in sugar beet and peach trees, respectively; Fig. 1). In field conditions, chlorosis in the orchards is often heterogeneous, with individual trees affected to different extents. Images of fruit tree field orchards affected by Fe-chlorosis are shown in Fig. 2 (A: peach tree orchard; B: pear tree orchard). Iron-deficient, chlorotic tree orchards are usually fertilized with Fe every year, because chlorosis causes decreases in tree vegetative growth, a shortening of the orchard lifespan as well as losses in fruit yield [2] and changes in fruit quality [3], [4]. The diagnosis of Fe deficiency, conversely to what happens with other nutrient disorders, cannot be adequately assessed using leaf elemental composition, because Fe-deficient field-grown leaves often have Fe concentrations as high as that of Fe-sufficient ones (the “chlorosis paradox”; [5]). This is possibly associated to an accumulation of Fe in or near the vascular system [6], [7]. Therefore, leaf chlorophyll (Chl) concentrations (generally monitored using a hand-held device) are used most of the times to assess the Fe nutritional status.

Iron fertilization in trees can be carried out in several ways, including the addition to the soil or irrigation water of Fe-containing compounds [8], as well as providing Fe directly to the plant by spraying tree canopies or injecting trunks or branches with Fe-compounds in solid or liquid forms [1]. There is a very large number (several hundred) of Fe-containing fertilizers, many of them containing the same active principles and others consisting of a mixture of Fe-compounds [8], [9]. These Fe-fertilizers often have different degrees of effectiveness due to many different factors [1], [8], [10]. Therefore, it is necessary to compare the efficiencies of Fe-fertilizers, and many studies are published every year assessing and comparing Fe-containing products (e.g., see [11], [12]). In particular, any new Fe-fertilizer must be assessed using this type of studies. The recovery after Fe-fertilization is generally monitored using the leaf Chl concentration, for the reasons explained above, although leaf Fe concentrations are still sometimes used. However, divergences in specific methodological details could be found in the literature, and this could make difficult the comparison of results obtained in different experiments.

This paper proposes good practices to assess the efficiency of Fe-fertilizers, by examining a number of factors that are crucial in this type of assessment studies. The rationale for most of them is provided by the current knowledge on plant physiology and biochemistry [13], which is not always taken into account in Fe-fertilizer efficiency agronomical studies.

Section snippets

Design of iron fertilization experiments

Iron fertilizers should be first tested for stability of the Fe-containing active agent in the conditions prevailing in the media to be used (e.g., spray solutions, nutrient solutions, soils, etc.) [14]. Many factors (light, pH, etc.) can affect its stability and availability for plants. Once the possible usefulness of a Fe-fertilizer is predicted, controlled conditions or field experiments must be designed for assessing efficiency.

Assessment of chlorosis recovery upon iron fertilization

The most appropriate way to assess the efficiency of Fe-fertilizers is to follow the evolution of leaf Chl after Fe-fertilization. This has been carried out traditionally by using visual scale ratings [21], [22], [23]. In the last two decades, however, hand-held apparatus, such as the SPAD (Soil and Plant Analyzer Development) from Minolta and others, have become popular for the diagnosis of plant nutrient status (of Fe and other elements; [24], [25], [26], [27], [28]). These devices measure

Analysis of the plant responses upon iron fertilization

Besides assessing the effectiveness of Fe-fertilizers in chlorosis recovery, the evaluation of their effects on crop yield and quality will be also desirable. However, the normalization of the tree Fe-status via corrective Fe-fertilization is likely to have major fruit quality effects only in the following growth season, and not in the season when corrective fertilization is carried out. The most likely explanation for this fact is that the recovery of the tree physiological processes after

Concluding remarks

When assessing the effectiveness of Fe-fertilizers, it is necessary to use sound practices based in the state-of-the art knowledge on the physiology and biochemistry of Fe deficiency [13]. This includes using appropriate choosing of experimental orchards and individuals (taking special care in assuring the presence of Fe deficiency and the homogeneity of chlorosis) as well as an adequate methodology to measure leaf Chl concentrations. It should be always taken into account that the

Acknowledgements

Study supported by the Spanish Ministry of Science and Innovation (MICINN; projects AGL2007-61948 and AGL2009-09018, co-financed with FEDER), the European Commission (Thematic Priority 5–Food Quality and Safety, 6th Framework RTD Programme, Contract no. FP6-FOOD–CT-2006-016279), the trilateral Project Hot Iron (ERA-NET Plant Genome Research KKBE; MICINN EUI2008-03618), and the Aragón Government (group A03). HEJ and JCM were supported by a FPI-MICINN grant and a JAE-CSIC postdoctoral contract,

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