Micronutrient Constraints to Crop Production in the Middle East–West Asia Region: Significance, Research, and Management

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 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.