Micronutrient constraints to crop production in soils with Mediterranean-type characteristics: A review

Mediterranean-type soils generally have free CaCO3, high pH, and low organic matter. Consequently, nutrient disorders in these soils are the most important limiting factor to crop production, second only to moisture stress. Major problems are deficiencies of nitrogen and phosphorus; however, recent research has revealed that micronutrient problems are also hampering crop production. Unlike major nutrient deficiencies, micronutrient problems are highly genotype-specific and location-specific. Zinc (Zn) deficiency is the most widespread problem, because of factors like alkaline soil pH, calcareousness, low organic matter, exposed subsoils, Zn-free fertilizers, and/or flooding-induced electro-chemical changes. Now zinc sulfate is commonly used in rice, wheat, barley, potato, citrus, deciduous fruits, and many other crops. Soil-applied Zn has a relatively long residual effect. Efforts are also underway to develop crop varieties for higher Zn efficiency. Iron (Fe) chlorosis, the second most important micronutrient disorder, is induced by high soil HCO3- in calcareous soils; high moisture and cold temperature accentuate it. Legumes, citrus, and deciduous fruits are more susceptible. Soil fertilization for iron is problematic; therefore, foliar feeding or breeding for tolerance remains the practical solutions. Boron (B) is a nonmetal micronutrient. While B deficiency has not been observed widely, research in the recent past has revealed this to be a widespread problem in crops like cotton, rapeseed, wheat, peanut, sorghum, and rice. The deficiency can be corrected by its soil application or by foliar feeding. As the concentration range between B deficiency and toxicity is exceptionally narrow, both are field-scale problems in the Mediterranean-type soils. Unlike B deficiency, B toxicity diagnosis is not simple, because of its accumulation in sub soils and resemblance of the symptoms with leaf fungal diseases. Also, B toxicity cannot be ameliorated for all practical purposes. Thus, breeding for B tolerance remains the only option. Luckily, many landraces of barley and wheat are quite tolerant to B toxicity. Deficiencies of manganese (Mn) and copper (Cu) are not a perceived problem, and availability of molybdenum (Mo) and chloride (Cl) is high in Mediterranean-type soils. Soil micronutrient deficiencies not only reduce crop productivity, but low Zn and Fe plant food is also adversely affecting human health and well-being. Though fertilizer use for micronutrients is highly cost-effective, actual use remains rather limited to a few specific crops in various countries. Thus, micronutrient problems are likely to be accentuated because of increased pressure on soil resources.