Mapping soil micronutrients

Abstract

Soils vary widely in their micronutrient content and in their ability to supply micronutrients in quantities sufficient for optimal crop growth. Soils deficient in their ability to supply micronutrients to crops are alarmingly widespread across the globe, and this problem is aggravated by the fact that many modern cultivars of major crops are highly sensitive to low micronutrient levels. Original geologic substrate and subsequent geochemical and pedogenic regimes determine total levels of micronutrients in soils. Total levels are rarely indicative of plant availability, however, because availability depends on soil pH, organic matter content, adsorptive surfaces, and other physical, chemical, and biological conditions in the rhizosphere. Micronutrient availability to plants can be measured in direct uptake experiments, or estimated with techniques that correlate quantities of micronutrients extracted chemically from soils to plant uptake and response to micronutrient fertilization. Rational management of micronutrient fertility and toxicity requires an understanding of how total and plant-available soil micronutrients vary across the land. A variety of approaches have been used to survey and map the geographic distribution of soil micronutrient content and availability at scales ranging from global to sites within single production fields. Soil micronutrient maps covering large areas improve our understanding of the nature and extent of micronutrient problems, and aid in determining their relationships with climate, soil properties, and soil genetic characteristics determined at similar scales, for example, Soil Taxonomy to the order, sub-order, or great group levels. Intermediate scale maps can be useful in delineating specific areas where deficiencies or toxicities are likely for agriculture, and in determining localized soil characteristics that may be associated with such problems. Highly detailed maps of soil micronutrient content and availability in individual fields are being developed for site-specific precision agriculture. Soil micronutrient maps have fostered discovery of relationships between soil micronutrient content and availability and some human and livestock health problems such as goiter, Keshan and Kaschin–Beck diseases, and cancer. Advances including the global positioning system (GPS), geographic information systems (GIS), inductively coupled plasma (ICP) spectrometry, geostatistics, and precision agriculture facilitate soil micronutrient mapping and provide quantitative support for decision and policy making to improve agricultural approaches to balanced micronutrient nutrition.