Micronutrients are of growing importance in crop nutrition because of: increased demand from higher yielding crops and intensive cropping; continued expansion of cropping onto marginal land with low levels of micronutrients; increased use of high analysis fertilizers containing low levels of micronutrients; decreased use of manures, composts and crop residues in some parts of the world; mining of micronutrient reserves in soils; nutrient imbalances. In addition there is an emerging shift in emphasis from the role of micronutrients in crop production, to their levels in the main staple foods in diets of humans and animals. The importance of micronutrients in agriculture can be defined as the product of the magnitude of impacts per unit area, and the area of impact. Impact is most commonly measured as crop yield. However, a variety of other properties may be more important for marketing the harvested plant products than yield, per se, including crop quality, and physical defects of the harvested products. For legumes, the main impact of micronutrients in a cropping system may be on amounts of N fixed. Another aspect of impact is the effect of micronutrient concentrations in planting seed on the vigor of the next season's crop. Finally, a major economic impact of micronutrients in a farming operation is through the increased efficiency of macronutrient fertilizer use. Areas affected by micronutrient deficiencies, the second component of importance, are challenging to estimate. Generally, approaches to defining the area of impact consider only the topsoil levels of micronutrients. Most current reports on the area of impact record micronutrient status in the topsoil at a point in time, but fail to recognise dynamic changes in micronutrient status or land use over time. For example, areas of southern Australia that were once mapped as almost entirely deficient in Zn, Cu and Mo are now mostly adequate in topsoils for crop growth. Moreover, the adequate micronutrient status in topsoils is no guarantee that sub-soil levels are sufficient for unrestricted crop growth. Changes in genotypes over time may also mean that an area once considered adequate in micronutrients is now deficient. Finally, as yield output from farming systems rise, areas that were previously adequate are now declining in micronutrient reserves in soils, and hence deficiency is reported with increased frequency. The challenge for the fertilizer manufacturers, distributors and agronomists is to find low cost-effective means to continually update information on the locations and areas affected by micronutrient deficiency. As a case study in addressing this need, we discuss a weight-of-evidence mapping approach that produces updateable maps of B deficiency and explore evidence relating to low sub-soil micronutrient levels.