Testing Soils for Copper, Iron, Manganese, and Zinc

This chapter reviews the current status of soil testing for copper, iron, manganese, and zinc. The three sequential steps generally followed to develop micronutrient cation soil tests are extractant selection, greenhouse evaluation, and field calibration. The first step is to select an extractant that will solubilize a proportionate part of labile forms of the micronutrient cations from different soils. Field calibration of a micronutrient cation soil test commonly entails the determination of a critical level, i.e., the soil test value that separates soils into responsive and nonresponsive categories. The critical level is based on yield response to application of the micronutrient under field conditions. An inherent limitation of soil micronutrient cation soil tests is the attempt to extract labile forms from a dry or moist soil sample, which may differ from labile forms in field soil from the same location during a growing season.