Willard L. Lindsay

TL;DR: The zinc content of soils, according to rather extensive surveys, is generally in the range of 10-300 ppm as discussed by the authors, which can be considered as a trace element in soil.

TL;DR: In primary soil minerals, Fe occurs largely as ferromagnesium silicates in which Fe is present as reduced Fe(II), and weathering of these minerals releases Fe which oxidizes and precipitates largely as Fe(III) oxides and oxyhydroxides as discussed by the authors.

TL;DR: The purpose of soil testing is to determine which nutrients are deficient in a given field and, if possible, to estimate how much fertilizer is needed as mentioned in this paper. But soil testing for micronutrients is particularly difficult because plant requirements for these elements are relatively low.