Figure 1

Most often, the method of supplying the trace elements to the soil has been to add the soluble salts of these elements as a direct soil amendment or by inclusion in a standard NPK fertilizer blend. That is, the metal ion in a SULFATE compound (SO4) is added. However, there are two significant problems with this approach. First, adding the sulfate complex of the metal does nothing to address the ability of the soil to sequester the element. For example, adding iron sulfate to alkaline soils will allow the following chemical reaction:

Fe2SO4 + O2 + H2O → Fe2O3 + H2SO4

That is, iron sulfate plus oxygen plus water produces iron oxide (rust) plus sulfuric acid. Most of the other metal sulfates have a similar reaction.

Secondly, when the trace metals are tied into sulfate compounds, most of the elements are not available to the plant. The ability of plants to directly absorb and utilize the sulfate forms of the trace metals is limited at best. The grower's response to this in the past has been to simply add more sulfate, leading to accumulations in the soil that eventually may reach toxic levels.