Soils 205- general soils

Lecture 29- micronutrients

I.  Micronutrients

Fe, Mn, B, Mo, Cu, Zn, Cl, Co

They are referred to as micronutrients because they are required in smaller amounts than are the macronutrients.  This does not mean that deficiencies are not significant.  A deficiency of B, for example, can cause just as many problems as a N deficiency. 

    a.  Concerns

    -increased yields have resulted in greater removal of micronutrients from the soil

    -high-analysis fertilizers (low impurities)

    -very narrow margin between deficiency and toxicity

    b.  sufficiency/deficiency ranges

    c.  sources of micronutrients

    -minerals

        examples:  Fe and Mn are found in structural positions within biotite and hornblende

            Fe, Mn, Co, and Zn can be found within silicate clay mineral structures

            sulfides and oxides of Fe, Mn, and Zn

    -organic matter (plant material and animal manure)

    d.  factors influencing micronutrient availability

        1.  pH

        Example of the influence of pH using Fe:

soluble                                                                                insoluble

        Example of the influence of pH using Mo:

        At low pH, Mo is present as an anion (HMoO₄^-) and is adsorbed by silicate clays and Fe/Al oxides (similar to P).

    2.  Redox status:  In general, micronutrients are more available in their reduced state.  For example Fe⁺² is more soluble than Fe⁺³.

    As a result, Fe and Mn toxicities are especially common in wet, low pH soils.

    e.  conditions favoring deficiency:

        1.  highly leached, acid, sandy soils

        2.  organic soils

        3.  soils of very high pH

        4.  soil that has been very intensively cropped and fertilized only with macronutrients

    f.  Chelates:  chemical compounds where a metallic ion is firmly combined with an organic molecule by multiple chemical bonds.  It protects the ion from reacting with mineral surfaces.