Soils 205- General Soils

Lecture 27 Sulfur and Phosphorus

I.  Sulfur- Sulfur is a required component of many plant compounds including proteins, enzymes, and chlorophyll.

    a.  Aspects of sulfur in soil:

    -can be leached

    -deficiencies are more common than in the past due to:

        1.  Enforcement of clean air standards

        2.  The use of concentrated, high purity fertilizers

        3.  Improved varieties have increased yields, so while less S is being added to the soil, more is            being removed through harvest

    b.  Deficiency symptoms include:

        -stunted growth

        -chlorosis

            i.  Conditions favoring S deficiencies

                -parent materials low in S

                -areas with extreme leaching and weathering

                -areas that do not receive significant S from atmospheric deposition

                - removal of vegetation

    c.  Sources of plant available S:

        -organic matter

        -soil minerals

        -atmospheric S

    d.  Sulfur sources and sinks

II.  Phosphorus- P is a macronutrient that plays a role in energy transformations within a plant.  It is found in important "biomolecules" such as ADP, ATP, DNA, and RNA.

    a.  Aspects of P in soils:

        -its form varies with pH:

        -problems with P nutrition

            1.  The total P level in soils is very low (1/10 to 1/4 that of N).

            2.  The P compounds commonly found in soils are insoluble.

            3.  Soluble sources of P added to soil are rapidly "fixed".

                i.  P fixation

        b.  Deficiency symptoms:

            -stunted growth

            -dark bluish-green to purple foliage (yellowing in severe cases)

            -sparse flowering and poor seed quality

        c.  Environmental problems- P is usually associated with the solid phase and is highly susceptible to losses due to erosion.  If the solid material winds up in lakes, the P becomes soluble again and is taken up by algae causing their numbers to rapidly increase.  Increased algae activity results in high oxygen demand and the lake may become oxygen deficient.  This process is termed eutrophication.

P is not usually a problem in groundwater because it does not get past the root zone.

        d.  Sources of P:

        -organic P

        -calcium-bound inorganic P

        -iron/aluminum-bound inorganic P

        e.  Phosphorus:  Sources and sinks