PLANT NUTRIENTS

                                               

1. What are the required elements?

2. How are plant nutrients stored in the soil?

3. How are nutrients obtained by plants?

 

 

CHEMICAL ELEMENTS REQUIRED BY HIGHER PLANTS

 

A. Absorbed from the soil solution

            Macronutrients:

                        Nitrogen, Phosphorus, Sulfur, Potassium, Magnesium, Calcium

            Micronutrients:

                        Iron, Molybdenum, Manganese, Copper, Boron, Zinc, Clorine, Cobalt

2. From CO2 and H2O:  Carbon, Oxygen, Hydrogen 

                       

NUTRIENT STORAGE IN THE SOIL

 

1. In relatively unavailable forms:

            a. in minerals

            b. in organic compounds

 

2. In relatively available forms as ions (cations or anions):

            a. adsorbed to soil colloids

            b. in soil solution

 

  Examples of relatively available forms:

            ammonium salts                                 NH4+

            nitrite salts                                           NO2-

            nitrate salts                                         NO3-

            phosphate of Ca, K, Mg                      HPO4--

            soluble organic phosphate                   H2PO4-

            potassium salts  (with sulfates and carbonates)

            potassium ions on colloids                 K+

            calcium on colloids                             C++

            calcium salts                                       C++      

            magnesium on  colloids                      Mg++

            magnesium salts                   

            sulfites                                     SO3--

            sulfates of Ca, K, etc.             SO4--

 

                                    INFLUENCES ON CATION EXCHANGE CAPACITY

 

            Cations are mainly adsorbed to negatively charged colloidal clay and humus; in much smaller quantities they are in the soil solution.

 

            Micelle = a minute soil particle; large surface area is normally negatively charged.

                       

            Cation exchange  is the exchange of adsorbed cations on a micelle (or colloid) with those in solution

 

A. Soil texture -- increased surface area of finer soils result in a greater CEC

 

B. Type of soil colloid

 

            The surface area and electrical charge of micelles largely determine the capacity of the soil to store cations.

 

1. Humus

            Very high CEC

            Humus is rapidly decomposed (unstable)

 

2. Mineral colloids

  a. Silicate clays and aluminosilicate clays

            Sheet-like structures of aluminum, silicon, oxygen, etc. give the clay crystal both an external surface and an internal surface (i.e. in between layers).

 

            i. non-expanding silicate clays -- rigid, fixed structure; less internal surface area and much less CEC (e.g. kaolinite typical of humid tropics, halloysite, illite, etc.)

 

            ii. expanding silicate clays -- shrinking and swelling clays; much greater CEC (e.g.'s montmorillonite and  vermiculite groups)

 

  b. Hydrous oxide clays  (iron and aluminum oxides with water)

            Non-expansive; smaller CEC

 

  c. Non-crystalline mineral colloids:

            e.g. allophane (high CEC)

 

MEASURES OF SOIL FERTILITY

 

1. Cation exchange capacity (but includes non-nutrient cations too).

 

2.  Soil reaction = acidity or alkalinity of the soil.

           

            pH = the logarithm of the reciprocal of H+

            acid if,               H+ > OH--

            alkaline or basic, if         H+ < OH--

 

  Sources of soil acidity:

            a.  CO2 in solution (carbonic acid)

            b. decay of organic matter (organic acids)

            c. weathering of granitic rocks rich in feldspars (K and Na)

            d. abundance of precipitation

 

3. Percentage base saturation = proportion of CEC occupied by bases rather than Al or H.

 

ANION NUTRIENT EXCHANGE

 

            Sulfites, salfate, nitrates and phosphates are negatively charged, mostly in soil solution and easily leached.

 

            very minor anion exchange capacity results from:

 

1. positive charges on part of the humus

2. positive charges on exposed edges of crystals in silicate clays where H+  groups are attached to OH--

 

            Anions are also slowly released by the decomposition of organic compounds.