Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Mineral Nutrition/Hydroponics

I.  Elemental Composition of Plants

A.  Table 6-1 from Salisbury and Ross.

B.  Conclusions: 

1. 90+% of plant C, H, O, N;

2. over 60 different elements have been found in plants including gold, arsenic, mercury, lead and uranium;

3. certain tissues concentrate elements (i.e., young tissues have higher levels of N, P and K);

4. similar elemental composition in different species.

II.  Essential Elements

A.  Definition - An element is considered essential if:

1. required for growth and development;

2. directly involved in plant metabolism;

3. an integral constituent of important constituents in the plants;

4. required for life cycle completion; and 

5. no other element can substitute for it.

B.  Specifics

• There are species specific differences

• Not all of the elements that occur in a plant are essential (i.e., some may be non-selectively absorbed). 

• There are 17 essential elements (see table 5.2 from Taiz and Zeiger)

• Mnemonic to remember:  "C. Hopkins Cafe closing; mob coming with machine guns" or in symbolic form - C HOPKNS CaFe ClZn; MoB CuMn Mg.

C.  Conclusions:  

1. some elements are required in high concentration (macronutrients), others low (micronutrients);

2. some supplied from the air (i.e., C and O via CO₂), most absorbed by the roots from the soil; 

3. lot of O and H - shows the importance of water.

D. Functions.
    Check the text (Table 5.3 in Taiz and Zeiger) for a nice summary.  Roughly speaking:

1. C, H, O, N, and S are required because they are the building blocks of the  molecules of life; 

2. P, B (Si) involved in energy transfer reactions; 

3. K, Na, Mg, Ca, Mn, Cl have various functions including maintaining osmotic concentrations and structures of enzymes;

4. Fe Cu Zn Mo prosthetic groups, electron transfer reactions.

III.  Determining nutrient elements.
    Use soil-less techniques.  Julius Sachs first showed could grow plants in solution culture in 1860.  Term hydroponics coined by W.F.Gericke in 1930's (from Greek, hydro = water; ponos = labor).  Many variants of the technique so long as: (1) support plant; (2) supply proper elements; (3) roots receive adequate aeration and moisture.

    The techniques include: (1) solution culture (+/- aeration); (2) nutrient film techniques; (3) aeration systems.

IV.  NPK
    Nitrogen, phosphorus and potassium are usually the most limiting elements because they are required in the highest concentration and are least likely to be supplied in a soil/growth medium in adequate amounts.  Hence the need for fertilizers.  Labels include the form and percent of each; i.e, 14 - 14 -14 means that a fertilizer has 14% nitrogen, 14% phosphorus and 14% potassium.  Fertilizer labels also include micronutrients.

    In class we will see some slides of "The Land" exhibit from Epcot.

V.  Calculating the cost of a fertilizer based on nitrogen content

• use N because it is the most important element in houseplant fertilizers

• Equation:  $/kg N  =  Cost of fertilizer ($) x 1/%N  x  1/pkg mass in kg

• Example using RaPid Gro:  $/kg  =  $1.99 x  (1/0.23)  x 1/0.227 = $38.10

• Table 1 lists some common fertilizers, type, cost.  Calculate the Nitrogen cost for the other fertilizers.  

(key:  S=soluble; C=controlled release; O=organic). 

References:

• This table is adapted from D. Hershey (1990) Science Activities 27:17-20.