name________________________    AGRO 2244, Soils, T2, F2007


____    The greatest pool of potential soil [H+] is:

            A. reserve acidity                                            B. labile acidity

            C. active acidity                                              D. exchangeable acidity


____    The two main sources of [OH-] generation in soils are the carbonates and:

            A. sulfates                                                       B. pH dependent charges

            C. reserve alkalinity                                        D. bicarbonates


____    An example of a 1:1 layer silicate clay mineral is:

            A. montmorillite                                              B. chlorite

            C. gibbsite                                                       D. kaolinite


____    The tetrahedral layer of clays is dominated by the cation:

            A. Ca2+                        B. Si4+                         C. Al3+                         D. Mg2+


____    Over time in warm, humid climates, smectite clays will weather into:

            A. humus                                                         B. Vermicullite

            C. oxides of Fe and Al                                   D. Montmorillite


____    The chemical quality of lime is determined from the:

            A. C.E.C.                                                        B. C.C.E.

            C. S.A.R.                                                        D. L.I.F.I.N.O.


Given the data below:                                     dominant clay mineral analysis

soil       pH       %clay              tetrahedral layer          octahedral layer           anion net

A         6.2         20                  Si3.5 Al0.5                     Al1.6 Mg0.4                   O10 (OH)2

B         6.2        20                  Si3.2 Al0.7                     Al1.5 Mg0.5                   O9.5 (OH)2______

Ml. Wts.:   Si 28.1,  Al 27.0,  Mg 24.3,  O 16.0,  OH 17.0

Charges :  Si 4+,  Al 3+,   Mg 2+,  O 2-,   OH 1-


Which one has the highest cation exchange capacity ?  1) Sum of the charges for each layer by multiplying charge by amount, ie., soil A, tetrahedral Si = +4 X 3.5.  1) Sum of the ml. wts likewise, ie, soil A, tetrahedral Si = 28.1 X 3.5.   Do the above for all layers and the anion net.  3) Divide sum of charges,  the one with the higher (-) wins.

                                  sum of ml. wts.


See notes for more examples. 








Consider the following data in answering the next set of questions.



          extractable bases
















              - cmol (-) kg-1  -

cmol kg-1



  dS m-1



















































(Use the space below to show any calculations you do.)


What soil has the highest CEC?  (1 pt)  add all the cmol kg-1 up.


Name one of the soils that is acidic. (1 pt)   any with pH < 7.0


What soil is saline-sodic? (2 pts)  Pr


Which soil will need dolomitic lime?  (2 pts) 


Which soil has the lowest % base saturation? (2 pts)  calc. % B.S. for each one.  see notes.


If the average CEC of the O.M. is 200 cmol (-) kg-1, which soil has the clay with the highest average CEC?  (4 pts)

   from notes.  STEP 1.  Subtract out CEC contribution from the O.M. from the total CEC:

                                    Tot. CEC  -  ((% O.M. / 100)  X  200) = CEC contribution from clay.

                        STEP 2. Divide the above by (% clay/100).  Highest one wins.



____    Which of the following will lower soil pH?

            A. CaO                                                            B. S

            C. CaSO4                                                        D. MgCO3


____    The leaching requirement (LR) compares the E.C. of irrigation water to:

            A. USDA standards                                       B. Ca+Mg/Na ratio

            C. C.C.E.                                                        D. E.C. of water leaching through soil


____    What two basic soil cations dominate the exchange sites of most soils?

            A. Al3+ and H+                                                            B. Na+ and Ca2+

            C. K+ and Mg2+                                               D. Ca2+ and Mg2+


____    Which of the following soil cations is responsible for destroying soil structure?

            A. H+                           B. Ca2+                                    C. Na+                         D. K+






1. Matching: units of measurements of soil chemical properties.  (5 pts)


            __B__ Na saturation                                                   A. cmol (-)  kg-1

            __A__             cation exchange capacity                                B. %

            __E__  salinity                                                             C. –log [H+]

            __B__ Ca + Mg saturation                                         D. g cm-3

            __C__ pH                                                                   E. mmohs  cm-1



2.  List three ‘pools’ of soil acidity.  (3 pts)  see notes



4. A mid-west prairie soil has a very high C.E.C. but a relatively low % clay (15%). 

    Why then is the C.E.C. so high?  (there are two main reasons)  (4 pts)


1. High O.M. due to grassland ecosystem.

2. Clays with high CEC.


7.  List five potential sources of H+ in soils.  (5 pts)  see notes




9. Soil colloids and cation exchange capacity are said to be the second most important

    factor of crop yield besides photosynthesis.  Why would anyone make this claim?  (5 pts)

see notes.  focus is on plant nutrient supply reservoir and ability to keep plant nutrients in ionic states and therefore plant available.