Last questions!!

1) Solenetzic soils have columnar structure when dry and massive when wet (because the sodium deflocculates the aggregates)? Right? What other relationships between soil orders and structures should I take note of?

2) Chernozemic soils have an Ah thicker than 10cm, have high base saturation and the main cation is Ca. But what about the Ap? Can this be in place of the Ah and still be a chernozem?

3) I am confused about pH and CEC. Does CEC increase when acidity increases (pH lowers)? How does this relate to dissociation, flocculation and dispersion?

- I think that CEC increases as pH increases. But I have a note next to losses in the sulphur cycle - it says that there is less loss of soluble sulphur in acidic conditions as it gets held to the colloid surface by adsorption. So is this indicating that there is a higher CEC in acidic soils?? - And then how does organic matter change pH and CEC?

4) Which soil organisms names should I know in more detail instead of just group (fungi, algae, bacteria, protozoa)? For example: Nitrobacter, rhizobium and nitrosomonas.

Soil organisms:

Carla, you should know the major groups (as per the class notes and text) as you indicate above, but within fungi and bacteria, as they include a wide range of organisms with different functions it would be useful to know important subgroups e.g. heterotrophic bacteria, nitrifying bacteria (your examples of nitrobacter and nitrosomas), N-fixing bacteria (your example of rhizobium with legumes), cyano-bacteria; similarly for fungi - heterotrophic fungi (decomposers), and mycorrhyzal fungi.

Thank you

CEC:

You are correct that CEC increases as pH increases (pH dependent charge), due to the deprotonation (loss of H⁺) of function groups (e.g. R-COOH + OH^- ---> R-COO^- + H₂O).

Soil Organic Matter (SOM), pH and CEC:

SOM has an important influence on CEC and pH as SOM contains a large number of functional groups (e.g. R-COOH, R-OH etc.) and deprotonation of these functional groups will 1) increase the net negative charge of the soil colloids (e.g. R-COO^-), and 2) increase H in the soil solution (buffer capacity).

Additionally, decomposition of SOM by soil organisms releases CO₂, which combines with water to form carbonic acid (lowering soil pH)

Sulfur:

Many sulfate compounds are quite soluble, so similar to nitrate, sulfate may be leached from the soil in humid regions. However, most soils have some AEC. Recall that AEC is greater at lower pH due to the protonation of hydroxyl (OH) groups. Consider the diagram in your text in the section of Chapter 12 on sulfur retention and exchange, the figure showing the effect of lowering pH on an Al-oxide. As pH decreases AlO^- --> AlOH --> AlOH⁺ giving the soil colloid net +ve change and AEC; thus SO₄ is attracted to the colloid

Soil orders:

The lab manual and soil web (http://soilweb.landfood.ubc.ca/classification/) provides a good overview of the characteristics of Soil Orders in the Canadian Classification system (supplementing our class notes). 1st I suggest focusing on the diagnostic horizons / diagnostic characteristics of the Soil Orders.

Columnar structure in the B horizon of Solonetzic soils is indicative of Na salts (as you indicate). Additionally in Luvisolic soils (Bt), the enrichment in clay may result in a blocky structure in the B horizon. What about Chernozems, developed under grasslands, would you expect an influence on soil structure? hint: earthworms. How about vertisols? consider wedge shaped soil peds - how are they formed.

Theoretically any soil order can have an Ap horizon.