Midterm 2, Q5
I lost a bunch of marks on Q5 on midterm 2 (the management question). I'd appreciate any feedback for how I could have answered this better (or, if you got full marks on this question, I wouldn't mind seeing your answer for comparison.) My answer from the exam is below.
a) L1: Has the best nutrient retention (highest CEC=35) and second best supply (percent org mattter=7)
L2: Has the second best nutrient retention (CEC=33), but the best supply (percent org matter=10)
L3: Has by far the worst supply (org matter=2) and the worst retention (CEC=15)
b) L1: Will have the highest water retention (since percent clay=50, and also good aggregation holds water in micropores)
L2: Will have the second highest water retention (percent clay=30, and also good aggregation holds water in micropores)
L3: will have the lowest water retention (percent sand=75, and no micropores in aggregates to retain water)
c) L1: should have the second best drainage. Although it's a clayey soil, the fact that it's well-aggregated means that it should drain efficiently via macropores. In addition, marine parent material has a higher inflitration rate than alluvial parent material.
L2: Should have the worst drainage (assuming the assumptions about L3 hold). Although the fact that it's well aggregated means that it should drain well via macropores, its alluvial parent material won't drain as well as L1's marine parent material.)
L3: should have the highest drainage, assuming the parent material is loose ablation till instead of basal till, and the soil has not been severely compacted.
d)
I would recommend L1 as the ideal site for a grassy picnic area, given that it seems to be the best compromise between the above factors. I would choose it instead of L2 given that it's likely to have better drainage, an important factor when considering a high traffic area that's likely to suffer from drainage issues due to compaction.
In addition to Maja's comments: Please be aware that there were different versions of the Data Interpretation Question randomly assigned to different students; thus the answers above are not application to other student's exams. Maja's comments below however are generic and useful.
Spencer,
In the part of the answer re. nutrient retention you did not mention differences in clay among the 3 soils, but based your very brief explanation of differences in CEC just on organic matter. And, those differences in organic matter contents among the 3 soils needed to be explained (you just stated what they are). The explanation should have included statements that both organic matter & clay particles cary lots of charges on their surfaces and are consequently responsible for nutrient retention.
Parts of answer on water retention & drainage were much better (ie complete). But the final selection of the most suitable location (or soil) was not correct. The best soil was #2 since it had the most balanced combination of nutrient and water retention properties for the city park.
So, here is an important advice to all of you who are reading this (and I sincerely hope that you are reading this) - in these so-called explanation type of questions, pls make sure that you provide both the reason for your observation and explanation.