Midterm Exam Questions
Questions: 1. Can I say the exchangeable ions are just ions located in DDL? Does the DDL include the soil solution ? 2. Does the AEV correspond to the matric potential at field capacity? 3. In the question 6 of 2006 old exam, I got the porosity is 0.363, but how can I see if it's good for plant growth?
Answers: 1. Yes, exchangeable ions are those present in the DDL. The DDL does NOT include ions in the soil solution. However, ions in the DDL are in constantly going into reactions of ion exchange with ions in soil solution, until the equilibrium between DDL and soil solution is reached.
2. AEV (air entry value) does NOT correspond to the matric potential at field capacity. AEV corresponds to matric potential at which water leaves the largest pores in your soil, and that condition is wetter than the field capacity.
3. Generally, mineral soils have porosity between 0.3 and 0.6 cm3/cm3 (or 30-60%) [see your lecture notes for lecture no.3]. The lower part of this range is an indication that a soil has been compacted, while the higher part of this range is an indicator of lack of compaction and good soil conditions for plant growth.
Hi Maja, based on my notes, Kaolinite is more stable than montmorillonite. I feel confused about that concept. Could you please briefly explain it? Thank you.
Both montmorillonite & kaolinite are members of the phyllosilicate group. However, they have a very different crystalline structure, which also translated into different stability and susceptibility to isomorphic substitution. With 2:1 type of structure, montomorillonite is characterized with weaker bonds within its crystallyne structure and is more prone to isomorphic substitutions and consequently larger number of charges than kaolinite (wiht its 1:1 type of structure)
More on these 2 minerals can be found at http://soilweb200.landfood.ubc.ca/soil-components/1-mineral-components/#Types_of_Soil_Colloids as well as in your textbook (see p.240-244)
Hi, Maja, in 2008 midterm , how to identify the soil structure, I can only tell that the first one is sand and second one might be clay since it has higher water retentive ability, for structure, should I answer something like spheroid, blocky? Thank you!
To have such a flat water retention curve, as shown in the 2008 midterm, soil #2 could only be a massive (i.e. structureless) clay. Any other clay textured soil with any formation of stable aggregates would not have such a flat water retention curve. Such shape of the water retention curve is indicative that the soil is only having small pores, and that can only be the case in a massive clay.