mid term

As I said before, topics up to (but NOT including) soil organic matter will be on the midterm exam.

According to the lecture notes, soil water content decreases when matric potential becomes more negative. I am not sure why this is the case. As matric potential decreases(becomes more negative), soil holds water more strongly. Wht would soil content decrease if the water is held more strongly? Thank you!

Water retention curve (which illustrates relationship between water content and matric potential) indicates that as matric potential decreases, the remaining water left in the soil is held more tightly by the soil matrix (or by soil solids).

You always need to know what is impacting what. In the case of the water retention curve, matric potential (plotted on x-axis, as independent variable) is impacting water content (potted on the y-axis as dependent variable). Hence, by applying lower and lower matric potential (as we did in our water retention lab) one is able to pull out more and more water. At the same time, water that is left in the soil is held more strongly.

1. Why would physical and chemical processes such as wetting/drying and freezing/thawing contribute to aggregate formation?

2. Would field capacity be lower in a sandy soil than in a clay soil?

3. Midterm 2008 question 5: Can we assume the density of the organic matter is 1.3g/cm^3?

1. Wetting/drying and freezing/thawing are physical (not chemical) processes that enhance aggregate formation. They do so, since they (namely thawing and wetting) bring individual particles closer together, allowing them to bond better (which leads to flocculation as the 1st per-requisite for aggregate formation).

2. Clay soil with its higher porosity (than sandy soil), will have the higher field capacity.

3. There was a typo in question #5 in 2008 midterm, hence I replaced it with another example

I don't really understand question 2 on the 2006 midterm. When you ask "what is the charge change resulting from the incorporation of the Mg2+ instead of the Al3+" are looking for something about how it becomes acidic?? Does in become negative? Thanks a lot!

this is related to the isomorphic substitution. And in this example the substitution leads to formation of a negative charge. This is the same example as I used in the lecture.

Low matric potential of a soil sample represents low water content inside the sample,and it also represents strong adsorption and capillarity between soil particles and water. Is that right?

Yes, that is correct

In the lecture notes it says massive clay and single grain of sand are structureless. Does this mean they will not form aggregates? In other words, massive clay only has micropores and sandy soil only has macropores. If this is correct, can I say that massive clay will have a higher field capacity than sandy soil because micropores hold water more strongly than macropores? You have mentioned that clay soil has a higher porosity than sandy soil, but I don't understand why this is the case.

Yes, that is correct - massive clay and single grain sand are both examples of structureless soils. And yes, massive clay only has micropores, and sand has only macropores.

Yes, macropores hold water more strongly than large (macro) pores. Consequently field capacity for clay would be higher than for sand.

Porosity (f) is defined as ratio of volume of all pores (Vf) relative to the total volume of soil (Vt). Hence, in a massive clay there will be a larger proportion of pore spaces (relative to total volume) than in the sandy soil.

For more explanation on this topic pls refer to your textbook