Midterm Exam Questions

hello Maja.

1.For the Q4 of 2008 mid term, could you please explain why those two ( sand and massive clay) soils differ in terms of water rentention?
2.why are the soil colliods the most reactive part of soil matrix? Is that because of they have permanent charges and there are many exchangeable ions bond to them? 

thank you .

1) Massive clay only has small (micro) pores, while sand only has large (macro) pores. Consequently, large pores in sand louse water very quickly (as indicated by a prominent drop in its water retention curve)with the deceasing matric potential. On the other hand, small proes of the massive clay, hold tightly on the water molecules and there is very little change in water content with a decreasing matric potential (as shown by its almost flat curve)

2)Soil colloids are small and as such they have large specific surface area and in addition they usually have lots of charges on their surfaces.