2 concept questions

-Term 'soil colloid' refers to a size of particles that are smaller than 0.002mm; and those particles can be very different in terms of their nature. As mentioned in the lecture on Jan 15, soil colloids include pyllosilicate minerals, Fe & Al oxides/hydroxides, amorphous minerals, and organic matter.
Pyllosilicate minerals, Fe & Al oxides/hydroxides, and amorphous minerals are all secondary minerals.

-Total volume of a soil is comprised of both coarse fragments (ie those with diameter larger than 2mm) and fine particles (ie those with diameter smaller than 2mm).

Calculating bulk density core vs excavation method

In agricultural soils, bulk density can commonly be determined using the core method. However in rocky forest soils (i.e. coarse fragments > 25%), the excavation method is often used. The excavation method involves digging out a small hole, then oven drying (at 105°C) and weighing the excavated soil. The volume of the excavation is determined by lining the hole with plastic film and filling it completely with a measured volume of water (or sand, or silicon beads). Coarse fragments (diameter > 2 mm) are sieved out and bulk density is calculated as the mass of dry, coarse fragment-free soil per volume of the excavated soil, where volume is also calculated on a coarse fragment-free basis. Full details on this method are online at https://labmodules.soilweb.ca/soil-compaction-bulk-density/