[USCC] soil texture testing procedures

[Wiebe, Brian]

Craig,

The texture analysis procedure may be giving correct readings (assuming that the organic component has been completely removed by peroxide treatment as mentioned in an earlier response).  The problem may stem from confusion as to the meaning of the term clay.

"the term clay may mean a material made of clay-sized grains (smaller than 2 µm or 0.002 mm) or of grains consisting of clay minerals. Grains smaller than 2 µm may be clay minerals or other materials such as finely ground quartz or rock flour."  (Geological Society, London, Engineering Geology Special Publications; 2006; v. 21; p. 73-138) 

Even the different clay minerals (montmorillonite, kaolinite, illite, to name a few of the most common in soil) have very different properties in regard to cation exchange capacity, swell/shrink, "stickiness", etc. but finely ground quartz or limestone would have properties extremely different from clay minerals.  Laboratory methods for determining the mineral particle size distribution of a soil sample (The Bouyoucos Hydrometer method and the pipette method are the most common) were developed to determine the amount of sand, silt and clay SIZED particles in a soil sample not their mineralogy.  It is quite possible that the rock fines are in this "clay sized particle size" (<0.002 mm effective diameter).  

The options that I see would be to either:
- get a mineralogical analysis to classify the fines as rock flour not clay minerals (this might satisfy the landscape architect), 
- use some of the engineering property based tests (as mentioned in an earlier post) such as the Atterberg limits (plastic limit, shrinkage limit, liquid limit) and the soil activity which is the Plastic limit divided by the percent clay-sized particles present to demonstrate that the material does not "act like" clay even though it is < 0.002 mm in diameter.  This test might even be something that you could suggest to the landscape architect as an important component in evaluating materials as the clay activity is more closely related to the physical properties of the material than is the amount of clay-sized particles in it.  
- or wash the clay-sized fines out of the rock fines before use (which is least desirable in my opinion as it creates a disposal issue and uses water).

Brian Wiebe
Winnipeg, MB


compost-request at mailman.cloudnet.com wrote:

Colleagues:

Can anyone recommend soil texture testing procedures other than the
Bouyoucos Hydrometer method?  A client of mine is making up manufactured
specialty soils from their compost plus rock fines from a nearby quarry.
Apparently, this texture analysis procedure is giving false readings on Clay
content in the manufactured soil sample due to the turbidity of the water
(the finest of the rock fines) in the hydrometer, which is causing the soil
mix to not meet the landscape architect's spec of no more than 5% clay
content.

Many thanks!

Craig