Soil Composition Analysis Soil Settlement Tests (Jar Tests) versus Drainage Estimates | |
Garden Tip TB1468 |
Technical Bulletin Series
|
The Importance and characteristics of Soil!
All soil is generally comprised of particles that
fall into three groups. The smallest is clay,
then silt and the largest is sand. Clay compared
to sand is as small as a baseball is to the White
House. Clay tends to drain very slowly while sand
drains too fast. Good "soil" has a mixture of these
that will allow for adequate oxygen throughout,
but also retain water long enough to prevent plant
drought. This is why "silt" is so nice, as a medium
sized particle it has the best of both worlds. 40 %
sand, 40 % silt and 20% clay makes a nice loamy soil.
|
Soil Drainage Tests
The most common method for evaluating soil composition is a "Drainage Test".
Usually this is performed in about an hours time and gives a rough estimation
of how fast the soil drains.
Problem here is that, if the soil has not been fully saturated for a period of at least 24 hours, the likelyhood for accuracy drops as much as 1000%. When looking at the Soil Triangle below, you will notice the 12 basic categories of soil composition. When comparing these with the Hydraulic Conductivity Triangle as published by the USDA - you will see the within the same group, soil moisture movement rate will vary as much as 1000%. By performing a "Drain Test" on your soil you can get a ballpark figure for your soil's Hydraulic Conductivity Rate [HCR], but really be in-the dark as regards how to optimize moisture depth for the soil you irrigate. Additionally, Drainage tests take a minimum of an hour and up to 24 hours of your valuable time. Jar Tests are actually "soil settlement tests" and are much more accurate than filling a hole with water. These can be completed in as little as 5 to 10 minutes.
|
|
The visual striations are not always distinct, like drawings and photos usually indicate.
This is the reason for utilizing "TIME" to determine the marking for separation
of larger and smaller soil particles.
Measuring based on TIME of SETTLING, uses the inclination of larger particles to settle out, while smaller ones are still afloat. After shaking, quickly set the jar on a table so you can Mark settling points. First measurement is at ONLY 6 seconds, so have the Marker ready to go. Most sand will settle (6 seconds) to the bottom. The silt (in 20 more seconds) and clay will settle (in 5 minutes or more) on top of that. You will Mark then later Measure the depth point for each and calculate the percentage. In this example we have approximately 7% clay, 20% silt and 73% sand. This is a "sandy loam" and will exhibit excellent mostly rapid drainage. Not a common soil here. |
At 6 seconds: Use a MARKER and MARK THIS POINT! This represents the amount of SAND in the sample. (in this example 5.5 inches) Now let settle for only 20 additional seconds |
At 26 seconds from start: Again use the MARKER and "MARK THIS POINT!" this represents the amount of SILT + SAND in the sample. (in this example 7.0 inches) Now let settle for 5 minutes (minimum) from start! At 5 minutes: Again use the MARKER and "MARK THIS POINT!" this represents the amount TOTAL SEDIMENT in the sample. You may want to "re-shake" and measure again. First measure your original Marks. Make new marks, and compare with your first measurement as it is not always clear where the sediment lines actually are. |