Soil Composition Analysis
Drainage Estimates
versus
Jar Tests
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 an approximation 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 arrive at an estimate of the type of soil composition that you have, but really be off-base 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 much more accurate, and can be completed in as little as 5 to 10 minutes.

For information on how to perform Drain tests:
Quick Drain (about an hour)
and the 24 hour Drain.
By clicking you will be directed to a page for instructions.

The "I&S-M Pro-I is designed for accuracy and efficient use of water, as well as plant health. Because of this the input data will require the Sand % and Clay % in order to function.


A Jar Test
 Instructions follow

Put enough dirt from your planting area in a quart size jar so that the level of the jar is about 2/3 full. Then add water until nearly full. Measuring begins in only 6 seconds. Use a Magic Marker (writes on glass) to indicate settlement points.

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.

Organic matter (like compost) will mostly float. The percent of compost can't be measured with the jar test. It can however, be measured by a soil laboratory. A bit complicated and expensive. Unless you have added it, organic material is nearly non-existent in our desert soil.

Now simply use the "Soil Triangle to find your Soil Type.

In order to use the Soil Triangle for determining which classification of soil you have;
You'll need to convert the above measurements in to percentage. Conversion formulas:

for Sand: Sand (inches) divided by Total (inches) = % Sand
for Silt: [Silt + Sand (inches) minus Sand (inches) ] divided by Total (inches) = % Silt
for Clay: [Total (inches) - Silt + Sand (inches)] divided by Total (inches) = % Clay

If this seems a bit difficult, confusing or complex;
You may want to utilize
Planting Guru's
Jar Test Analyser

To use our ON-LINE Soil Composition Analyser to easily convert
your jar test measurements to percentages; click here:

Soil characteristics directly affect proper watering and fertilization.
In fact; without knowledge of the type of soil being watered
IT IS NOT POSSIBLE to provide accurate recommendations for watering.
Any watering schedule offered without knowledge of your soil type is really just guessing!