UNIT 3: Soil Particles

Objectives
1.  Identify the clay, sand and silt ratio for balanced tomato soil,
2.  Identify why soil particle imbalance affects plant calcium intake,
3.  Identify two methods for particle soil testing,
4.  Identify one solution for bringing soil into particle balance,
by correctly answering true/false and multiple choice items.

Humor
So you see this is a lesson with ratios, fractions and common denominators!  How many mathematicians does it take to screw in a light bulb?  Answer: Just one, once you've presented the problem in familiar terms.  Ha. Ha.  We too will do this in familiar terms.

Content
What is balanced particle soil? 

Physical (particle) soil structures include measurements of clay, sand and silt.  Particle balance is important for plant calcium uptake.  A combined ratio too high in clay may hold water and stress roots, contributing to BER.  A ratio that is too high in sand may allow too much water to drain away from the roots, carrying important nutrients away as it drains.  

Generally an ideal ratio for quality tomato soil is 20% clay, 40% silt and 40% sand.  Physical soil needs differs in many areas of the United States, for instance in coastal areas soil moisture rate differs from other US areas.  It is important to consult your state agriculture extension center to see if your area differs from the norm.

Content	Percent	Fraction	Ratio
Clay	20 %	1 / 5	1
Silt	40 %	2 / 5	2
Sand	40 %	2 / 5	2

• In studying the above chart for ideal physical soil content, the first column lists percents.  Notice that the sum of the percent column equals 100%. 
• If we were to write 20% as a fraction, it equals 1/5.  The 40% equals 2/5.  Notice the fraction column.  The sum of these fractions equal 5/5. 
• If we were to write these as a combined ratio, the ratio is one part (clay) to two parts (silt) to two parts (sand).  It is often written 1:2:2 or 1-2-2.
• If we were to stack these sections of soil in a mayonnaise jar, the clay would be half as tall as the silt and also half as tall as the sand.

Practice #1
1.  Name the three particles of physical soil.
2.  Name the ideal ratio.

How do unbalanced soil particles affect tomatoes?
A physical imbalance of clay, sand and silt prevent calcium uptake into the tomato plant.  Excess sand allows minerals to filter out.  Excess clay retains water and causes root rot.  Both contribute to low calcium uptake and to BER.

Practice #2
1.  Why is particle content of soil important to have in balance?
2.  What soil has a high clay content holds excess water, this contributes to ...?

How do I perform a particle test?

Particle soil testing can be done at home or professionally.  One common test is the particle soil ratio test, the 'Mayonnaise jar' method.  To obtain soil samples, dig to 8" to 12" deep. 

• Place three inches of dry, crushed garden soil in a tall clear jar.
• Fill jar with water.
• Shake jar well and let contents settle for three days.

Sand and silt will settle in five hours, but clay will remain suspended for up to three days.  After three days, study the content without disturbing the layers.  Sand will have settled to the bottom, silt will be in the middle and clay will surface to the top. (Above these layers will be the water.)  Measure the height of each layer. Remember the ratio should be two-parts sand, two-parts silt and one-part clay. 

The second method of physical soil testing is the professional method.  It is important to learn soil sample collection information from the testing site, and to follow their recommended soil sample gathering methods.  If you have several gardens, it is important to test each area.

Practice #3
1.  How long should you wait to read soil particle level?
2.  When gathering soil samples, you should dig to ___ inches  deep. 

How do I treat soil to get particle balance?

When clay, sand and silt are not in balance it is important to consult your state agriculture extension center as to the type and amount of additives needed to prevent BER.  Appropriate amounts differ per area of the country.

Generally:
With high clay soil add humus or gypsum.
With high sand soil add humus or peat moss.
With high silt soil had humus.

Assessment
On a separate sheet of paper, answer True or False, or select the appropriate letter (a, b, c...).  Then check your work using the link below.

T   F   1.   The particle soil ratio test, using a clear jar is a two hour test.
T   F   2.   The ideal particle ratio of tomato soil is 20% clay, 40% sand and 40% silt.

           3.   The following high level of particle often filters calcium from the soil.
                 a.  Sandy soil
                 b.  Clay soil
                 c.  Silt soil

Link to Answers

References

Proceed to Unit 4

Revised December 3, 2001
Copyright Anne Premont