SOIL TEXTURE – Refers to proportions of sand, silt and clay size particles. These proportions determine water infiltration rates, permeability rates, and water holding rates.

Us ing a so i l tex ture t r iang le . You will need to read the attached explanation on how to interpret the soil texture diagram and THEN answer the following questions, using your triangle and your newfound knowledge. EXAMPLE Po int A : Sandy Loam 65% Sand 20% S i l t 15% C lay Po int B : ___________ ____%Sand ____% S i l t ____% C lay Po int C : ___________ ____%Sand ____% S i l t ____% C lay Po int D : ___________ ____%Sand ____% S i l t ____% C lay Po int E : ___________ ____%Sand ____% S i l t ____% C lay

Now w i th that pract ice , cont inue naming the fo l low ing so i ls . % Sand % S i l t %Clay Textura l C lass/ So i l Name 1. 33 33 34 2. 55 30 15 3. 80 5 15 4. 25 60 15 5. 30 20 50 6. 60 10 30 7. 40 40 20

A

B

C D

E

Reading  a  Soil  Triangle  

Using  the  soil  texture  triangle,  scientists  have  created  classes  which  break  the  distribution  of  particle  sizes  (soil  textures)  into  12  categories:  clay,  sandy  clay,  silty  clay,  sandy  clay  loam,  clay  loam,  silty  clay  loam,  sand,  loamy  sand,  sandy  loam,  loam,  silt  loam,  silt.    

The  soil  texture  triangle  is  one  of  the  tools  that  soil  scientists  use  to  visualize  and  understand  the  meaning  of  soil  texture  names.  The  textural  triangle  is  a  diagram  which  shows  how  each  of  these  12  textures  is  classified  based  on  the  percent  of  sand,  silt,  and  clay  in  each.  Note:  these  percentages  are  based  on  the  USDA  definition  of  sand  and  silt  only.    

Follow  these  steps  to  determine  the  textural  class  name  of  your  soil  sample:    

1. Determine  the  percent  sand  of  your  sample  and  find  that  number  on  the  bottom  of  the  triangle.  Note  that  the  numbers  read  from  right  to  left,  not  left  to  right.  For  example,  if  your  sample  is  65%  sand,  then  you  need  to  pick  a  point  to  the  LEFT  of  the  60  mark.    

2. Draw  another  line  to  correspond  to  the  percent  clay.  Let’s  say  you  had  27%  clay.    

 

 

3. Where  the  lines  intersect  should  also  indicate  percent  silt.  On  the  graph  above,  you  can  see  that  it  is  about  8%  silt.    4. Wherever  your  lines  intersect  indicates  the  soil  type  you  have.  In  this  situation,  with  65%  sand,  27%  clay,  and  8%  silt,  it  is  sandy  clay  loam.    

F ind ing so i l tex ture by f ragmentat ion . This design is based on the principle that larger particles having a greater mass will settle out first. Sand (the largest particle) should settle out faster in a suspension than silt or clay. We can use this technique, called fragmentation, to find the general composition of our soil samples. Sand will usually settle in about 1 minute, silt settles in about 10 minutes, and any remaining particles still suspended after 10 minutes are clay. Organic material may float.

1. 25 mL of soil, added with water up to 75mL mark, was shaken vigorously. The mixture was then allowed to settle over time. 2. After 1 minute, the sand settled to the bottom of the cylinder. 3. After 10 minutes, the silt particles had settled on top of the sand layer. 4. Identify the three layers in the graduated cylinder. 5. Determine the volume of each layer (record):

a. TOTAL VOLUME = 25 mL b. Layer A (Sand) Volume = c. Layer B (Silt) Volume = d. Layer C (Clay) Volume =

6. It takes a long time for clay to settle out. If it hasn’t complete settled, subtract your sand and silt measurement from 25mL to determine your clay volume.

7. Draw, label, and describe a diagram of the particles in the cylinder. 8. Why did the particles settle out this way? 9. Calculate the percentage of each component using the following equation:

Amount o f each component X 100 = % Component Tota l Vo lume o f so i l

10. NOW identify the type of soil in your sample, using your % combinations of Particles and the soil triangle.

SOIL DENSITY. Soils that are very dense have high strength, low porosity, and poorly support plant growth. Soils can become compacted in many ways (overgrazing livestock, high traffic). We will now calculate the density of your soil sample.

PROCEDURE: 1. Mass 15 grams of your soil sample using a coffee filter. 2. Transfer 15 grams of soil into a DRY 100 mL graduated cylinder. Tap gently on the sides to settle the particles on the

bottom. 3. RECORD the volume of the soil in the cylinder. VOLUME = ______mL 4. Pour the sample from the cylinder onto a dry paper towel. 5. Calculate the bulk density and record your answer in the chart below. 6. Repeat with second soil sample.

Sample 1 Mass (g) Volume (mL) Bulk Density (g/mL) Sample 2 Mass (g) Volume (mL) Bulk Density (g/mL) ANALYSIS:

1. Which sample had the greater density?

2. Of the two, which would have the greater porosity?

3. Which sample has the greatest water holding capacity?

4. What is the relationship between texture and porosity?

SOIL POROSITY: The space between the particles is called the pore space. It determines the amount of water that a given volume of soil can hold. Porosity is the percentage of the total volume of soil that consists of pore space. PROCEDURE#1

1. Fill beaker A to the 200 mL mark with dry soil or sand. Place it on a table or flat workspace. 2. Fill beaker B with 200 mL of tap water. 3. SLOWLY pour the water from the beaker B into the soil sample in beaker A. STOP pouring when the water level reaches the top of the soil. The soil has now reached

saturation and cannot hold any more water. 4. How much water is now left in beaker B? _________mL 5. How much water is now held in the pore spaces of the soil sample in beaker A? ______mL 6. Use these numbers to calculate the porosity of the soil sample. Poros i ty = Vol o f water added to the so i l (mL) X 100

In i t ia l vo lume of so i l (mL)

PROCEDURE#2 1. Place enough soil in a coffee filter to fill it about ¾ full. 2. Find the mass and record this data on the Data chart below. 3. Put the soil filled coffee filter into a plastic funnel. Place the funnel on top of a large beaker. 4. POUR in 400 ml of tap water and let the water drain through the soil into the beaker. 5. POUR the water collected in the bottom beaker into a graduated cylinder and measure the volume. RECORD this value below. 6. Compute the amount of water being stored in the pore space of the soil sample. RECORD this value. 7. Place the saturated soil filled filter back on the balance to find the mass. RECORD this value. 8. Compute the mass of the water in the soil. RECORD THIS VALUE. 9. What is the percentage of water in the soil sample? RECORD this value.

Sample Number or

Name

X Soil Sample Mass (g)

A Water added

(ml)

B Water

recollected (ml)

A-B Water

Absorbed (mL)

Y Saturated Soil

Mass (g)

Z =Y-X Water Mass in

Soil (g)

(Z/Y)*100 Percent water

in soil

SOIL PERMEABIL ITY : Soil permeability is the property of the soil pore system that allows fluid to flow. It is generally the pore sizes and connectivity—how well those pore spaces allow water to flow through the soil—that determines whether a soil has high or low permeability. Procedure : 1. Take one of the plastic cups and poke three holes in the bottom. 2. Fill that cup halfway with one of the soil samples. Pack it down lightly to avoid having large air spaces. 3. Have one of the lab partners hold the cup with the soil above an empty cup with no holes so that they can catch the water as it drips through the soil. 4. The other lab partner should measure out 50mL of water and slowly pour it on top of the soil. 5. Time for 1 minute and then set the cup with the soil aside. (Be careful where you place the cup. It will continue to drip water, so put it in a sink or over a cup or beaker.) 6. Pour the water from the second cup into the graduated cylinder to see how much of the 50mL came through. 7. Record the amount in the following chart; repeat the procedure for the remaining two soil samples.

Data :

So i l Sample 1 So i l Sample 2 So i l Sample 3

Vo lume of Water (mls)

Ana l ys is :

1. Why would a farmer need to know the soil permeability of his land? Why is this important? 2. Which of the soil samples held more water and why? 3. Which soil sample has the best permeability? 4. Which soil sample would have the greatest porosity? Explain your reasoning.

SOIL HORIZONS

• Soil Profile - A vertical section of the soil extending vertically through all its horizons and into the parent material.

• Soil Horizon - A layer of soil, approximately parallel to the surface, with properties that differ from the horizons above or below it – the properties (characteristics) are produced by soil forming processes.

• Soil Layer - A layer in the soil deposited by a geologic force (wind, water, glaciers, oceans, etc.) and not relating to soil forming process.

There are 3 primary soil horizons called master horizons. They are A, B, and C. These are part of a system for naming soil horizons in which each layer is identified by a code: O, A, E, B, C, and R. They will be discussed as follows:

1. “O” horizon. This is an organic layer made up of partially decayed plant and animal debris. It generally occurs in undisturbed soil such as in a forest.

2. “A” horizon. This is often referred to as topso i l and is the surface layer where organic matter accumulates. Over time, this layer loses clay, iron, and other materials due to leaching. This is called e luv ia t ion . The A horizon provides the best environment for the growth of plant roots, microorganisms, and other life.

3. “E” horizon. This is the zone of greatest eluviation. Because the clay, chemicals, and organic matter are very leached, the color of the E horizon is very light. It usually occurs in sandy forest soils with high amounts of rainfall.

4. “B” horizon. This horizon is referred to as the subso i l . It is often called the “zone of accumulation” since chemicals leached from the A and E horizons accumulate here. This accumulation is called i l luv ia t ion . The B horizon will have less organic matter and more clay than the A horizon. Together, the A, E, and B horizons are known as the so lum . This is where most of the plant roots grow.

5. “C” horizon. This horizon is referred to as the substratum . It lacks the properties of the A and B horizons since it is influenced less by the soil forming processes. It is usually the parent material of the soil.

6. “R” horizon. This is the underlying bedrock, such as limestone, sandstone, or granite. It is found beneath the C horizon.

Answer the fo l lowing quest ions:

1. Draw a soil profile and label each of the three major horizons.

2. Explain how each horizon is different from one another in terms of color and content.

3. Explain how “losses” might occur that will cause a change in the soil profile.

 

 

 

 

 

 

Using  the  table  below,  determine  the  properties  of  each  of  the  soil  samples  analyzed  above.    

Soil  texture  

Nutrient-‐holding  capacity  

Water-‐  infiltration  capacity  

Water-‐holding  capacity   Aeration   Workability  

Clay   Good   Poor   Good   Poor   Poor  Silt   Medium   Medium   Medium   Medium   Medium  Sand   Poor   Good   Poor   Good   Good  Loam   Medium   Medium   Medium   Medium   Medium  

Using  the  table  below,  determine  the  properties  of  each  of  the  soil  samples  analyzed  above.    

Soil  texture  

Nutrient-‐holding  capacity  

Water-‐  infiltration  capacity  

Water-‐holding  capacity   Aeration   Workability  

Clay   Good   Poor   Good   Poor   Poor  Silt   Medium   Medium   Medium   Medium   Medium  Sand   Poor   Good   Poor   Good   Good  Loam   Medium   Medium   Medium   Medium   Medium