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### Teacher Notes On Soil Permeability

Objective: Evaluate effectiveness of soils to hold water.

Materials:

The Soil fractions from last lab.
Funnels
Cheap paper towels as filters
100 ml graduated cylinders.
250 ml beakers to collect dripped water.
Ring stand to hold funnels

This is the lab where they get to see the important effect of particle size played out before their eyes. Typically, the sand just passes the water through, while the finer particles won't even pass water.

An alternate to this is a demonstration using two glass tubes with cloth rubber banded onto the bottoms filled with sand and clay. With a ring stand clamp, dunk the cloth bottoms in a pan of water and watch the level that the water rises. The sand goes quickly but it doesn't go far, and the clay goes slowly but it eventually reaches the top. Then remove the water from the bottom and see which one dries out more quickly. The sand is bone dry in a day while the clay is still wet for a week or more.

See A-5 for assessment.

Statement of Investigative Question: What fraction of the soil holds the most water?

What I already know: Different soils hold different amounts of water.

Experimenting and collecting data:

Take 50 ml of each soil and add 25 ml of water to it to check its permeability.

Materials:

*Four Soil Fractions from last Activity
*Ring Stand
*Water
*Gravel Funnels
*Four Hole Stand Clamp
*Four 100 ml beakers
*Paper towels

 Set up your lab table to look like this: Line the funnels with paper towel shaped like a cone. Wait at least 10 minutes or until the water stops dripping through.

 Name Amount of Water Added Amount of Water Passed Through Water Retained Rock 25ml _ = Gravel 25ml _ = Sand 25ml _ = Silt 25ml _ = Clay 25ml _ =

Conclusion:

The soil fraction that could hold the most water was ____________________________

The second most water retained was the _____________________________________

The third most water retained was the _______________________________________

The fourth most water retained was the ______________________________________

### Teacher Notes for upper level students

This lesson should be reserved for the upper level students. This is the officially recognized particle size analysis method. As you will notice, the values that are obtained are far different than the sieve method simply because the sieves don't actually get down to silt and clay sizes.

Background Teacher Information:

Soil texture is the most important factor for nutrient and moisture availability. It is also a key factor in soil aeration which is a limiting factor for many desert plants.

I. Surface Area.

A. For a given weight of sample, the smaller the particle, the greater the surface area.

B. Clays: Increased Surface Area

1. Chemical reactions: Clays exchange ions -Sands do not.

2. Water Retention: Clays have a very strong hold of water.

a. This means subsurface clays can be moist even after long droughts.

b. Only strong suction from a plant's roots can separate the water from clays. [(Mesquites) they also have mycorhizae (fungus) associated with them that increases the root surface are by thousands of times and also helps suck the water off of the clays as well as add nitrogen to the soil]

3. Decrease aeration of roots. It is very important for some roots to get a good supply of air. Aeration is a limiting factor for the ubiquitous creosote. Mesquite's tap root doesn't care about air.

C. Sands: Increased Pore Size

1. Greater water flow rate: Sands pass water quickly- many plants with a surficial roots uptake water quickly also.

2. Greater evaporation: Sands do not hold onto the water. The plants with surficial roots also retain the water in their tissues (cacti and succulents)

3. Increased aeration: Creosotes and paloverdes are sensitive to soil air. Sands provide plenty of air.

Soil lichen are usually found on sandy loam. They hold onto the moisture longer than the surrounding soils because of their gelatinous bodies, but they also have a shallow rhizoid and hyphae (less than a centimeter). They need the good aeration of sandy soils because many can start rehydrating in high humidity.