Richard R. MacMahon, Ph.D.
For this laboratory you will work in groups and will submit one laboratory report for each group. The format for the lab report is a short hand-out which will be provided. Part of your lab report should be the data sheet that is page three of this paper.
When we discuss soils, we are talking about the abiotic factor that furnished water and minerals to plant roots. Soil type is often the limiting factor in the distribution of plants. How does soil structure influence the availability of water and minerals? The soil has a texture that results from the combination of the
organic matter
in the soil with the
mineral material
present. The organic material results from the
decomposition of plant material
in the soil. The mineral materials are based on the
size of the soil particles
. The finest particles are called clay, the next finest silt, then sand and finally gravel, which is the coarsest. The various combinations of these substances give rise to the various kinds of soils.
How does a soil absorb and hold water? When it rains, the spaces between the soil particles are filled with water, as shown in Fig. 1 below.
Figure 1 Ð Soil saturated with rain water.
(figure available in print form)
All of the dark areas represent water between the soil particles, which are the light areas. In this condition, there is no air present from which the roots may obtain oxygen. After the excess water (called the
gravitational water
) has drained off, what is left is a shell of water around each soil particle. The soil at this point is said to be at
field capacity
. The plant roots absorb this water for plant use. The water is held to the soil particles by hydrogen bonding. See Fig. 2 which shows the soil at field capacity.
Figure 2 Ð Soil at Field Capacity
(figure available in print form)
As the water is absorbed by the roots, the shell of water around each soil particle becomes thinner. And finally, when almost all of the water has been absorbed by the roots, the force needed to remove the water from the surface of the soil particles becomes so great that the roots can no longer absorb the water (Fig. 3).
Figure 3. Ð Soil depleted of water available to plants.
(figure available in print form)
What we will do today is start to measure how much water our soil samples will hold. We will do this by first
wetting
a sample of soil. When the gravitational water has run off we will
weigh the sampl
e and its container. We will then leave the soil to
air-dry over night
. The air-drying approximates the amount of water removed from the soil by the roots. The second day we will
weigh the sample
and then place it
in an oven
. This will remove the remaining water from the soil. The soil will remain in the oven overnight.
The last day we will take the oven-dried soil,
weigh it
, and
heat it
in a crucible to
get rid of
all the
organic material
in the soil. This must be done with caution. Be sure you
wear goggles
and stand well back when you are heating the soil. It will have to be stirred while you heat it. Let the soil
cool thoroughly
and then
weigh it
one last time. This will give you the amount of mineral material in the soil. We will, of course, record these findings (see the attached data and calculation sheet).
The organic material in the soil is important in helping plants absorb minerals. This humus gives the soil a “crumbly” texture and ensures the soil is open and well aerated. A soil without organic material is generally poor and very hard.
Data Sheet
Weight of wetted soil in container:
|
SOIL WEIGHTS:
|
Weight of air-dried soil in container:
|
Wetted Soil:
|
Weight of oven-dried soil in container:
|
Air-dried Soil:
|
Weight of container:
|
Oven-dried Soil:
|
Weight of wetted soil:
|
Heated Soil:
|
Weight of air-dried soil:
Weight of oven-dried soil:
Weight of oven-dried soil in crucible:
Weight of heated soil in crucible:
Weight of crucible:
Weight of heated soil:
Calculations
How much water is available to plants? ___________ ml/gram of soil
How much water is present at field capacity? ___________ ml/gram of soil
How much water is not available to plants? ___________ ml/gram of soil
What is the percentage of organic material in the soil? _____________ %