SOIL WATER CONTENT LAB
(adapted from Laboratory Manual for Soil Science by Thien and Graveel)
water retention curve
Soil water status can be reported on either an energy or content basis. The four benchmarks described in the Figure below (field capacity, wilting point, air-dry (hygroscopic coefficient, and oven-dry) all define the energy status of soil water. A satuated soil has water filling all of the soil's pore volume. When a soil is retaining all the water it can against the pull of gravity, it is at field capacity. When the water film on soil particles is so thin that the energy exerted by plants fails to extract water in sufficient quantities to prevent wilting, the soil is at the wilting point. When evaporative forces have removed water down to a layer held with 31 bars of tension, the soil is at the hygroscopic coefficient (air dry condition). Heat energy of 10,000 bars is required to remove the most tightly bound water molecules and create an oven dry soil that will not lose additional water with further heating.
Expressing Soil Water Content:
Percent soil water by weight is calculated by weight loss during oven drying.
Percent soil water by weight (%w) = wet weight
- oven dry weight x 100
oven dry weight
Percent water by weight can be converted to water content expressed as percent water by volume if the soil's bulk density is known. This conversion can be stated as:
Percent soil water by volume (%v) = (%w) (Bulk density)
Equivalent surface depth describes soil water content by how deeply it would cover the sol if removed and set on top of the sample (i.e., how many cm of rain or irrigation would be needed to replace this amount of water).
Equivalent surface depth = (%v) (sample thickness in cm)
For water management, the available water fraction is important. Irrigation decisions are based on knowing the portion of the available water that has been depleted (available water depleted %). A rule of thumb suggests that irrigation should begin when 50% of the avialble water has been depleted. This value can be calculated as follows:
Available water depleted, % = field capacity
(%w) - current water (%w) x 100
field capacity (%w) - wilting point (%w)
This available water depleted value suggests when water replenishment should begin; it does not provide information about how much water should be added. That value can be determined by calculating the field capacity deficit, as follows:
Field capacity deficit, cm = (Field capacity,
%w - Current water content,%w) (sample thickness, cm)
Effect of Texture on Soil Water Content
Soil texture affects water content through its influence on binding
sites and storage volume. Water binds to the soil particle surfaces,
so those soils with the largest
surface area per unit volume have the greatest potential for storing water. Surface area is directly proportional to clay or organic matter content (colloids). The water-holding ability of sandy soils can be increased by adding residues that raise organic matter content.
Soil texture also has an impact on water content through its influence
on storage volume as characterized by aggregation and soil porosity. Water
enters soil through surface pores, moves through internal pores, and, once
bound to soil particles, resides in soil pores. The samllest pores within
a soil exert the greatest tension on water and so would fill first and
empty last. At field capacity, all but the largest soil pores are flled
with water. Loss of aggregation and compaction can dimnish a soil's pore
volume, and as a consequence, reduce infiltration , volumetric water storage
capacity, water movement and alter patterns of water distribution.
PART I. WATER CONTENT AND DISTRIBUTION IN A SOIL PROFILE.
Several physical and hydrological properties can be determined from
collecting a soil sample's volume, moist weight, and dry weight. This exercise
illustrates data collection and calculation of percent water by weight,
percent water by volume, bulk density, total pore space, and the equivalent
depth of water contained in a sample.
Determining Water Content in a Soil Profile.
1. Extract a core sample from a soil using a hand- or power-operated sampling tube. Wrap the core in plastic to prevent water loss between sampling and weighing.
2. In the lab, label sufficient covered containers to accommodate 3-cm segments of the profile (0-3 cm, 3-6 cm, 6-9 cm, etc.). Weigh and record the empty weight of each container and lid.
3. Unwrap the core, measure and record its diameter, and section it into 3 cm segments. Place each segment into the appropriately labeled container and replace the lid to prevent any water loss. Weigh and record the container and wet soil weight.
4. Crumble the soil for faster drying and place the open container into either a microwave or hot-air oven (105 C). Remove samples at suggested intervals, close the container, and weigh. Continue drying and weighing until a constant weight is attained. At that point the soil sample is losing no more water and is considered to be oven dry. Record the weight of the container and dry soil.
5. Before discarding your samples, determine the texture by feel for each sample.
6. Calculate the information requested below:
|Soil Profile Segment , cm|
|Diameter of core|
|Volume of core, cm3 (V=pr2h)|
|Weight of container and lid,g|
|Weight of wet soil, container and lid,g|
|Weight of dry soil and container and lid,g|
|Weight of water in soil,g|
|Oven dry weight of soil,g|
|Percent water by weight, %w|
|Bulk density, g cm-3|
|Percent water by volume, %|
|Equivalent surface depth,cm|
|Texture (by feel)|
PART II. EFFECT OF SOIL TEXTURE ON WATER CONTENT
1. Prepare soils with different textures by air-drying and sieving through a 2 mm. screeen.
2. Determine the texture by feel of the soil.
3. Prepare wetting columns from plastic pipe (15 cm long). secure screen over one end of column with a rubber band. Suspend each tube over a drainage pan. Label each column for the soil textures being used.
4. Put an equal amount of soil into each column.
5. Add water to each soil column until water stops dripping from the bottom of the column.
6. When water stops dripping from a column, the soil is cosidered to be at saturation. Remove the soil from each column onto a non-adsorbent surface and mix the soil.
7. Put approximately half of the soil from each column into each of two previously weighed beakers, one labeled for air-drying and one labeled for oven-drying. Immediately rewiegh the beaker (with soil) for oven-drying before appreciable water loss can occur and record the data. Set aside other beaker for air-drying.
8. Oven dry the oven-dry sample for 24 hours at 105oC. Air dry the other sample at room temperature for 4 - 7 days. (Soil should be stirred several times during this interval to facilitate drying).
9. Weigh both samples after oven and air drying.
10. For the air-dry sample, oven-dry it after it has air-dried and has been weighed.
11. Determine the water content of oven-dried soil at saturation. (Saturation does not equal field capacity under these conditions because the tension in this short column does not equal the 333 cm column of water needed to reach 0.3 bar tension).
12. Determine the water content of air-dry soil to approximate the hygroscopic coefficient.
Enter data into a table as follows:
Saturation Determination: The Oven-Dry Soil
|Soil1||Soil 2||Soil 3||Soil 4||Soil 5||Soil 6|
|weight of saturated soil + beaker,g|
|weight of beaker,g|
|weight of oven-dry soil + beaker,g|
|weight of water in saturated soil,g|
|oven-dry weight of soil,g|
|% water by weight at saturation|
Hygroscopic Coefficient Determination: The Air-Dry Soil
|Soil 1||Soil 2||Soil 3||Soil 4||Soil 5||Soil 6|
|weight of air-dry soil + beaker,g|
|weight of beaker,g|
|weight of oven-dry soil + beaker,g|
|weight of water in soil at air-dry condition,g|
|oven-dry weight of soil,g|
|% water at air-dry, %w|
PART III. SOIL WATER CALCULATIONS.
1. Use the equations in this exercise and fill in the data
Use the information in Columns 1-6 to calculate soil water values
in Columns 7-12.
|Sample volume,cm3||Wilting point,%w||Field capacity,%w||Present
|Present water content,%v||Equivalent surface depth,cm||Accumulated surface depth,cm||Available water depleted,%||Field capacity deficit,cm|
1. How does a saturated soil differ from a soil at field capacity?
2. Explain the relationship between soil texture and water content at field capacity. Did your results from lab support this relationship? If not, why not?
3. A 400 cm3 sample from the top 25 cm of a soil weighed 686 g when collected and 576 g when oven dry. This soil contains 34%w water at field capacity. What is the bulk density? What is the percent water? What is the field capacity deficit of this soil zone?
4. How does air-dry soil differ from oven-dry soil?
5. Describe the relationship between soil texture and plant-available water. Did your results from lab support this description? If not, why not?
6. How could the plant available water capacity of a soil be increased?