Exercise 2: Soil Water Movement

Soil water movement and related characteristics are important properties of soil for plant growth. Soil structure, including the size and arrangement of pores, greatly affects water movement into (infiltration) and through (permeability) the soil profile. We will consider some fundamental aspects of soil water movement in this exercise, and we will be especially concerned with ways in which soil water movement can be influenced by management practices such as compaction and the presence or absence of a protective surface cover.

The permeability rate is the rate that water can move downward through a soil profile and the infiltration rate is the rate that water enters soil, which really is a special case of permeability of the soil surface. Both permeability and infiltration rates affect the rates of soil water movement through a profile. Various layers in the soil profile have their own permeabilities, and these may differ from one soil to another. Usually the layer with the lowest permeability will control the passage of water and, therefore, have the greatest significance for soil water movement, especially when the limiting layer is at or near the soil surface. A limiting layer at a greater depth within the soil profile can cause water to back up and create a saturated zone above it. Saturated zones that exist for extended periods cause reducing conditions that have important effects on soil chemistry, soil biology, and soil fertility.

Soil water movement is influenced by such factors as soil aggregation, percentage pore space, the pore-size distribution (especially the size, orientation, and continuity of the largest pores), and the presence or absence of a surface crust and/or compacted zones within the profile. The amount and size distribution of pore space in a tilled soil can vary considerably during the year. A soil that has recently been tilled contains many sizable cavities through which water can pass easily. These cavities are unstable and collapse as the days and weeks pass. Disking, harrowing, rototilling, or other means of tilling the soil breaks the clods into smaller pieces and reduces the size of the pores between them. Gradually the soil becomes denser until it stabilizes, usually with less pore space than it had in the virgin state.

A surface crust is a common phenomenon that is more serious than many realize. Extreme cases of crusting can cause greatly increased runoff and may interfere with the emergence of germinating plants (legumes such as soybeans that must push large cotyledons up through the soil are especially susceptible to emergence problems). The direct cause of crusting is usually pounding by raindrops, resulting in destroyed or damaged surface soil structure. Water standing on the soil surface can also be a factor in decreasing the strength of surface structural peds. Furthermore, a thick crust with low permeability can be produced by either animal or vehicular traffic, especially when the soil is wet. The ability of a soil to resist crusting is influenced by the amount and type of clay and organic matter it contains. Soil microbes actively decomposing fresh organic materials produce gums and exudates that are very helpful for stabilizing soil structure and in giving soil more strength to resist crusting.

Procedure

The procedure that follows is designed to measure the effects of various soil treatments on soil water movement. Movement will be quantified by calculating the constant for Darcy’s Law for each of five soil treatments. A comparison of the results should enable you to answer these questions: 1) how much influence do the fine particles have on soil water movement, 2) how important is it to protect the soil surface from the dispersing action of raindrops and flowing water, and 3) what is the effect on soil water movement of compacting either the soil surface or the entire soil column?

1. Place a rubber stopper, with a drain tube in one end, into each of five percolation tubes. Add an approximate 1-cm layer of vermiculite over the stopper inside each percolation tube. Place the tubes in a wooden rack that holds them upright.

The vermiculite will serve as a support for the soil column and to filter the soil out of the water to minimize plugging of the drain tube.

2. Using a 10-mesh sieve, remove the fine material from at least 240 g of the soil (< 2 mm). (Keep at least 160 g of unsieved soil that still contains its fine material).

Loose, fine particles plug soil pores and thereby make a big difference in the movement of water. Testing the soil with and without these fine materials present will show the magnitude of this difference.

3. After adding the vermiculite, fill percolation tubes with 15 cm of soil: three replicates with dry, sieved soil and two replicates with dry, unsieved soil. Label the tubes as follows:
   1. Sieved, compacted by dropping, covered with paper.
   2. Sieved, surface packed, covered with paper.
   3. Sieved, unpacked, covered with paper.
   4. Unsieved, unpacked, covered with paper.
   5. Unsieved, unpacked, uncovered.

We will use a 15-cm height of air-dry soil as a standard amount for measuring soil water movement.

4. Mark the soil height in tubes ‘a’ and ‘b.’ Compact the sieved soil in percolation tube ‘a’ by dropping the tube onto a solid support 5 times from a 2.5-cm height. Measure the reduction in column length. Compact the soil in percolation tube ‘b’ a similar amount by adding water to the surface to moisten the entire soil column, then press down on the surface with a wooden dowel.

Compacting the soil reduces the amount of pore space, especially the larger pores, and thereby reduces water movement. The dropping procedure should compact the soil throughout the column, whereas ­pressure from the dowel should compact mostly the upper part of the soil column.

5. Cover the surface of the soil with a piece of wadded-up paper towel in four of the percolation tubes (all except one of the unsieved soils).

Pouring water directly on the soil surface is likely to puddle it and form a crust much like that formed in the field by pounding raindrops.

6. Mark the top of each percolation tube at a point 10 cm above the soil surface and fill the tubes with water to that level. Note the starting time. Place 250-mL beakers under the tubes to catch the water.

The 10-cm line will give a consistent amount of water in each tube and a consistent water pressure (head) above the soil at the beginning of the test period. The head will decrease with time as the water level drops.

7. Watch the columns and determine how long it takes for water to begin dripping from the drain of each tube. Note the time and refill each individual tube to its 10-cm line when that happens and record the water column start length (total height from the upper water surface to the bottom of the drain tube).

Water movement measurements begin when water fills the whole column and begins dripping out of the drain tube.

8. Determine the rate that the water level drops in each tube as either 1) the time it takes for the water level to drop 10 cm (if the flow is fast), or 2) the distance the water level drops in 30 min (if the flow is slow). Record the water column end length.

The two different techniques are needed because some tubes may have much higher permeabilities than others.

9. Calculate the permeability constant (k) for Darcy’s Law, v = k h/l, in which v = observed rate of water level drop in cm/hr, h = the average water head (cm) from the water surface to the bottom of the soil column, and l = the length of the soil column (cm). We will use a value of h that is the average of its starting and ending values; this result is slightly smaller than the theoretically exact value obtained by using logarithms:

k = ln (h₁/h₂) × I/(t₂-t₁) or k = 2.303 log(h₁/h₂) × I/(t₂-t₁)

Darcy’s Law describes the flow of a fluid through a porous medium. The permeability constant can be calculated by transforming Darcy’s Law into k = v l/h so that k can be calculated readily. The value of k is equal to the rate of flow in cm/hr if the soil column and the water column are both the same length.

Name____________________

Date_____________________

Section__________________

Do the treatments affect the length of time for each tube to begin dripping? Explain. How much influence do the fine particles have on soil water movement? How important is it to protect the soil surface from the dispersing action of raindrops and flowing water? What is the effect on soil water movement of compacting either the soil surface or the entire soil column?

Notes or comments: