By Francis Belohlavy & Steve Hartung
Introduction:
This is an activity to demonstrate water erosion on soil surfaces and the beneficial effect of conservation practices. Specifically, it can show: that crop residue left on the soil surface slows erosion as compared to erosion on bare soil; how using conservation terraces can slow the speed of water running off the soil surface and thus allowing more water to soak into the soil and slowing erosion; how farming on the contour can slow water runoff speed because each crop row works as a miniature terrace; and how combinations of these conservation practices can significantly reduce the amount of topsoil lost each year.
Needs:
- One 5 quart pail of topsoil and one of sand for each student or small group (2-3). Preferably the topsoil is a loam, silt loam, clay loam, or silty clay loam texture. In other words, a soil with a "nice" feel that one might want in ones garden as compared to something that is "heavy" with clay and hard to work or something that is very sandy.
- Shallow containers. These are boxes, food service trays, pop flats or whatever to put the soil or sand into for the exercise. Food service trays have been found to work well, but the soil (especially the sand) tends to slide off after it becomes saturated. Generally the containers are about 1 foot wide by 1.5 feet long and « to several inches thick. The containers may be larger if desired. One may construct wooden boxes that can be reused. Cardboard boxes would probably be destroyed by the wetting.
- A bag or box of grass clippings. This material is put on the soil surface to demonstrate the effect of crop residue.
- A source of "rain", such as a sprinkler can or a hose with a fine mist setting. Water needs to be applied to the finished "soil surface" in a manner that is akin to a light-to-moderate rain and is that not so heavy or so forceful as to immediately wash the soil away.
- A place such as the out of doors to do the exercise. Since this exercise calls for the mixture of students, soil, and water; it is messy.
- Grass seed or sod may be needed if a growing crop demonstration is to be used.
The exercise itself may be begun in a number of ways and is not meant to be the only way to conduct the exercise. The instructor may want to prepare several examples of soil surfaces and then demonstrate how each reacts to rain or the instructor may want to briefly explain the various conservation practices and allow the students to make their own displays. Brief descriptions of several examples that may be used. After each is set up, one end is propped up to simulate a hillside.
- Bare soil-no conservation: Take a container and place a sufficient amount of soil on it to cover the surface to a depth of 1 to several inches (depending on the depth of the container), smooth the surface so it looks like a bare field and lightly pack the soil. When water is applied to this example, it should show the most runoff and the greatest amount of erosion. Note the more the soil is packed the easier it is for the water to run off, conversely, the looser the soil surface is the more water enters it and the more "rain" necessary to begin erosion.
- Bare sand-no conservation: Take a container and set this example up like #1. When water is applied it rapidly soaks or infiltrates into the sand and very little runs off or causes erosion. This example is used mostly to show how rapidly water infiltrates into the coarse (sand) soils as compared to the loam etc. soils. It might be explained that while the water infiltrates rapidly, the sand can actually hold very little. It also shows why water erosion is not considered as much a problem on very sandy soils as on loamy, silty and clayey soils. It also demonstrates what happens when an impervious layer (the bottom of the tray) stops water movement and the soil becomes saturated (stability is lost and the sand slips as in a landslide).
- Bare soil-terraces: Set up the example as in #1. Make sure there is enough soil so one can make a series of terraces which should be parallel to the slope. When water is applied the series of terraces should slow or stop the water flow. This slows erosion and allows water infiltration particularly in the terrace "channel". As additional water is applied the terraces will overflow and erosion will be intensified. (See original for drawing)
- Bare soil-contour farming: Set up the example as in #1. Using ones fingers or some utensil make a series of ridges parallel to the slope such as would be formed from row cropping. When water is applied the rows act like small terraces, but overflow much sooner. Mostly likely when the overflow small erosion rills will be formed going down the slope and may do additional cutting. (See original for drawing)
- Soil+crop residue-no other conservation practices: Set up example as in #1. Sprinkle grass clippings onto the soil surface. One may set up an additional number of examples to demonstrate what varying amounts of crop residue can do to slow erosion. For this example, sprinkle enough grass clippings to completely cover the soil surface. When water is applied the crop residue breaks the force of the water droplets. Thus the droplets don't break apart the soil particles which is a condition enabling the soil to erode. Also the crop residue slows the water flow into the soil allowing more to infiltrate.
- Soil+crop residue-terraces: Set up as in example #3. Sprinkle grass clippings onto the soil surface as in example #5. When water is applied one should observe less erosion than in either #3 or #5.
- Soil+crop residue-contour farming: Set up as in example #4. Sprinkle grass clippings onto soil surface. Apply water as in the other examples. There should be less erosion than in #4.
- Soil with a crop: Planning ahead is necessary for this one as a crop of grass must be planted and growing to show how growing crops reduce erosion potential. A plot of sod could be substituted.
- Rows running up and down the hill: Any of the above demonstrations can be done, but with the rows or terraces running up and sown the hill. This should show increased erosion potential. (See the original)
Modifications:
- Grassed waterways can be used to channel water from the terraces to the lower parts of the slopes (end of trays).
- Varying the slope of the trays will also have a effect on the erosion potential.
- Intensity of the rainfall can also effect the potential of the soil for erosion.
- If the water is shut off before the soil is washed away, students can examine the cross section of the demonstration. Packing by raindrops may be visible on the bare soil versus less packing when protected by residue.
Publications of interest:
Available from your County Extension Agent;
Nebguide, G85-750, Terrace Systems for Nebraska Available from your USDA Soil Conservation Service;
Nebraska Conservation Planning Sheets
- No. 1, Contouring
- No. 2, Terraces
- No. 3, Grassed Waterways
- No. 4, Crop Residue Management
- No. 6, Contour Strip cropping
- No. 7, Contour Barrier Strips
- No. 9, Filter Strips
A Productive State in Harmony With The Environment, NE-MP-11
Farming and Maintaining Terraces, Leaflet Number 570 (Some figures do not match with described practices for grassed back slope and narrow based terraces.)