Effect of Raindrop Size on Soil Erosion

Soil erosion has always been an agricultural problem. It is caused by the effect of wind and water on unprotected soil. Rain falling on sloping bare ground can remove a considerable amount of valuable top soil in a short period of time.

Fill two 8" pie plates (of aluminum or other metal) with a 1/2" layer of very fine, dry soil which has been sifted through a piece of window screening. If the soil contains a great deal of clay, it is better to mix some fine sand with it. Smooth the top of the soil layer with a small piece of cardboard. The plates should be propped up so that they have a slope of about 1:3. It is easiest to set them on a piece of 1/8" or 1/4" plywood, 6 x 20", along one edge of which you have nailed a strip of wood, 3/4 x 2 x 20". Hammer two nails for each plate from the bottom side of the board to prevent sliding. Cover each plate with a sheet of aluminum foil.

Raindrops of different sizes can be produced by making holes in the bottle about 1/2" from the bottom. A fine pin or sewing needle at an angle of about 45 from the vertical will produce a very fine spray of drops of about 1/32" or less. A nail (such as a 1" brad) is similarly pushed through the bottle at another place will produce a spray of relatively large drops of 1/8" or more. Fill the bottle with water to the top and test the "rain" issuing from the two holes. The pin-hole should produce, whereas the nail-hole should produce If the holes are not large enough, widen them; if they are too large, cement them shut again or cover them with tape and make new holes.

Since the amount of soil erosion also depends on the amount of rain, make sure that the same amount of water is used for the production of the two different "rains". Place a piece of tape near the top of the bottle. Make two water-level marks 1" apart on the tape. Then cover the two holes near the bottom of the bottle with tape. The bottle is then wired or taped to the top of a 6ft. stick of wood, so that the height from which the drops fall onto the soil is the same in both cases.

The experiment is best performed outdoors at a wind-protected place, or indoors where water on the floor does not matter. Cover both plates with the aluminum-foil sheets, fill the bottle with water open one of the holes and let the spray fall on one of the covered plates. When the water level in the bottle reaches the top mark, quickly uncover the plate and let the "rain" fall on the soil, until the water level in the bottle reaches the lower mark, when you quickly cover the plate again. This process is repeated with the other hole and the other plate. Observe the effects of the "rains" and describe them in terms of pitting and splashing of the soil.

Material: Two aluminum pie plates

Fine soil or soil-sand mixture

A piece of 12 x 12" window screening

1/8" plywood, 6 x 20",one strip of wood, 3/4 x 2 x 20"

A 6-ft. stick or pole of wood

Small piece of cardboard

Two sheets of 10 x 10" aluminum foil

One 2-liter clear plastic bottle

Tape, nails, water

diagram for soil erosion activity