EXERCISE 16 



HOW WATER GETS INTO THE SOIL 



Statement. Drought is the universal disaster. The crops of nearly every part of the globe are 

 injured by drought at some season of every year. In some regions, two days after a heavy rainfall 

 the crops suffer from lack of moisture because the soil is too shallow to absorb and hold much 



Fig. 21. Apparatus to illustrate how water gets into the soil 



moisture and nearly all the water which fell ran off the surface. The water run-off on many soils is 

 greater than the water penetration. It is the water which enters the soil and is held within reach 

 of the plant roots that is of value to growing crops. 



Object. To study the conditions which favor the penetration of water into the soil and the 

 percolation of water through the soil. 



Materials. Three percolation tubes or lamp chimneys ; three glass tumblers ; graduated measuring 

 glass ; clay, loam, and sandy soils ; finely pulverized manure ; balances ; three boxes, each of the 

 same size; cheesecloth; string; a sprinkling can. 



Directions, i. Cut one end of each box slightly V-shaped, as shown in Fig. 21, and fill each 

 level full with clay soil firmly compacted. Level the surface the same shape as the end of each box 

 and then make a soil mulch in one of the boxes by stirring the soil to a depth of three inches. Leave 

 the second box of soil firm and smooth. Mulch the third with finely pulverized barnyard manure 

 mixed into the surface inch of soil. Place the boxes on an inch strip (Fig. 21) to represent a natural 

 slope, and place vessels under the end of each box to catch the water that runs off. With a sprinkling 

 can apply to each of the boxes of soil a known and equal amount of water. Sprinkle slowly. Measure 

 the amount that runs off and compute the percentage of absorption and penetration in each case. 

 Explain results. 



2. Perform a similar experiment, using compacted clay loam and sandy soil respectively. Com- 

 pare results and explain. 



3. Place a piece of cheesecloth in the bottom of each percolation tube or tie pieces over the bottom 

 of lamp chimneys if they are used. Fill one tube with clean, dry sand to within an inch of the top. 

 Likewise, fill the second tube with dry, finely screened loam, and the third with finely screened clay. 

 Jar each tube lightly to settle the soil and suspend them in a percolation rack or in some manner so that 

 tumblers may be inserted under them. Pour water into each tube, keeping the water level near the top 

 of the tube. Note the time it takes the water to begin to drip from the bottom. After the water 

 has been dripping from a tube for a time and the dripping has become constant, catch the water that 

 percolates from each soil in fifteen minutes. Measure the water and compare the percentage that perco- 

 lates from each soil. This is known as free or gravitational water. 



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