EXERCISE 17 



THE FILM, OR CAPILLARY, WATER OF SOILS 



Statement. After the free, or gravitational, water has passed out of the surface soil and the soil is 

 dry enough to be cultivated, there yet remains considerable moisture as a thin film around the soil 

 particles and in the small soil pores. It is this film, or capillary, water which growing plants absorb and 

 which brings to the plant its mineral food dissolved from the soil. In general, the more such moisture a 

 soil can hold, the better the crops growing upon it can withstand drought. 



Object. To show the relation of the size of the soil particles and the presence of organic matter to 

 the capacity of the soil to hold film water. 



Materials. Glass tumblers ; a deep vessel, as a four-gallon crock ; three soil tubes, or lamp chim- 

 neys ; soils of various types ; some small pebbles ; cheesecloth ; balances. 



Directions, i. Fill one tumbler half full of pebbles and place in another tumbler an equal volume 

 of soil. Pour enough water into each tumbler to cover or saturate the pebbles and soil respectively. 



Pour off into separate tumblers that water which will pour from each 

 tumbler. Measure and compute the percentage of water retained. 



2. Place a piece of cheesecloth in the bottom of each of the per- 

 forated soil tubes or tie it over the bottom of lamp chimneys. 

 Number and weigh the soil tubes or lamp chimneys thus prepared. 

 Fill one tube with washed sand, another with loam, and another with 

 clay. Jar each slightly to settle the soil, which should compact to 

 within about an inch of the top of the tube. Reweigh each tube and 

 determine by subtraction the weight of its contents. Place the tubes 

 in the large vessel provided and pour water around them until it 

 stands a little higher than the soil in the tubes. Note the time re- 

 quired for the water to reach the surface of each soil. Explain the 

 cause for the differences. When all the tubes show water at the sur- 

 face, remove them and wipe the tubes dry. Immediately weigh each 

 tube and compute the amount of water absorbed by each soil. Cover each tube and set aside where 

 the water will drain away. Weigh each tube at the end of an hour, then at the end of six hours, and 

 daily thereafter until each of the tubes remains at a constant weight. Tabulate and explain all results. 



Questions. When the soil in the tubes showed moisture at the surface did it contain only film 

 water or gravitational water as well? After all the water has drained away that will, what kind 

 of water does the soil contain? Do your results show any difference in the time required for 

 the water to reach the surface in the tubes in different soils? In which was the longest time 

 required? In which was the shortest time required? What type of soil will retain the greater 

 amount of film water, clay, loam, fine sand, coarse sand, or gravel, and why? In which type of soil 

 may the water table be the deepest and still be of service to the crops ? Which type of soil requires 

 that the water table be near the surface ? Why will a fine soil retain more film water than a coarse 

 one? On which of the following soils will plants best withstand wet weather : (i) soil underlaid with 

 rock, near the surface ; (2) stiff clay; (3) silt or loam; (4) fine sand; (5) loam sand; (6) coarse gravel ? 

 Give reasons for your answer. On which of these soils will plants best withstand drought and why ? 



References. Waters, H. J. Essentials of Agriculture, pp. 64-66. Ginn and Company. Mosier and 

 Gustafson. Soil Physics and Management, pp. 194-215. J. B. Lippincott Company. Burkett, C. W. Soils, 

 pp. 37-38. Orange Judd Company. Stoddard, C. W. The Chemistry of Agriculture, pp. 179-183. Lea & 

 Febiger. Lyon, Fippin, Buckman. Soils, their Properties and Management, pp. 206-213. The Macmillan 

 Company. 



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Fig. 22. The structure of the soil 

 0, soil grain ; b, water film ; c, pore space 



