30 BULLETIN 355, U. S. DEPAETMENT OF AGKICULTTTRE. 



keeps up a protective mulch over the surface. This mulch and the 

 destruction of weeds largely prevent the loss of moisture from the 

 soil and it is held for use by the crop of the following year. 



Other means used to control the water supply of the soil are irri- 

 gation and drainage. 



EXERCISES, LESSON IV. 



Materials required. — One small balance or scales; four 1-pound baking-powder cans; 

 4 quarts each of dry sand, dry muck or peat, dry clay or silt loam; one 2-quart pail; 

 a small piece of cloth; two cups; two or three pie tins; two or three small shallow 

 dishes (saucers); a small quantity of lump and powdered sugar; six fine sewing needles; 

 two pieces of | or 1 inch glass tubing 2 feet long; one-half bushel of moist loam or silt 

 loam; two 2-gallon crocks. 



Water-holding capacity of soils (See reference in lesson). — Turn four 1-pound baking- 

 powder cans upside down and punch three holes in the bottom of each. Obtain the 

 weight of each can. Fill can No. 1 with dry sand, can No. 2 with dry muck or peat, 

 can No. 3 with dry clay or silt loam, and can No. 4 with a mixtiire of one part (by 

 volume) of dry sand and one part of dry muck or peat. Determine the weight of dry 

 soil in each can. Saturate all with water, let stand until no more water drips from 

 them, then weigh again. Determine the percentage of capillary water retained by 

 each land of soil. Account for the variation in water-holding capacity of the several 

 samples. How may the water-holding capacity of a sand be increased? Of a heavy 

 clay? "VvTiich class of soil will give -up its water the easier, sand or clay? Why? 

 On which soil do crops suffer more for want of water during a drought? 



Percolation of water through soils (Ref. Nos. 2, pp. 170-173; 4, p. 32). — Punch a 

 half-inch hole through the side and near the bottom of a 2-quart tin pail. Cover the 

 opening on the inside with thin cloth and fill the pail with sand. Put a stopper in 

 the opening and saturate the soil with water, measuring the quantity of water used. 

 When saturated, remove the stopper and catch and measure the water that runs out. 

 When dripping ceases compare the quantity of water caught with that used to satm'ate 

 the soil. What name may be given to the water retained by the soil? 



Capillary rise of soil ivater. — Pour a cupful of dry sand on a pie tin in a conical pile. 

 Pour about a third of a cupful of water into the tin (not on the sand pile) and obseive 

 results. What name is given to this phenomenon? Of what importance is it in 

 agriculture? Is this the only direction in which film water moves in the soil? What 

 determines the direction of movement? In what kind of soil will water rise the 

 higher, sand or clay? Explain. Repeat this experiment, if possible, by using 

 2-foot glass tubes filled with dry sand and clay loam. Cover the lower end of each 

 tube with cloth, tamp the soil carefully, and stand tubes in a tray. Pour about 

 half an inch of water into the tray and observe results. Note carefully the rate of 

 rise and the height to which the water will rise in each tube. 



Resistance of dry soil particles to water films. — Fill a small dish with water; place a 

 perfectly dry, fine needle carefully on the ^rface film of the water. The needle 

 will float. Explain. Take a pinch of road dust and let it drop carefully into the 

 water. What happens to the finest dry particles? Explain. Why do water drops 

 roll off a dusty board like so many shot? 



Conserving soil moisture (Ref. No. 3, p. 264). — Sprinkle as much powdered sugar on 

 top of a lump (do not press down the powdered sugar) as it will hold, and place the 

 lump in a pool of about 12 drops of water poiu'ed out on a smooth surface. What hap- 

 pens? Explain fully. Let the lump of sugar represent a portion of soil inamediately 

 underneath a thoroughly cultivated surface. W^hat does the powdered sugar repre- 

 sent? Is this principle of moisture conservation practiced in connection with all farm 

 crops? Repeat the experiment using dry caked and powdered clay. 



