30 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
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 mixture 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 kind 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? Which 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 saturate 
the soil. What name may be given to the water retained by the soil? 
Capillary rise of soil water. — 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 observe 
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 surface 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 poured out on a smooth surface. What hap- 
pens? Explain fully. Let the lump of sugar represent a portion of soil immediately 
underneath a thoroughly cultivated surface. What 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. 
