106 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



of dust and let drop carefully into the water. What happens to the finest, 

 dry particles? Explain. Why do drops of water roll off a dusty board like 

 so many shot? 



To Note the Effect of a Soil Mulch on the Rate of Evaporation from a 

 Silt Loam or Loam. Procedure. Fill one gallon crock level full of air-dry silt 

 loam or loam. (In filling establish good contact between the soil particles.) 

 Determine weight of soil in the filled jar. Moisten with water until a good 

 supply of capillary water is held throughout the soil. Weigh again and deter- 

 mine the per cent of capillary water contained in the soil. 



Fill a second crock in a similar manner as No. 1, only fill within two inches 

 of the top. Water, and determine the per cent capillary water as in No. 1. 

 Now fill crock to level full with dry, crummy soil (soil mulch). 



Place both crocks in a breezy place and determine the per cents of capillary 

 water in the two soils in the two crocks at the end of a week or ten days. Do 

 not water. 



Have students record results. 



Questions. (a) What constitutes a good soil mulch? On sand? On 

 silt loam? 



(6) When is a soil mulch most effective? (Consult text.) 



Laboratory Exercises. Mate-trials Needed. Three large and 3 small baking 

 powder cans; some cheese cloth; one 20-mesh screen; one 40-mesh screen; one 

 pie tin; one soil auger; one balance; 4 quarts each of dry coarse sand, fine sand, 

 and a mixture of coarse and fine sand; one quart each of dry crummy silt loam, 

 peat, muck and pulverized peat. 



To Determine the Porosity of Soils. Procedure. Punch a hole in the 

 bottom of a baking-powder can. Place a piece of cheese cloth over the bottom 

 on the inside, and fill the canfull of dry, coarse sand (screen out all fine material). 



Now set the can into a dish of water so that the surface of the soil is about 

 on a level with the surface of the water on the outside. Allow the soil to 

 become saturated so that free water is noticed on the surface of the soil. Place 

 finger over the hole in the bottom of the can, take can out, and allow all free 

 water to drain into another can to be measured. The amount of water in cubic 

 inches represents the amount of pore space in the soil. Determine the per cent 

 of pore space in the coarse sand. (3! X square of radius of soil column X height 

 of soil column = cubic inches.) Record results as follows: 



Repeat the experiment by using a fine sand (screen out all coarse material), 

 and a mixture of coarse and fine sand. 



Questions. (a) What is the relation between the size of soil grains and 

 porosity of a soil? 



(6) Would the pore space in clay be greater or less than in sand? Why? 



(c) How does'porosity of a soil affect the rate at which water will percolate 

 through it? 



(d) Will dry clay loam weigh more or less than fine sand? Why? 



(e) What are the conditions producing the least amount of pore space in 

 a soil? (Note pore space in mixed sand.) 



To Observe the Movement of Capillary Water in Soils. -Procedure. Pour 

 a cupful of dry sand or loam on a pie tin in a conical pile. Pour about a 

 third of a cupful of water into the tin (not on the pile of soil) and observe results. 



