458 Introduction to the Study of Science 



Different soils from coarse to fine have different capacities 

 for soil water. This is important in agriculture. A simple 

 test' will illustrate the principal differences in the moisture 

 capacity of soils. 



Exercise. Weigh out from the sample soils, before they have 

 lost their moisture by evaporation, about four ounces of each. Dry 

 each portion thoroughly, but without burning the humus, over a flame 

 in the laboratory or in the oven at home. After each is dried, weigh 

 and compare the weight with that of the fresh soil. This shows the 

 amount of soil water each contained. 



Next put the dried soils into glass tumblers and pour over each 

 just enough water to reach the surface. Drain off the free water. 

 Weigh the soil. This gives the total capacity of the soil for moisture. 

 Arrange the samples of soil on the basis of their respective capacities 

 for moisture. What does this show as to the texture of the soil? 



A fine clay soil holds more moisture than a coarse, sandy 

 soil or a gravelly soil. The size of the soil particles seems to 

 determine the soil's capacity for water. You will find by 

 examination that the particles hold moisture only on their 

 surface. Consequently the smaller the particles, the greater 

 must be the moisture capacity of the soil, for the greater is their 

 combined surface area. 



Exercise. This fact may be clearly illustrated in a simple way. 

 A stone measuring one cubic inch has a surface area of six square 

 inches. Break it into two equal parts ; its surface area is increased 

 by two square inches. Break it again and again until it is made into 

 particles the size of coarse silt. The total surface area of these par- 

 ticles is about twenty-two square feet. Now if moisture is held only 

 by the surface area, obviously more of it is held by the minute particles 

 than by the original cube. 



Does water readily penetrate to the interior of gravel or of larger 

 rock fragments ? Put a pebble into water and let it soak for a while. 

 After taking it out, wipe with absorbent material to remove the thin 

 film of water that envelops it. Break it and examine the inner sur- 

 faces for moisture. Has the moisture penetrated ? Test by breaking 

 some of the large gravel which you may have found in digging where 

 moisture is plentiful. Select a handful of pebbles, dry thoroughly 

 by wiping, weigh accurately, and put into an oven to dry. When re- 

 moved, weigh and compare with former weight. 



