154 STATE BOARD OF AGRICULTURE. 



THE IMPORTANCE OF SOIL MOISTURE. 



The plant obtains all of its food, excepting carbon, from the soil or through it. 

 In nature all these excepting a part of the nitrogen are derived from the soil 

 materials, both mineral and organic, of which they form a part. These foods 

 are dissolved in the soil water and the water with its dissolved materials is taken 

 in through th5 roots of the plant and thence conveyed to the leaves where the 

 food materials are reconstructed and much of the water thrown off into the air^ 

 From the leaves the remaining water with the reconstructed food moves out 

 through the plant to the growing parts where the food is transformed into plant 

 tissue or is stored for future use. 



But the food materials in the soil before they can be dissolved by the water 

 must undergo a change by which they are made soluble. Whatever this change 

 may be, water is one of the agents by which it is produced. 



Water then is important: 



1. In the changing of plant foods into soluble form. 



2. To dissolve and convey plant foods to and into the roots of the plant. 



3. To convey plant foods from the- roots to the leaves for elaboration. 



4. To convey the elaborated foods from the leaves to other parts of the plant 



for use. 



Nothing has been said of the importance of water in the germination of seeds. 

 Without water the seeds would not swell to burst the coat, nor could the food 

 stored in the seed be transformed, dissolved, and transported to the different 

 parts of the young plant. 



It requires large quantities of water to dissolve and convey the food to plants. 

 To produce one pound of dry matter of crop requires the passage into the plant 

 of from nearly 300 pounds to nearly 500 pounds of water. See Table II. 



An inch of rain on an acre of ground (one acre-inch) weighs about 113.4 tons. 



CONDITIONS INFLUENCING AMOUNT OF CAPILLARY WATER. 



The amount of capillary water a soil will hold will depend upon: 



1. The size of the mineral particles in the soil. 



2. How these particles are combined into compound grains. 



3. The amount of organic matter in the soil. 



4. The degree of mellowness, or the tilth of the soil. 



1. The finer the grains in a soil the greater is its water-holding power, other 

 things being equal. This is true because the finer the particles of soil the larger 

 the total surface the grains of a .given weight of soil will present for the moisture 

 to gather upon. For example, one pound of marbles one-half inch in diameter 

 will present just twice as great total surface as one pound of marbles one inch 

 in diameter, and theoretically would hold twice as much water on their com- 

 bined surfaces under the same conditions if not in contact. If in contact they 

 would probably hold more than twice as much. 



If the surfaces of all the grains in a cubic foot of some of our finer soils 

 could be measured they would be found to amount to as much as four, five and 

 sometimes more than five acres — five acres of surface to which water may cling, 

 and all this in a single cubic foot of soil. 



Cut II shows in a graphic way how the total surface varies with the size of soil 

 grain. The four squares represent the exact total surface of one-tenth of a gram. 

 =.0035 of an ounce, respectively of a typical truck soil, a typical potato soil, a 

 typical wheat soil, and a typical grass soil. The average diameter of the soil 

 grain is indicated in each case. A. millimeter (mm.) is about one twenty-fifth of 

 an inch. 



The small cube in each square represents closely the volume of one-tenth gram 

 of the soil indicated. 



The figures in the squares indicate the actual areas of the squares. 



A cubic foot of the truck soil Avould have a total surface of about .3 acre, 

 while a cubic foot of the grass soil would have about 4.12 acres. 



