FIFTH ANNUAL YEAR BOOK — PART VI. 521 



irregular shape, varying sizes and different properties. These particles 

 range from those too small to be seen with the naked eye to those the size of 

 pebbles, or even small stones. There is a large amount of pore space, or 

 space intervening between the soil particles, which in an average soil equals 

 nearly half its volume. When all these pore spaces are filled with water, 

 the soil is said to be saturated. A soil which is completely saturated with 

 water can not grow cereals and other valuable farm products. 



If tile drains are placed in the ground they afford an outlet for the sur- 

 plus water which is carried downward by the force of gravity. But the 

 tiles do not render the soil absolutely free of moisture, for a force which is 

 known as surface tension, holds a film of water over the entire surface of 

 each particle of soil. In this way a supply of water is held in a drained soil 

 for the use of plants for it is this capillary water, which constitutes from 

 fifteen to twenty per cent of all the water a soil will hold, which will not pass 

 off as drainage, but furnishes moisture for the plant and aids in the prepara- 

 ation of plant food. 



S^''«; which are made up of fine grains retain the greater quantity of water 

 because the fine grains possess a larger surface area in a given volume of 

 soil. Soils of this class also require more artificial drainage than coarser 

 soils owing to the fact that the closeness of the particles reduces the rate 

 of percolation downward under the force of gravity. 



With this knowledge of the structure of the soil and the relation of soil 

 water to plant growth, it is not difficult to understand that the soil is greatly 

 benefited by adequate drainage. 



Adequate drainage increases the firmness of the soil and thus renders 

 it fit for cultivation earlier in the season. The surplus water readily per- 

 colates through theunderdrained soil and permits the surface to become firm 

 and solid. This firmness of the soil, which makes it possible for teams to 

 pass back and forth without injuring the texture or rendering the surface 

 uneven, can not be secured when the spaces between the soil grains are filled 

 with water, for then there is a lack of surface tension and the soil grains 

 move so easily upon one another that a team mires. Near the northern 

 limit of the corn belt the advantage gained by working and planting the soil 

 a week or ten days earlier in the spring and thus lengthening the season to 

 that extent, is an exceedingly important one. There are a few seasons when 

 the larger and more profitable varieties of corn are not frosted in northern 

 Iowa and southern Wisconsin and Minnesota. 



Oxygen in the soil is as essential for the life of the plant as it is for the 

 animal. Without free oxygen in the soil the seed fails to germinate and in a 

 short time rots; the roots of the plants fail in their appointed tasks; the 

 innumerable host of soil bacteria, whose work it is to change the nitrogen 

 of decaying organic matter into an available form, perish, and the germs on 

 the roots of the red clover and other leguminous crops, which supply avail- 

 able nitrogen at the lowest cost, do not accomplish their important work. 



When the soil is full of water to within a few inches of the surface, there 

 can be no circulation of air among its particles. Drainage ventilates the soil 

 by lowering the ground water three or four feet and thus makes it possible 

 for the roots of plants to penetrate the soil more deeply. In time these roots 

 die and decay, and afford passage-ways throughout the soil for the ready 



