622 IOWA DEPARTMENT OF AGRICULTURE. 



movement of the air. Thus conditions are secured which promote the 

 growth of plants, facilitate the work of the unlimited host of soil bacteria 

 and hasten the formation of available plant food. 



Wet soils are cold soils. This is true for the reason that a large amount 

 of heat is used in the process of evaporation of surplus water. Therefore a 

 well tiled field, from which the water is drained, must necessarily prove 

 warmer than one which is watterlogged. King states that the amount of 

 sunshine which will warm a given weight of water ten degrees Fahrenheit 

 will raise the temperature of an equal weight of dry sand 52.38 degrees Fah- 

 renheit, clay 44.58 degrees and humus 22.60. In the spring it often occurs 

 that a drained soil is ten to twelve degrees warmer than the undrained soil. 

 This is due to the circulation of air in the soil and the absence of evapora- 

 ting water. An early, dry, warm seed bed is an essential factor in success- 

 ful farming. It has been shown by experiments that sixteen days elapsed 

 before corn appeared above the ground when the temperature of the soil 

 was sixty degrees Fahrenheit and that an equal growth was made in three 

 days with the soil temperature at seventy-two degrees. 



When a field is poorly drained at the time of germination and early 

 growth of the crop, the root system must of necessity develop near the sur- 

 face. The result is that the feeding area is too restricted, and later, when the 

 crop needs a large supply of water, the surface soil becomes very dry be- 

 cause capillarity can not act with sufficient rapidity to meet the demand for 

 moisture. In well drained fields the deeper soil is occupied by the roots 

 earlier m the season and not only is the ground water more accessible, but 

 the upper soil is not so readily dried out by a multitude of roots near the 

 surface, and hence capillarity more easily maintains favorable moisture 

 conditions. 



The experience of farmers shows that crops suffer less in time of drought 

 on well drained clay or alluvial soils than they do on the same type of soils 

 not drained. Underdrained soils dry out very completely near the surface 

 in time of drought and therefore water rises by capillarity in them very much 

 more slowly than in moister soils. For this reason it is very important that 

 the soil just beneath the surface be kept as moist as the growing crop will 

 permit. Furthermore in a well drained soil the roots of the plants spread 

 out and go deeper in the early spring and summer. For this reason they 

 are located nearer the ground water supply and do not exhaust the moisture 

 near the surface to such an extent that capillarity is seriously impaired. 

 This supply of water furnished by the action of capillarity aids in bringing 

 the plant food into an available form and carrying it to the plant while the 

 deeper growing roots secure from the ground water supply much of the 

 water given ofif by transpiration. Hence well drained soils are better fitted 

 for crop production in time of drought than the same class of soils not 

 drained. 



Again, underdraining not only carries off excess water, butit also renders 

 the soil more moist when it is comparatively dry, because in time of drought, 

 the air upon the surface is heated by the sun's rays that are absorbed by the 

 soil. The heated air expands and a certain volume is expelled from the 

 soil. The space made vacant by the air which has been forced from the soil 

 is gradually occupied by warm air which enters the drain, and steadily 

 ascends through the soil. But the soil a few inches below the surface is 



