8o ENGINEERING ON THE FARM 



grain effectively stopped evaporation and the ground was 

 so soft that the binders would not operate, and a large part 

 of the wheat crop was lost. It was found that in the fields 

 where the binder had started before the rain one round 

 could be cut in the morning because the surface from which 

 the grain had been removed was sufficiently dry to hold up 

 the binder, but the machine could not run on land from 

 which the grain had just been cut. Twenty-four hours' 

 time was required to dry the soil so as to hold up the binder. 

 This illustrates the effect of evaporation in drying the soil 

 as well as the effect of vegetation on evaporation. While 

 evaporation is one of the drying agencies and very effective 

 under certain conditions, it is uncertain and cannot be 

 relied upon to remove moisture that is interfering with 

 crop production. 



Plant growth. The growth of plants requires a large 

 quantity of water. Water is taken from the soil by the 

 plant roots during the growing period and used in the struc- 

 tural system of the plant or evaporated from the surface of 

 the leaves and the stems into the air. 



Run-off. All rainfall not disposed of by absorption, per- 

 colation, evaporation, and plant growth is known as run-off 

 and is the water that collects on the surface and flows over 

 the surface into ditches and streams. On frozen or hard 

 ground which tends to limit absorption and percolation, or 

 where evaporation is limited by checking of the circulation 

 of the air, or the quantity of water used in plant growth is 

 limited because the rain comes at a season when plants are 

 not growing, there is a tendency for the run-off to increase. 

 A large run-off may be expected in the spring when the 

 ground is frozen and the temperature is cold, and a small 

 run-off in the middle of the summer when plant growth is 

 the greatest, evaporation is at a maximum, and the ground 



is dry. 



Injury to the growing crops from excessive moisture is 

 due to the fact that they are drowned out by the run-off 



