Necessity for Drainage 417 



soil is a fundamental necessity to plant life, and 

 thorough drainage secures this. 



The third demand for drainage is to render the 

 soil sufficiently firm and solid to permit the field or 

 road to be moved over without difficulty or incon- 

 venience. If the spaces between the soil grains are 

 completely filled with water, then there is no surface 

 tension, and so only a slight friction to bind the 

 grains together, and hence they move so easily upon 

 one another as to be unable to sustain much weight, 

 and the horse or wagon mires. 



Everyone is familiar with the hard surface pos- 

 sessed by wet beach sand, from which the water has 

 just withdrawn, and how yielding it is when under 

 water and also when it becomes dry. In the first 

 case, the sand grains are bound together by the thin 

 films of water which surround them ; in the second 

 case, there is no free water surface between the grains, 

 and the sand tends simply to float and so moves 

 easily ; while in the third case, when the sand is 

 dry, the binding water films have either drained 

 away or have been lost by evaporation, hence there 

 is nothing to hold the grains together. 



The hard, firm character of a clay soil when it 

 loses its moisture is due to the fact that the grains 

 are so small and so close together that the little 

 material which is held in solution in the soil water 

 cements them together when dry. Were the grains 

 large like those of the sands, with few of the fine 

 particles between them, the contact areas would be so 

 few and so small that little binding could result. 



