84 



SOILS AND FERTILIZERS 



it will not interfere with root growth. The more permeable 

 the soil and the deeper the drains, the further apart they may 



<St/aFACE 



sS^/VDY LO/1M 



Fig. 16. — Cross sections of two soils, a sandy loam and a clay, both of 

 which have drain tiles laid at right angles to the sections. Owing to the 

 more rapid movement of water through the sandy loam, the tiles are laid 

 twice as far apart as they are in the clay. They are also deeper in the former 

 soil. The water gradient is steeper in the clay. The tiles should be suf- 

 ficiently close together to keep the water table below the plowing line. 



be placed. The position of the water level between drains 

 is shown in Fig. 16. 



Ditches may be dug or partly dug by means of spades, 

 ditching plows or traction ditchers. The last named, while 



Trie Elements or Soil Water Control 



WATER CONTROL. 



MOISTURE CONSERVATION 



DRAINAGE 



WILTIN6 POINT. 



MAXIMUM CAPILLARY 

 CAPACITY 



3— H OPTIMUM 50IL MOISTURE*! — j 



UNAVAILABLE 

 WATER 



AVAILABLE 

 WATER 



! 5URPLU3 

 WATER 



Fig. 17. — Diagrammatic explanation of water control in a humid region. 

 On the one hand we have drainage reducing the surplus water to the maxi- 

 mum capillary water capacity or below and thus bringing it within the range 

 of available water. On the other we have moisture conservation by means 

 of which the moisture is kept above the content of unavailable water or the 

 wilting point. Somewhere within the limits of available water lies the 

 optimum moisture content for plant growth. 



