Conditions Modifying Effectiveness of Rainfall 111 



to prevent the crops from suffering for lack of moisture, while 

 once in seven days met the needs of plants growing upon soils 

 of the finer texture used in the experiments. 



The difficulty in the case of soils of coarse texture is, not 

 that the water evaporates more rapidly from the surface of them, 

 nor is it because more water must be present in them in order 

 that plants may utilize it, for it is true that the surface evapora- 

 tion from them is slower than with most other soils, and that 

 plants may use the water more closely from them than is 

 possible when the grains are smaller. The real trouble is found 

 in the fact that when they are underlaid by a coarse subsoil, and 

 when standing water in the ground is more than 5 feet below 

 the surface, the water drains out so completely in a short time 

 that not enough remains to keep the crop from wilting. 



We do not yet know how closely the water may be used up 

 in field soils of different textures before crops of different kinds 

 will begin to suffer, or will have their rate of growth checked ; 

 but the writer has found that clover, timothy, blue -grass and 

 maize have their growth 'brought nearly to a standstill in a clay 

 loam soil underlaid with sand at 3 to 4 feet, when the amount of 

 water left in it was that stated in the table below: 



Table showing the amount of ^vater in a clay loam in the field when crops wilted 

 and growth was brought nearly to a standstill 



Timothy and 



Clover Blue-grass Maize 



Depth of sample PER CENT PER CENT PER CENT 



0- 6 inches loam 8.39 6.55 6.97 



6-12 " clay loam 8.48 7.62 7.8 



12-18 " clay 12.42 11.49 11.6 



18-24 " clay 13.27 13.58 11.98 



24-30 " clay 13.52 13.26 10.84 



40-43 " sand 9.53 18.37 4.17 



Nothing more definite can be said regarding the data of this 

 table, than that under the moisture relations there shown, growth 

 was practically at a standstill, and that when very considerably 

 larger percentages of water were present in the soil the normal 

 rate of growth was checked, 



