Planting 139 



the soil under the tilled area and the surface. Until 

 capillary action is again established by the settling to- 

 gether of the tilled soil, but Httle water can be lost by 

 evaporation. A thin layer only of loose soil is suflBcient 

 to prevent evaporation. A thin layer settles together 

 sooner than a thick one and needs breaking more often. 

 More emphasis has usually been placed on tillage of 

 potatoes after planting than before planting. The 

 great soil supply of water in late winter and early 

 spring presents opportunities for conservation that exceed 

 those during the growing period of the crop. In recent 

 years a change has come about in the attitude of inves- 

 tigators towards the value of summer tillage. Extended 

 experiments have shown that one factor which was 

 largely neglected by early experimenters is of impor- 

 tance — this is the ability of the plant root to seize 

 upon the use of soil moisture. When the soil is not 

 occupied by a growing crop, there is no doubt that a sur- 

 face mulch of loose soil does conserve moisture by pre- 

 venting evaporation, but when the soil is well filled with 

 plant roots, there is little more water evaporation from 

 untilled soil than from tilled. The gains in yields ob- 

 served from tillage come from another source — weed 

 killing — rather than from prevention of the evapora- 

 tion; This means that the farraer will till practically 

 as much as under the former practice, but will have in 

 mind weed killing as his principal object, instead of pre- 

 venting loss of water. In practice this will mean that the 

 tillage of the potato crop during the growth of the crop 

 will be done earlier than has been the custom. Water 

 will be conserved by fall plowing where possible ; by the 

 addition of organic matter to the soil ; and by early spring 

 tillage to save the water supply. The soil will not be 



