DISCUSSION OF PLANT IXDUSTRIES 439 



of \\'ater ^\•hil•.h adhere to soil particles are not removed by 

 drainage, and plant roots are better located to avail themselves 

 of this supply than if they are placed near the surface, as 

 happens in -wet soils. 



Withm the last decade it has been shown that at least some 

 of the cereals secrete and leave within the soil substances 

 which are injurious to the kinds of plants which produce 

 them.i Adequate drainage probably assists in removing some 

 of these poisonous materials. Drainage ditches help to aerate 

 the soil, and in this way are of great benefit to the growth of 

 economic plants. 



405. Tillage and water supply. In olden times agriculturists 

 ad\ised against cultivating corn and other crops during times 

 of drought, bet'ause they thought that if constantly stirred the 

 soil would lose its moisture more rapidly. People now know 

 that it is of the greatest importance to till the soil durmg 

 droughts m order that it may not lose its moisture. An illus- 

 tration will help in studying this matter. If two pieces of loaf 

 sugar are placed one upon the other, the lower one held in 

 the thumb and finger and the other left lying loosely upon 

 the first and not tor^chmg the fingers at all, and if the lower 

 one is then placed in contact with ■\^■ater, two important facts 

 are sho^\•n. The lower piece takes up water freely, but the 

 upper one, though l_ving upon the lower wet piece, becomes 

 wet only after a long time. Close connection between the 

 solid paiticles is necessary for the rapid upward passage of 

 the water. 



"When soils are compact, moisture from the deeper portions 

 passes up\^ard freely, as in the lower lump of sugar, and evap- 

 orates into the air. If, howe^"er, the surface is kept loose and 

 finelv pulverized, so that the particles are less closely con- 

 nected, moisture does not readily pass through it and there 

 is less loss from evaporation. The roots of plants being more 



1 Schieiuer, 0., and Reed, H. S.. Some Factors influencing Soil Fertility. 

 "Bur. Soils," Bulletin40, U. S. Dept. Agr., 1907. Also, "The Production of 

 Deleterious Excretions by Roots," Bulletin 34, Torrey Bot. Club, 1907. 



