TEL' WORK OF BOOTS 133 



making a surface mulch, in which case it soon dries 

 out; (2) moisture may be increased by a surface mulch 

 which hinders the rain-water from running off and 

 largely prevents loss by subsequent evaporation ; by 

 keeping the soil in good tilth; by the addition of 

 humus ; by artificial application of water. 



Other things being equal, the yield of a crop is di- 

 rectly proportional to the amount of water it receives 

 within the limits mentioned above. ^ 



How thoroughly do roots explore the soil for mois- 

 ture? This may be investigated by carefully re- 

 moving the earth in successive layers, or, better still, 

 digging a trench around the plant and lifting out a large 

 ball of earth, which should be carefully washed away 

 by means of a hose. The roots of Corn, Barley, etc., 

 examined in this way show a dense mat extending 

 downward three or four feet. In a root- system of this 

 kind we find practically every cubic inch of the soil 

 explored by one or more (sometimes by very many) 

 roots: a conservative estimate of the extent of the 

 root- system of four well -developed Corn plants gave 

 an aggregate length of over a mile of roots, not count- 

 ing root-hairs. The estimate was made by calculating 

 the number of cubic inches of soil (the roots occupied 

 a cube of earth a little less than three and one -half 



iSee Bailey, "Principles of Acrrieulture"; King, "The Soil" and "Irrigation 

 and Drainage"; also articles in the Year-Book of the U. S. Dept. of Agriculture 

 for 1895 by Taft; for 1898 by Briggs; for 1900 by Johnson and Stannard; 

 for 1902 by Beals. 



