10 Irrigation and Drainage 



is a matter of universal experience in humid climates 

 that on clayey soils heavy protracted spring rains con- 

 tribute more to the production of large crops of grass 

 than all the manure which farmers can put upon their 

 lands, and that with dry springs fertilizers, of what- 

 ever sort and however applied, are of but little avail. 

 So, too, four weeks of copious, timely, warm rains fall- 

 ing upon fields of potatoes after the tubers begin to 

 set, and of corn after the tassels and silk begin to 

 form, are certain to be followed by enormous yields, 

 even when the soil is not rich, unless frost or disease 

 intervenes. On the other hand, let the tuber and grain- 

 forming period of these crops be one of drought, and 

 it is only those soils which are most retentive of mois- 

 ture, and which have been most skillfully handled, that 

 are able to mature even moderate yields, though the 

 land be very rich. 



What, then, do warm spring and summer rains and 

 warm, sweet irrigation waters do in the soil which con- 

 tributes so much to plant growth? In the first place, 

 it is only through the soil, where very extensive absorb- 

 ing surfaces of root hairs are developed, <that plants 

 are able to obtain the very large amounts of water 

 they need for food and for the maintenance and carry- 

 ing forward of the physiological processes which are 

 associated with plant growth. 



But it is not alone for the crop which is being grown 

 upon the ground that water is needed in the soil ; for 

 it must never be forgotten that there are living within 

 the dark recesses of the soil organisms of various kinds 

 upon whose normal and vigorous activity depends, in 



