28 THE ESSENTIALS OF AGRICULTURE 



trees and grapevines in the early spring. The sugar-maple tree 

 is bled by tapping, and its sap is collected and boiled down to 

 sirup or sugar. 



When the ground is warm and moist the root hairs absorb 

 water most readily. Furthermore, the roots will not grow and 

 produce the many rootlets which bear the root hairs unless the 

 soil is well supplied with air, since the roots grow only when 

 they can secure the oxygen of the air that circulates through 

 the soil. 



30. Only part of the soil water used by plants. Not all the 

 water in the soil can be absorbed by the root hairs. Some 

 water is left in the soil during the severest droughts, after 

 all the plants have withered. This amount may be as much 

 as 5 or i o per cent in ordinary agricultural soils. The amount 

 of this unavailable water varies in different soils, being greater 

 in a fine fertile loam than in a sandy soil, because the greater 

 the number of the soil particles, the greater the number of 

 films of water that will be held. Different kinds of plants, 

 moreover, vary with respect to the degree to which they can 

 exhaust the water of the soil. Lettuce and cucumbers leave in 

 the soil from 8 to 10 per cent of water which they are unable 

 to withdraw, while corn and cabbage in the same soil leave 

 only 6 per cent. 



31. The amount of water required by plants. In more than 

 half the area of the United States, and indeed of the world, 

 there is not enough water available in the average season for a 

 maximum yield of crops. In the central part of the United 

 States agricultural plants, on the average, take about four hun- 

 dred and fifty pounds of water from the soil for every pound 

 of dry matter produced. It is of the greatest importance, there- 

 fore, to find out what plants will produce a pound of dry matter 

 on the least water drawn from the soil. By careful experiments 1 

 the pounds of water which under ordinary conditions are drawn 

 from the soil for every pound of dry matter manufactured by 

 certain plants has been ascertained to be as follows : 



1 Bulletin 284 and Bulletin 283, United States Department of Agriculture. 



