218 Bulletin 37. 



The chief aim of irrigation by any method should be to get the 

 maximum amount of the water applied back through the crop being 

 grown, and let the minimum amount escape downward or directly 

 from the soil into the atmosphere. Only the water passing throiigh 

 a tree, for example, benefits it. That which is taken up by the 

 roots and exhaled from the leaves is of service to the tree. The 

 water that does not take this course does not benefit it. Another 

 important point to be kept in mind in irrigating is that the en- 

 trance of air into the soil should not be interfered with while the 

 plant is growing. The proper aeration of soil is very important. 

 That the necessary biological and chemical processes may proceed 

 properly in the soil, a constant supply of oxygen is essential. The 

 method of irrigating trees that interferes least with the soil aera- 

 tion is the desirable one. A consideration of the winter irrigation 

 system, as outlined in the foregoing pages, will disclose that it 

 answers the two above requirements. 



During the winter the lower temperatures and the higher rel- 

 ative humidity cause evaporation to be much slower than during 

 the remainder of the year. In applying water, therefore, compar- 

 atively little escapes into the atmosphere. The supply of water 

 being greatest at that time of the year makes it possible to apply 

 large amounts at short intervals, thus avoiding the loss that oc- 

 curs if small amounts are applied at greater intervals. Then, too, 

 the trees are dormant and the roots need little air; hence, no in- 

 jury is done them by keeping the soil supermoistened, or by let- 

 ting the surface bake to some extent. Consequently, cultivation 

 after each irrigation is not necessary, much time thus being- 

 saved. 



When the soil is of the proper character, the roots of orchard 

 trees penetrate to great depths, enabling the trees to thrive, though 

 the surface stratum be quite dry. In making the underground 

 investigations in this orchard, roots were found in abundance at a 

 depth of 12 to 16 feet, and many penetrated to a depth of more 

 than 20 feet. This characteristic is what makes it possible to 

 store in the soil, during winter, much, if not all of the water 

 needed during the summer. 



Trees make use of, and consequently need water much earlier 

 than is commonly supposed. An examination made February 



