88 APPLIED BIOLOGY 



potato plants, and also in order to keep the surface of the soil 

 in a dusty condition. This checks the capillary attraction 

 a few inches below the surface and moisture comes up to 

 within reach of the roots of potato plants but not to the 

 surface where it will be wasted by evaporation. Such a 

 condition is known in agriculture as a dust-mulch. On many 

 farms which are conducted according to modern science one 

 may see on hot dry days in the summer the cultivating- 

 machines at work pulverizing the surface of the soil in order 

 to keep in the moisture. This is the main secret of success 

 in the " dry farming " in many western states. 



(2) The second kind of mulch is illustrated by the following 

 method, often successfully used in growing potatoes in dry 

 regions and seasons. The potatoes are planted shallower 

 than in the usual method and the entire field is covered with 

 several inches of straw. The potato plants grow up through 

 the straw, but most of the ordinary weeds do not. The 

 straw prevents evaporation of water from the surface, 

 and in dry weather the soil is found to be moister than soil 

 treated by the dust-mulch method. In fact, a great ob- 

 jection to the method is that the soil often becomes too moist 

 after heavy rains. The same method is often used in or- 

 chards, cutting the weeds and grass and spreading them over 

 the ground around the trees, instead of cultivating the soil 

 to keep in the moisture. 



It is obvious from the foregoing discussion of water in 

 soil that this is one of the most important problems connected 

 with growing useful plants, and horticulturists and agricul- 

 turists have found it important to understand the scientific 

 facts concerning the water of the soil and its use by plants. 



86. Absorption of Water by Roots. That pressure is 

 developed when water is absorbed by the roots may be 

 demonstrated as follows : 



(D) Cut the stem of a plant near the ground, and attach a glass 

 tube by means of rubber tubing as shown in Fig. 35 ; but use a 



