AN INTRODUCTION TO PLANT BIOLOGY 87 



84. Water in Soil. How can plants get water from soil 

 which appears to be very dry? The answer to this question 

 is that the apparently dry soil is not without water. It is 

 not necessary, or even desirable, for many plants that the 

 soil be wet ; that is, contain free water which might be 

 drained off. On the contrary, it is best for most plants 

 if the soil contains moisture in the form of thin films ad- 

 hering to the particles of the soil. We have already noticed 

 that the root-hairs adhere to the soil particles (Fig. 26), and 

 therefore they are in the best possible position for absorbing 

 the moisture of the soil.* 



85. Rise of Water in Soil. For our plant studies the most 

 important points concerning soil moisture are as follows : 

 The water stored in the deeper layers of the soil supplies 

 moisture to the layers nearer the surface. This is due to 

 capillary attraction, of which good illustrations are the rise of 

 oil in an ordinary lamp-wick and of coffee into a lump of sugar 

 which just touches the liquid. Water is continually being 

 lost at the surface of the soil, owing to evaporation and to 

 absorption by plants, and it is as constantly coming up 

 from below. But it may be lost by evaporation from the 

 surface soil more rapidly than it can come from the deep 

 layers, especially if the surface is hard-packed, as on a road, 

 or if the surface is covered with large numbers of plants, such 

 as weeds or grass, which take much water from the soil. 



Mulching. Excessive loss of water from the surface of the 

 soil may be prevented by mulching, two kinds of which may 

 be illustrated by two ways of raising potatoes : 



(1) In the usual way the soil is cultivated on the surface 

 in order to kill weeds, which use water needed by the growing 



* If the students working with this book have not in some previous science 

 work had lessons on water in soil, it is suggested that some work in this line 

 be here introduced. Read Osterhout's "Experiments with Plants," pp. 

 103-121; and Burkett, Stevens and Hill's "Agriculture for Beginners," 

 pp. 10-15 ; and perform the experiments suggested. 



