PLANT-FOOD MATERIALS IN SOILS 105 



Without going over this table in detail, it may be noticed 

 that the effect of both farm manure and commercial fertiliz- 

 ers is to increase the percentage of plant-food materials in 

 the drainage water. 



123. Drainage water from different soils. — The composi- 

 tion of the drainage water varies with different soils. 

 Table 20 in which the composition of the drainage water 

 from Dunkirk clay loam and Volusia silt loam is given, is 

 an illustration of the very considerable differences that 

 may occur in this respect. The more productive soil has 

 lost the greater quantity of plant-food material. The rates 

 of loss, however, are not proportional to the amounts of 

 plant nutrients that the soils contain. The Dunkirk soil 

 contains less nitrogen than the Volusia, but has lost more in 

 the drainage water. 



124. Absorption of food materials by plants. — It is only 

 when substances are in solution that they may be absorbed 

 by agricultural plants. This means that the soil from which 

 plants draw their nourishment must contain water. Plants 

 absorb both water and nutrient salts through their roots, 

 more especially through the root-hairs, as these have very 

 delicate walls through which solutions may readily pass. 

 The movements of water and of salts through the walls of the 

 root-hairs are independent of each other. When the weather 

 is very hot and dry, a larger proportion of water to salts will 

 pass into the roots than when the weather is cool and moist. 



125. How plants absorb nutrients. — When a solution 

 of plant nutrients is brought in contact with roots, there is 

 a tendency for the solution in the inside of the root and that 

 on the outside to become of the same strength for each par- 

 ticular substance in the solution. Thus, if there is much 

 available nitrogen in the solution, it will be absorbed in 

 greater quantity than if there were very little. Then, when 

 the nitrogen in the plant juice is utilized by the plant to 



