THE SOIL. 73 



every other particle through the medium of the water - 

 envelopes. This flow of water from particle to particle 

 explains how a plant may be able to supply itself with 

 water from a comparatively dry soil, and how it may be 

 nourished by particles with which its roots never come 

 in contact, by the gradual solution of these particles in 

 the soil water. 



When a soil is very wet the water is easily drawn from 

 its particles, but as it dries it becomes more and more 

 difficult, as a thin layer of water is more firmly retained 

 on a particle than a thick one. A plant can take a very 

 large proportion of the water from soil but cannot com- 

 pletely dry it. A time at last comes when the osmotic 

 movement into the cell cannot overcome the attraction, 

 between water and particle. 



In a fertile soil there must always be air spaces from 

 which the soil water may constantly dissolve and carry a 

 supply of air to the plant. In a rice field, constantly 

 irrigated, there may be no air spaces, but their loss is 

 compensated for by the air which is always present 

 dissolved in running water. 



The micro-organisms in the soil live in the soil water 

 and in the wet organic matter, and doubtless they are 

 able to move about in the larger spaces between the soil 

 particles. 



When the farmer buys a soil he obtains a certain capital 

 of raw plant food, a physical apparatus more or less 

 suited for the growth of crops, and certain micro-organisms 

 to carry on the preparation of plant food. Further, he 

 stipulates for a water supply, and an annual deposit of 



