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stated that, so far from water passing through the soil 

 being able to wash out of it "the goodness," the soil 

 exercises the most powerful attraction over the food 

 constituents of plants, and not only retains those ex- 

 isting in it but seizes these out of solutions passing 

 through it. Thus phosphoric acid, potash, ammonia, 

 and (except in soils verj poor in lime) silicic acid, the 

 most important of the food elements, obtained from 

 the soil, cannot normally be washed out of it by any 

 amount of subsoil drainage ; on the contrary, the soil 

 acts like a filter to arrest these and decomposes solu- 

 tions containing these which pass through it in order 

 to retain them. 



Of course there are limitations, most prominent 

 amongst which is the fact that for each soil there 

 seems to be a point of saturation, if we may use the 

 word, of each food constituent ; up to that point the 

 soil seizes all it can get of that element ; beyond that 

 point it allows it to pass away in solution. Nothing is 

 more greedily seized by most soils than potash, but 

 you may get bog-earths that allow potash solutions 

 to pass through them untouched. 



Of course, you may raise the point of saturation for 

 any element by adding more of some other element, 

 but this is in effect changing the soil. For each soil 

 the point of saturation for each food constituent is 

 liable to vary. 



Other minerals, not useful to plant-life, pass freely 

 in solution through the soil, and food constituents 

 even, when by the chemistry of the soil they have 

 assumed forms useless as the food of plants, or forms 

 of which there is already a full supply, pass away. 



