416 SOILS: PROPERTIES AND MANAGEMENT 



nent part in the treatment, as it keeps the soil in a mechan- 

 ical condition favorable to retention of moisture, which 

 vegetables require in large amounts, and it also supplies 

 needed fertility. The very intensive method of culture 

 employed in the production of vegetables necessitates the 

 use of much greater quantities of manures than are used 

 for field crops, and the great value of the product justifies 

 the practice. 



336. Fruits. — In manuring fruits, with the exception 

 of some of the small, rapidly-growing ones, it is the aim 

 to maintain a continuous supply of nutrients available 

 to the plant, but not sufiicient for stimulation except 

 during the early life of the tree, when rapid growth of 

 wood is desired. An acre of apple trees in bearing re- 

 moves as much plant-food material from the soil in a 

 season as does an acre of wheat. Farm manure and a 

 complete fertilizer may be used, of which the constituents 

 should be in a fairly available form, as a constant supply 

 is necessary. A young growing orchard requires con- 

 siderably more nitrogen than does an old orchard. Some 

 nitrate of soda in early spring is desirable. 



337. Mineral substances absorbed by plants. — The 

 plant, in its process of growth, withdraws from the soil 

 certain mineral substances that are presented to its roots 

 in a dissolved condition. As the salts in solution are 

 rather numerous, and as the osmotic pressure by which 

 the absorption is accomplished does not admit of the entire 

 exclusion of any ion capable of diosmosis, there are to be 

 found in the plant most of the mineral constituents of the 

 soil. Some of these are concerned in the vital processes 

 of the plant and are essential to its growth ; others seem 

 to have no specific function, but are generally present. 



The substances commonly met with in the ash of plants 



