WHAT PLANTS NEED 303 



vigorous growth, if continued too long, will delay ripening. 

 No definite rules for watering can be given, for it is only 

 experience in observing the condition of plants that can 

 suggest the amount of water necessary. 



In determining what crops can be grown to the greatest 

 advantage on a given area, the supply of water must be taken 

 into account. For example, where there are drier and wetter 

 areas, wheat would be appropriate for the drier ground and 

 meadow grass for the wetter, and so for each crop. The 

 ability to select the most suitable plant for a given area is one 

 of the first things a cultivator of plants must acquire, unless 

 the water supply is under control. 



In considering the other materials needed by the plant, it 

 will not be necessary to include all that have been found to 

 be used by plants, but only those that have proved to be the 

 most important. 



11. Carbon dioxide. Of course carbon dioxide must be 

 ranked with water as of first importance, because these two 

 materials are used in the manufacture of carbohydrate food, 

 upon which also depends the manufacture of other foods. 

 Since carbon dioxide enters the plant from the air, its supply 

 is usually sufficient, and in any event there is nothing to do 

 in the way of regulating the supply. 



12. Nitrogen. The other materials needed by the plant 

 are obtained from the soil, and with these the practice of 

 agriculture has very much to do. Chief among these ma- 

 terials are the compounds of nitrogen. Although nitrogen 

 forms a very large part of the air, it cannot be used by plants 

 in its free condition, but must be obtained from its com- 

 pounds. Of all the compounds of nitrogen, it appears that 

 the nitrates are most used by plants. It is evident, there- 

 fore, that nitrates must exist 'in the soil if plants are to be 

 grown ; and that if they become insufficient in amount, 

 they must be restored in some way. As in the case of other 

 things that plants need, the nitrates may be not only insuffi- 



