304 ELEMENTARY STUDIES IN BOTANY 



cient in amount, but they may be excessive in amount, so 

 that too much as well as too little is injurious to the crop. 

 The plants show the symptoms of both these troubles, lack 

 of nitrates resulting in starved-looking plants, and an 

 excess of nitrates resulting in weedy-looking plants and 

 delayed ripening. For example, with an excess of nitrates, 

 cereals " run to straw " and ripen poorly. Of course, if 

 rank growth rather than grain or fruit is desired, as in the 

 case of cabbages, a larger amount of nitrates is helpful. 

 This will serve to illustrate how the use of nitrates will depend 

 upon the kind of plant, the age of the plant, and the product 

 desired. This means that the successful cultivator of plants 

 must develop an experience in the observation of his plants 

 that will enable him to recognize their symptoms. 



In connection with the problem of nitrogen supply, it is 

 important to know that leguminous plants have developed 

 an unusual method of securing nitrogen. These plants 

 include such crops as clover and alfalfa. In these cases, 

 therefore, the problem of nitrogen supply is very different 

 from that in non-leguminous plants. Such plants as clover 

 and alfalfa can draw upon the free nitrogen of the air, as it 

 circulates in the soil, by means of their association with 

 certain bacteria of the soil which have the power of " fixing " 

 free nitrogen in its compounds. Leguminous plants, there- 

 fore, are very useful in increasing the amount of nitrates in 

 the soil ; while non-leguminous plants, if removed as crops, 

 diminish the amount of nitrates. This fact explains the 

 significance of a very common form of " rotation of crops," 

 for it is obvious that the supply of nitrates in a soil may be 

 kept sufficient by alternating crops .of leguminous and non- 

 leguminous plants. The clovers and alfalfa are used exten- 

 sively in this way. 



13. Phosphorus. Another important group of substances 

 needed by plants and supplied by the soil are phosphates, 

 from which the growing plants obtain such phosphorus as 



