CROP COMPOSITION 223 



a slow rate. In the spring and early summer when the 

 demand of the young plant for protoplasmic material is 

 greatest, the relatively small quantities of available 

 nitrogen in the soil, accumulated since the last harvest, 

 are eagerly and quickly absorbed. From then on, the 

 small supply of nitrogen is that made available by nitrifi- 

 cation and similar processes. Moreover, after the proto- 

 plasmic materials have once been made, it is doubtful if 

 the plant's demand for nitrogen continues unabated. 



The protein in the plant is thus obtained in the early 

 stages of growth. Carbon assimilation, however, con- 

 tinues until the period of ripening, and the more water 

 used the more dry matter produced. In any case, after 

 the first periods of growth, dry matter is formed more 

 rapidly than protein. Consequently, the percentage of 

 protein in the dry matter decreases as the plant grows 

 older and as more water is used. This seems to be the 

 simplest explanation of the important law that the larger 

 the irrigation, the smaller the percentage of protein in the 

 dry matter produced by plants. 



Whatever explanation may be found for this law, the 

 fact remains that the decreasing percentage of protein 

 with increasing irrigation is another strong argument in 

 favor of the use of little water in the production of crops 

 under irrigation. 



131. Fat. The quantity of fat in plants, except in 

 the few crops especially grown for their fat content, is 

 so small as to be of little consequence. Little is known of 

 the effect of irrigation on the content of fat in plants. 

 The more water used in irrigation, the larger, usually, the 

 percentage of fat in the plant. This, however, is subject 

 to revision as more knowledge concerning the matter is 

 obtained. 



