48 THE CHEMISTRY OF THE FARM. 



sufficient for his turnip crop, or should guano or nitrate be 

 employed as well ? What is the smallest quantity of 

 superphosphate sufficient for the crop ? Will it pay to 

 use potassium salts for his seeds or pasture ? These and 

 many other questions can only be answered by trials on 

 his own fields, and on the farmer's knowledge of such facts 

 will depend the economy with which he is able to use 

 purchased manures. 



Influence of Climate and Season. — The influence of 

 weather upon crops is far greater than the influence of 

 manure. 



As a plant contains water as its largest constituent, and 

 as the whole of the plant food obtained from the soil is 

 taken up through the medium of water, while the amount 

 of water daily lost by the plant through evaporation is very 

 large, the necessity of a sufficient supply of water in the 

 soil during the growing period of a crop is very evident. 

 On the other hand, an excess of water in the soil prevents 

 root development, and causes a loss of nitrates and other 

 soluble plant foods in the drainage water. Deeply rooted 

 crops, as wheat, red clover, and mangel, are those best 

 fitted to resist drought ; while shallow-rooted crops, as 

 grass and turnips, are those which suffer most from it. 



We have already seen that carbon, which forms the 

 largest ingredient of all vegetable substances, is obtained 

 by plants exclusively from the atmosphere under the in- 

 fluence of light, and that a certain temperature is necessary 

 for this assimilation of carbon, and for the other chemical 

 processes which proceed in a growing plant ; a sufficient 

 supply of light and heat is therefore plainly required 

 for the production of a crop. In a season of de- 



