INFLUENCE OF CLIMATE AND SEASON. 59 



crop ? Will it pay to use potassium salts for his seeds, 

 his pasture, or his potato crop ? These and many other 

 questions can only be answered by trials on his own 

 fields. On the farmer's knowledge of such facts will de- 

 pend the economy with which he is able to use purchased 

 manures, which are too often wastefully employed. 



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 evapora- 

 tion is very large, the necessity of a large 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, 

 lucerne, sainfoin, 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 from the atmosphere under the influence of 

 light, and that a certain temperature is necessary for this 

 assimilation of carbon and for the other chemical pro- 

 cesses which proceed in a growing plant a sufficient 

 supply of light and heat is therefore required for 

 the production of a crop. In a season of deficient light 

 and heat the harvest is always late, growth having taken 



