46 



General Botany 



It is therefore evident that to obtain the best crop yields it is 

 not only essential to have sufficient nutrient salts in the soil but 

 that the temperature and light conditions be favorable for photo- 

 synthesis. Water must always be available. It is possible to 

 increase crop yields by increasing the supply of carbon dioxide 

 and water, as well as by adding more mineral salts (fertilizers) 

 to the soil. 



Amount of food produced per acre. Since the food supply of 

 all living beings depends primarily upon these synthetic processes 

 that are carried on in plants, it is of interest to inquire how much 

 food may be derived from an average acre of land when planted 

 to different crops. It must be remembered that the plants that 

 produce this food take a considerable part for their own main- 

 tenance, and that the part which the farmer harvests is the 

 plant's surplus. The following table shows the average yield per 

 acre, its food value calculated in Calories, and the number of 

 men that i acre planted to different crops might feed for i day, 

 assuming that each man required 3000 Calories per day : 



If the plant products of an average acre are fed to cattle, the 

 dressed beef produced amounts to only 125 pounds, yielding an 

 energy equivalent to the food of 43 men for i day. If trans- 

 formed into pork, the yield is 273 pounds, or sufficient food for 



1 A Calorie is the amount of heat necessary to raise the temperature of i kilo of 

 water to i degree Centigrade. 



