STUDIES IN NATURE 



People who have much to do with land can 

 often tell by the look of the ground and by the 

 appearance of the crop on it what substances 

 it requires, and of what treatment it is in need. 

 They will know what sort of manure to put on 

 it, and the best time to lay it on ; for land that 

 is properly manured one year and has crops 

 grown on it in the right order, will not require 

 further attention in the way of manure for some 

 time. It will be several years before the sub- 

 stances added to it are taken out by the different 

 crops grown on it. Indeed, some plants, such 

 as beans, peas, and clover, have actually the 

 power of improving the soil in certain ways, 

 and leave it in a better state for the next year's 

 crops. They are able to take the nitrogen out 

 of the air and store it up round their roots in 

 the ground ; and nitrogen is one of the sub- 

 stances which it is most necessary to plants and 

 animal life. 



(102) 



