SOIL EXHAUSTION AND EOTATION OF CROPS. 167 



soil which he used was not a large absorbent or fixer of the 

 substances furnished to the plant. The nitrogen which he 

 used was in the form of nitrates, which are never absorbed 

 by soils, so far as we know. The matters with which he en- 

 riched the sand, therefore, were soluble and entirely available 

 to the plant. The latter had only to stretch out its roots to 

 obtain its food, and the quantity of soil was small, so that the 

 roots had not far to travel, and could so completely occupy 

 the soil as to come in contact with all the nourishment it con- 

 tained. 



Question. Does nitrogen form a part of the plant ? 



Peof. Johnson. Yes ; an important part, always. 



Question. How large a part ? 



Prof. Johnson. In the entire plant, when dry, from one- 

 half to two per cent. In the different parts of plants it va- 

 ries greatly. You have fifteen per cent, of nitrogen, for 

 example, in the gluten of wheat; one and one-half to two per 

 cent, in the wheat grain ; you have no nitrogen whatever in 

 pure cotton fiber ; there is no nitrogen in the sugar or in the 

 starch of the plant. 



Question. Does it exist in the wood in the form of 

 nitrate ? 



Prof. Johnson. No ; but in the form of what is called al- 

 buminoids ; something which is similar to the albumen or 

 white of the eggs of animals.* 



Mr. S. L. Goodale, of Saco, Maine. What is the compara- 

 tive value of a given amount of nitrogen in ammonia salts and 

 in animal substances, such as blood, flesh, dung? 



Prof. Johnson. It is very difficult to say ; but these ex- 

 periments of Mr.Lawes show that in order to get thirty-six 

 bushels of wheat to the acre, he used two hundred pounds of 

 nitrogen, in the form of stable manure, whereas eighty 

 pounds of nitrogen, used in the shape of salts of ammonia, 

 gave the same crop. The reason of that is, that the nitrogen 

 of the salts of ammonia is in a condition to be made immedi- 

 ately available to the plant, whereas the nitrogen in animal 



* See " How Crops Grow," pages 94 to 109. 



