SOIL EXITAUSTION AND ROTATION OF CROPS. 215 



never absorbed by soils, so far as we know. The matters with 

 which he enriched 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? 



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



Question. How large a part ? 



Prop. Johnson. In the entire plant, when dry, from one-half to 

 two per cent. In the different parts of plants it varies greatly. 

 You have fifteen per cent, of nitrogen, for example, in the gluten 

 of wheat ; one aud one half to two per cent, in the wheat grain ; 

 you have no nitrogen whatever in pure cotton fibre ; 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 albumi- 

 noids ; something which is similar to the albumen or white of the 

 eggs of animals.* 



Mr. S. L. Goodale, of Maine. What is the comparative 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 experi- 

 ments 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 immediately available to the plant, whereas 

 the nitrogen in animal manure exists in a form or in forms such 

 that much of it cannot be taken up by the plant at once, if at all. 

 It must undergo an alteration to become of use, and much of it, 

 instead of passing into an .available condition, doubtless becomes 

 permanently inert. 



Mr. S. L. Goodale, of Maine. What are the conditions under 

 which the nitrogen of manure is converted into ammonia, which 

 is retained in the soil, and what the conditions in which it is con- 

 verted into nitrates, which may pass out of the soil ? 



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



