IO 



larger quantities of phosphates and potash, the land will be 

 so much the poorer in those substances. If its fertility is to 

 be kept up, they must be returned to the soil in the form of 

 manure. But that is no reason why nitrogenous manures 

 should be considered as mere stimulants, and not as plant 

 food. A favourable season might equally well be considered 

 exhausting to the land ; for it produces large crops which, of 

 course, remove more plant food from the soil than small 

 ones would take up. With regard to lime, however, manures 

 in which the nitrogen is in the form of salts of ammonia are 

 exhausting ; for they cause it to be carried away in the 

 drainage from the land. This is, of course, of most practical 

 importance on soils deficient in lime, which in other ways 

 are less suitable for ammoniacal manures than those in which 

 lime is abundant. The whole question as to the advis- 

 ability of using nitrogenous manures becomes one of money. 

 If the cost of the extra manures used, both to produce the 

 crop and to compensate for the extra demands made upon the 

 land, is greater than the increased returns that may be 

 expected from their application, nitrogenous manures will 

 not be remunerative. This, however, is very seldom the case 

 on ordinary farm land, for the judicious and moderate use of 

 nitrogenous manures enables the crops to make use of mineral 

 matter which would otherwise lie idle in the soil. 



EFFECT OF NITROGENOUS MANURES ON THE PLANT. 



Having satisfied ourselves that the proper use of nitro- 

 genous manures is advantageous, we may pass on to con- 

 sider generally their action on plants. 



Comparing plants grown with a full supply of nitrogen 

 with those receiving little, the most noticeable difference is 

 that, if all other conditions for healthy growth are fulfilled, 

 the plants receiving nitrogen produce a much larger leaf 

 growth of a very dark colour. The importance of this will 

 be recognized when it is remembered that the leaves are the 

 organs by which the plant feeds on the air taking the 

 element carbon from the carbon dioxide which the air con- 

 tains; and that the green colouring matter, called chloro- 

 phyll, is the necessary agent in this action. The nitrogen 

 may thus be said to increase the apparatus of the plant for 

 the assimilation of carbon. Mineral substances are neces- 

 sary for the building up of the carbon into such compounds 

 as starch, sugar, and cellulose, which are called carbo- 

 hydrates. Potash particularly seems to be needed for this 

 purpose an important fact, to which we shall refer again. 



Another effect resulting from the use of nitrogenous 

 manures is that the maturing of the plant is somewhat 

 hindered, especially if the manure is applied late in the 

 crop's growth. Provided the crop is grown early enough 

 in the season to allow of its maturing properly, this is no 

 disadvantage, as it gives the plant a longer time in which 



