take it up when it is fixed or combined with oxygen or some 

 other element. Combined nitrogen is supplied naturally in 

 rain-water, and in dew and hoar-frost ; but the total average 

 quantity so added to the soil was estimated at Rothamsted* 

 to be only about 4^ Ibs. per acre per annum a good deal 

 less than is required by any ordinary farm crop. 



Clover, beans, and other leguminous plants make the soil 

 on which they grow richer in combined nitrogen. It was 

 first shown by Hellriegel and Wilfarth that this family of 

 plants normally have on their roots small nodules, contain- 

 ing bacteria which have the important power of using the 

 free nitrogen of the air, and preparing it for the use of their 

 hosts. The amount of nitrogen thus obtained, and left in 

 the soil as root and leaf residue, is often very large. These 

 facts must be remembered when we come to deal with the 

 effects of nitrogenous manures on leguminous crops. 



LOSSES OF NITROGEN FROM THE SOIL. 

 Whatever the quantity of nitrogen in the soil it is always 

 liable to loss, apart from its removal by crops. Nitrogen in 

 the soil may be said to be always in one of three forms viz. 



i. Nitrogenous organic matter. 



2. Ammonia. 



3. Nitrates. 



In the first form, as organic matter i.e., the remains of 

 plant or animal life the nitrogen is for the most part safe 

 from loss. Sooner or later, however, it is liable to be acted 

 upon by bacteria in the soil, and changed into the second 

 form, when in the majority of soils it is still retained ; clay 

 and humus being able to hold or "fix" ammonia, while 

 sand, gravel, and lime have no absorptive power, as it is 

 called. Eventually organic and ammoniacal nitrogen will 

 undergo a further change, through the action of micro- 

 organisms, and will be converted into the third form that 

 of nitrates. It is then that serious loss takes place ; for 

 nitrates cannot be retained permanently by any soil, but are 

 lost by drainage following rain. More will be said with 

 regard to both the absorption of ammonia by soils and the 

 formation of nitrates, when considering the action of sulphate 

 of ammonia. 



INCREASE FROM NITROGENOUS MANURING. 

 Seeing, then, that the natural supply of nitrogen to the 

 soil is generally insufficient for the farmer's needs, and that 

 loss is continually occurring, it is obvious that the supply of 

 nitrogen in suitable form and quantity must increase the 

 return from the land, and, indeed, must be an essential part 

 of farm management. Both field and laboratory experiment, 

 as well as general agricultural practice, fully bear out this 

 conclusion. Out of the great mass of evidence as to the 



* lt Journal of the Koyal Agricultural Society of England," Vol. XIX., S.S. 



