CHAPTEE VI. 

 MANURING AND GENERAL MANURES. 



FROM what has been said in Chapters III, IV and V, it is evident 

 that the fertility of a soil depends upon a number of circumstances ; 

 among others, upon the existence of suitable relationships between its 

 properties regarded from 



(1) Mechanical, 



(2) Chemical, 



(3) Biological, 

 points of view. 



(1) The possession of the proper fineness of subdivision, porosity, 

 water-holding power, and other physical properties is essential to per- 

 mit of the ready growth of a plant's roots, to afford the necessary 

 mechanical support and access of air to the roots, and to provide them 

 with a due supply of moisture. If these properties are lacking, no 

 matter how rich the soil may be, it cannot possess, in the highest de- 

 gree, true fertility. 



(2) So, too, from a chemical standpoint, the possession of a suitable 

 store of plant food in an available condition is obviously necessary. If 

 the plant food, or any part of it, be present in insufficient amount, or 

 in a completely insoluble or unavailable form, the soil, however suit- 

 able in mechanical structure, must be incapable of yielding the best 

 results. 



(3) The conversion of one of the most important constituents of 

 plant food, nitrogen, from a state of combination in the insoluble, highly 

 complex organic substances associated with humus into the directly 

 assimilable nitrates, is, as already described (Chap. IV), accomplished 

 by the action of micro-organisms, so that it is evident that the realisa- 

 tion of one of the above conditions as to availability of plant food in 

 the soil can only be effected if the necessary micro-organisms or 

 their spores are present. Even more dependent upon the presence of 

 suitable micro-organisms is the utilisation of the free atmospheric 

 nitrogen by the leguminosce. Fungi, yeasts and other low forms of 

 vegetable life, as well as worms, caterpillars and larvae, also play an 

 important part in altering both the physical and chemical properties of 

 soil. Unfortunately, in the case of the latter organisms, their influence 

 on the plant itself often exceeds in importance their effect on the soil. 



Improvements in the soil are naturally the aim of the agriculturist. 

 As regards its physical condition, much can be done by tillage opera- 

 tions ploughing, harrowing, draining, etc. also by the introduction 

 of materials which affect the mechanical properties of its constituents, 

 e.g., lime, and bulky vegetable refuse. 



With reference to the actual plant food in a soil, improvements can 

 be effected in two ways 



(103) 



