544 Joiinial of A^i;ric///f//ri\ Victoria. [lo Sept., 191 2. 



indispeiisible fraction, and, as will l)e seen later, on this simple fact rests 

 the whole practice of manuring. 



So far as chemical considerations are concerned, it would appear that 

 a fertile soil is one which contains a suflficiency of plant foods — and par- 

 ticularly nitrogen, phosphoric arid, and potash — in an available or soluble 

 form. J'>mphasis must be laid on the presence of available plant food, 

 becau,se the greater portion of the plant food is locked up in inert forms, 

 i.e., in forms incapable of being used by the plant. It may, therefore, he 

 readily underst(x>d that, while a soil may contain large reserves of nitrogen, 

 phosphoric acid, and potash, it may yet be unable to support the growth of 

 a decent crop. Such a soil would pos.sess potential fertility, but could 

 only become really fertile bv methods of soil treatment which would bring 

 about a conversion of the dormant plant food into available forms. 



It must be clear, also, that under certain circumstances the amount of 

 available plant food in the soil may be reduced to such a low level that 

 there is insufticient for the requirements of a normal crop. Under these 

 circumstances, the soil is said tO' be " worn out,'' or " exhausted." 



It was formerlv supposed that the fertility of a soil could be determined 

 bv chemical analvsis. Soils were analysed, and the analyses referred to 

 arbitrary standards, and judgment was given on the agricultural possil)ili- 

 ties of the soil under re\'iew. If, for example, a given soil was analysed 

 and found to contain o.i per cent, of nitrogen, o.i per cent, of phosi)horic 

 acid, and 0.2 ])er cent, of potash, it was judged to be a good soil. But no 

 agriculturist who knew his business would attempt to judge a soil merely 

 bv the results of its chemical analvsis. and to say what crops should be 

 grown and what manures should be a])[)lied. 



Soil analysis can tell us the total reserves of food stored up in the soil, 

 or it may inform us as to the amount of food soluble in dilute solution of 

 citric acid, but it can throw very little light on the forms in which the 

 elements of plant food actuallv exist in the .soil, and as to the amount 

 actually available for a wheat crop. 



Physical Condition, of the Soil. — A soil exceedinglv rich in total 

 nitrogen, phosi)horic acid and potash, and also in its available i)lant food, 

 may yet be incapable of growing a blade of grass, much less a satisfac- 

 torx crop. This may be the case with naturally rich but Ijadly <lrained 

 soils, and .sour land, such as, e.g., one would expect in a swamp. 



Hence, to be fertile, a soil must not only be rich in plant food, but it 

 nmst possess a satisfactory physical condition. Howe\"er rich a soil may be 

 in plant food — total or available — its fertility will be low. if it is sour, or 

 in need of draining, unretentive of moisture, or of poor texture, and low 

 capillary power. The physical condition of the soil, if unsatisfactory^ and 

 inimical to successful cropping, may be improved by tillage, drainage, and 

 the use of soil amendments. These, however, lie outside the scope of the 

 present article. 



Closelv correlated and intimately bound u}) with the physical condition 

 and chemical composition, is what may be termed the biological condition 

 of the .soil. 



Biological Condition of Soil. — The soil used to be regarded as a mere 

 inert mass of matter void of all vitality. We now know that it is really 

 a vast laboratory swarming with millions of microscopic bacteria, and 

 that these bacteria play a most important part in the nutrition of plants. 

 The importance of these lowlv organisms may be appreciated bv briefly 

 referring to the functions of the more important bacteria found in every 

 soil. 



