42 R. GREIG-SMITH. 



Part III. — Some Aspects of Soil Fertility. 



When we consider the fertility of a soil, we naturally 

 take into account all the factors that go to produce or are 

 likely to be employed in obtaining a good crop. The measure 

 of the fertility is the crop itself, and this is influenced by 

 the nature of the soil, its texture, its capacity for holding 

 a suitable amount of water and air, its composition which 

 determines the food materials likely to be at the disposal 

 of the plant roots, the labour required to maintain its 

 physical condition, the climate which guarantees an 

 adequate rainfall and suitable temperature, the drainage 

 either natural or artificial to remove an excess of water 

 and so on. 



These factors have an influence upon the changes that 

 are taking place in the soil, whereby the constituents are 

 altered into substances, some of which are injurious, some 

 of which are beneficial. The nature and extent of these 

 changes may be controlled, and experience has shown how 

 the soil should be treated to obtain a maximum amount of 

 the beneficial substances. 



We may look upon the soil as being a vast laboratory in 

 which the workers are the micro-organisms, while the 

 management sees to it that the ventilation and the water 

 and drainage arrangements are efficient, and that the 

 quantity of material necessary for the workers is always 

 ready. The workers are specialists. Some for example, 

 attack proteid, reducing the combined nitrogen to ammonia 

 and no further, others attack the ammonia and convert it 

 to nitrite, while yet another group change the nitrite to 

 nitrate. Some forms can only attack urea converting it 

 into ammonia. Many convert nitrate to free nitrogen, 

 while others reverse the procedure in part and elaborate 

 the useless free aerial nitrogen into valuable combined 

 nitrogen. There are very few substances in the soil that 



