60 MICRO-ORGANISMS IN SOILS 



reduced to the ferrous condition by the organic matter, dissolve as 

 ferrous bicarbonate in the water of the soil ; the solution on exposure 

 to air, absorbs oxygen and deposits its iron as ferric oxide or basic ferric 

 carbonate, forming an incrustation in the soil. Such iron pans are 

 often very hard. 



" Pans " of any kind are very objectionable and greatly diminish 

 fertility. They interfere with drainage and with the rise of water from 

 below, as well as proving impassable to roots. It is, therefore, neces- 

 sary to break them up by subsoil ploughs or other mechanical means. 



CHANGES IN THE ORGANIC MATTER. Important as the 

 complex and little-known changes which affect the inorganic portions of 

 soils undoubtedly are, quite as much interest and perhaps more fruitful 

 labour have been directed to the study of the changes undergone by 

 the carbonaceous and nitrogenous constituents. The organic matter in 

 a soil is continually undergoing alteration, attended by the absorption 

 of oxygen, and the consequent evolution of heat. This heat emission 

 becomes greater when farm-yard manure is applied to land, and in 

 some cases has been sufficient to raise the temperature of the soil 1 or 

 2 C. (Wagner). The air in the interstices of a soil is always poorer 

 in oxygen and much richer in carbon dioxide than the air above it. 



It is by changes of this kind that humus is produced from vege- 

 table fibre (vide p. 48). Other organic acids are also formed by oxida- 

 tion of vegetable matter, and if the soil be deficient in basic materials 

 these acids may exert a baneful influence (as in so-called " sour " 

 land). 



Humus itself is not a permanent substance, but is continually being 

 oxidised and broken down in a soil, a portion of its carbon being 

 evolved as carbon dioxide, while its nitrogen passes eventually into the 

 form of nitrates. 



Biology of the Soil. These chemical changes are brought about 

 chiefly through the life processes of minute organisms existing in the 

 soil. An ordinary soil is teeming with living beings, the majority of 

 which belong to those lowest forms of life bacteria, but other organisms, 

 yeasts, moulds, algae, larger fungi, protozoa and amoebae are also present. 



These are engaged in a perpetual struggle for existence and 

 whenever the circumstances become favourable, are busily engaged in 

 carrying on the chemical changes in the organic matter essential to 

 their growth. In some cases, the processes of one set of organisms 

 are favourable to those of others, while in others they are inimical and 

 a fierce battle for supremacy may be going on. 



It would be out of place to attempt to give here any detailed ac- 

 count of the micro-organisms of soil, or any reference to the methods 

 used in investigating their nature and functions. Such matters belong 

 to the domain of biology and bacteriology, subjects which now have 

 extensive literatures of their own. 



But the influence of the micro-organisms in a soil upon fertility is 

 of great importance and it is advisable to consider the character of the 

 chemical changes which are induced by their lite processes and to pay 

 some attention to the circumstances which affect their activity. 



