66 THE FARMERS HANDBOOK. 



formed in the rock by the mechanical action of wind and rain. As the water 

 evaporates the salt crystallises out, expanding in the process, and thus aiding 

 the work of disintegration. 



The Action of Vegetation. 



As soon as a very small quantity of disintegrated soil is produced, it 

 becomes at once the medium for the germination of small seeds, wind-blown 

 or dropped by birds. As these seeds germinate and grow they assist in the 

 disintegration of the rock below, mechanically by forcing their roots into its 

 mass, and chemically by the solvent action possessed by their roots. Even 

 on the solid rock vegetation can gain a foothold. The first forms of vegetation 

 are the lichens which cover the surface of the rock in moist and sheltered 

 places. These plants secrete oxalic acid — a much stronger acid than car- 

 bonic acid — which attacks the lime in the rock. Crystals of oxalate of lime 

 are frequently found in lichens. By their growth they exclude light and air, 

 and help to keep the rock moist, thus favouring its further denudation. 



When a certain amount of soil has been produced by the growth and decay 

 of these lichens other forms of vegetation can obtain a foothold — mosses, 

 ferns, stone-crops, and saxifrages, followed by other shallow-rooted plants. 

 By the decay of these and the continued disintegration of the rock, the soil 

 gradually increases in depth until higher plants and trees are able to find 

 support and nourishment. 



Bacteria. 



Bacteria and micro-organisms generally play a highly important part, not 

 only in the transformation of organic material within the soil (which will be 

 discussed more fully under the heading " The Plant's Supply of Nitrogen " 

 (page 75, et seq.), but also in the formation of soil from the disintegration of 

 rocks. The bare surfaces of rocks contain large quantities of organisms 

 which, with the aid of the oxygen, ammonia, watery vapour, and carbonic 

 acid of the air, are capable of decomposing the rock and of forming nitrates 

 and accumulating carbon. 



Factors which determine Fertility. 



The fertility of a soil depends only partly on its contents of plant-food 

 (potash, lime, phosphoric acid, and nitrogen), and partly on the power the 

 soil possesses of making use of these compounds. We have to take into 

 account the other characteristics which conduce to fertility, especially the 

 power possessed by the soil of changing the condition of the plant-food. 



The soil is not an inert mass of material, out of which the plant picks 

 whatever material is present for its nourishment, and, having exhausted that, 

 dies. When we talk of changes that take place in the soil, we must realise 

 that the changes are constantly going on, that the material of which the soil 

 is composed is continually altering, that the growth and decay of plants, 

 the movements of underground animals and of minute organisms, the fall of 

 rain, the evaporation of moisture, alternations of temperature, of night and 

 day, of summer and winter, even alterations of atmospheric pressure, the 

 passing of clouds, and countless other phenomena of which we take no heed, 

 or whose action we do not yet fully understand, all these agencies produce 

 an incessant series of changes within the soil. When we add to these the 

 changes produced by human agencies — by cultivation, by ploughing, liming, 

 manuring, &c, &c. — it will be seen at once that a mere statement of the 



