164 SOILS AND MANURES 



in cultivating it. It does not imply that the soil has been 

 rendered absolutely barren by the removal of the last 

 vestiges of available plant foods. Such a condition is prac- 

 tically unattainable, and, if it were, it would not be per- 

 manent. The conditions which originally gave rise to the 

 formation of the soil are in constant operation, and avail- 

 able plant foods are constantly being produced from the 

 original sources. The reserves of plant foods in a non- 

 available state are practically inexhaustible. 



The natural productiveness of soils is, of course, an 

 indefinite term. It refers to the amount of produce that 

 can be obtained from a given area of land when nothing 

 in the shape of manure is added to it. This, however, 

 depends very largely upon whether or not the produce is 

 removed. Let it be supposed that the physical and bio- 

 logical conditions of fertility are satisfied, and that pro- 

 ductiveness is limited only by the supply of available plant 

 foods. If the crops are not removed from the land, all 

 the ingredients will be, in time, restored to it, and again 

 become available for the growth of crops. During the 

 interval occupied by the growth and decomposition of the 

 plants, a further quantity of available plant food is formed 

 from the original sources and is added to the ingredients of 

 the previous crops. A larger growth is thus rendered pos- 

 sible, and the productiveness of the soil is increased. 



The formation of available plant food in the soil, the 

 abstraction of the same by growing crops, and the decom- 

 position of the plants with the reconversion of the ingre- 

 dients into the available state, constitutes a cycle of changes 

 which might, theoretically, go on for ever without gain 

 or loss, thus : 



Crops - > Humus 



Soil 



