14 SOILS 



appreciably to it, and that by forking up or ploughing the surface 

 layers so as to maintain these in a loose state, their action would 

 reach to a greater depth, and be therefore more pronounced. It 

 will often be seen in the tropics that after a long period of drought 

 the soil will show signs of extraordinary fertility. It has been 

 , found in the case of soils which have become exhausted by inten- 

 sive culture in glass-houses, that by heating these to a temperature 

 of 140 Fah. fertility may be largely restored and all the eel- worms 

 and other injurious insects present killed, the soil becoming again 

 capable of producing healthy vigorous crops. 



Exhaustion of soils. The term is based on money values 

 rather than on scientific conceptions, and has no precise definition. 

 Although no soil can be entirely exhausted, generally speaking it 

 becomes so, as regards any particular crop, whenever the cost of 

 cultivation comes to as much as the crop is worth. Thus the word 

 "exhaustion" refers to the available character of the plant-food 

 present in the soil. Most cultivated soils contain a store of plant- 

 food which it would take many crops to exhaust ; but a large 

 proportion of the elements of this food is present in such a form 

 that plants are unable to make use of it. Thus an acre of land 

 may contain many thousand pounds of phosphoric acid, potash, or 

 nitrogen, and yet be in "poor condition;" while an application of 

 a suitable fertiliser, supplying 50 pounds of readily available 

 phosphate or nitrogen may greatly increase its productiveness. 



Theory of toxic substances in the soil. According to recent 

 theory, infertility in a soil, i.e., inability to produce good crops, 

 may sometimes be traced, not necessarily to the absence of plant- 

 food, but to the presence of compounds injurious to plant-growth. 

 These compounds are attributed to decomposition products of plant- 

 tissue, excretions from growing roots and germinating seeds, etc. 

 Crops are thus supposed to form toxic substances in the soil which 

 are deleterious to the crop following. The presence of excessive 

 quantities of organic matter in the soil, as applied in the form of 

 organic manures, is also considered capable of producing similar 

 toxic effects, and the action of suitable artificial manures, proper 

 rotation of crops, good drainage, etc., is to neutralise or destroy 

 these. A special advantage of crop rotation is that, while the 

 excreta of one kind of crop may not be harmful to another, 

 different species assist, it is considered, in the destruction and 

 removal of the excretions of others. 



