56 THE PHYSIOLOGY OF PLANTS 



in our present system of agriculture, they cannot replace 

 stable manures entirely. Our agricultural efforts must aim 

 at getting as much animal dung as possible, and to use the 

 mineral manures as additions to the former. For besides 

 taking into account the chemical constitution of the soil, we 

 must also bear in mind its physical or mechanical structure, 

 and the latter is greatly improved by stable manure, but is 

 rarely benefited — sometimes indeed deteriorates — through 

 mineral manures. As an example of the deterioration of good 

 soil under such conditions, the continuous manuring with 

 saltpetre and common salt might be cited. After successive 

 manurings of a clay soil with saltpetre, the finest particles of 

 clay will be found washed together and deposited in a solid 

 and compact form. 



As the nitrates are so easily washed out of the soil, the 

 latter often becomes so caked that plants cannot be grown in 

 it. Manuring with saltpetre may, therefore, at first produce 

 fine green crops, but soon a sudden deterioration will be 

 noticed. Similar results follow on the manuring with common 

 salt, which is carried on in the case of certain crops. Soils 

 which contain basic carbonates in large quantities (alkaline 

 soils) have a consistency which renders their working quite 

 impossible. 



(d.) Stable Manures. 



As it is the first endeavour of an agriculturist to produce 

 the most favourable physical conditions of a field and to 

 preserve them, because otherwise chemical changes take place 

 which are detrimental to the life of plants, the chief point 

 which has to be considered in manuring a field is to find a 

 manure which will serve the above purpose best, and this is 

 certainly stable manure. 



The ideal condition of a soil is one in which it resembles a 

 sponge, and in which it will retain the greatest amount of 

 nutritive substances and water, without losing its capacity of 

 absorbing air. 



Heavy soils can be made to approximate this condition by 

 addition of straw-containing manure, while a light soil requires 



