SOIL MAKING. 



5 



be even more harmful. Undecayed vegetable matter opens up the 

 soil much and facilitates evaporation so as to render difficult the 

 maintenance of proper moisture conditions. If there were no soil 

 population to act as scavengers it would be necessary to gather up 

 the leaves not only from paths and lawns, but from the flower beds 

 and vegetable ground as well. 



A further great effect of the soil population is to keep in circula- 

 tion the plant nutrients, and especially the nitrogen compounds. It 

 so happens that nitrates, which are by far the most important nitro- 

 genous foods, are easily washed out from the soil, so that there is 

 never any very large reserve there. The stock is quickly taken up 

 by plants and immobilized by conversion into protein. Once the 

 plant residues get into a normal soil they begin decomposing : the 

 process is now reversed and the proteins are converted into nitrates. 

 The dead remains of one generation of plants thus provide food 

 materials for the next generation. 



But they do more. Some of the soil bacteria have the remarkable 

 property of fixing gaseous nitrogen from the air and changing it into 

 protein : other bacteria can then attack the protein and convert it 

 into nitrates. The nitrogen fixation requires a source of energy, and 

 this is provided by the organic matter of the plant residues. Thus 

 the decomposition brought about by the soil organisms not only 

 makes available the nitrate stored in the old dead vegetation, but it 

 helps to increase the supplies by bringing in newly fixed nitrogen. 



Unfortunately Nature is full of reverse processes : the vegetable 

 remains not only bring about the fixation of nitrogen, but also the 

 loss of nitrogen. This reversal is quite orderly, and the conditions 

 are definite ; they have recently been discovered in the laboratory. 

 The useful process — the fixation of nitrogen — occurs after certain 

 decompositions of the plant residues have taken place ; the wasteful 

 process — loss of nitrogen — occurs before or during these decomposi- 

 tions. Thus, when fresh vegetable products, such as straw, starch, 

 or sugar, are added to soil in spring immediately before sowing, there 

 is a depression in growth due to loss of nitrogen ; when the addition 

 is made in autumn some months before sowing there is no depression 

 but a gain in crop. Thus we can justify the autumn application of 

 farm-yard manure, especially when unrotted, rather than the spring 

 application. 



The fixation of nitrogen is intensified when a leguminous crop is 

 grown. If one pulls up a root of clover, peas, or beans, one finds 

 little nodules present, which, when examined under the microscope, 

 are seen to contain numerous bacteria. These organisms are busily 

 engaged in seizing nitrogen from the atmosphere and building it up 

 with compounds of use both to themselves and to the plants. The 

 necessary energy comes not from decaying plant residues, but from 

 the juices of the living plant : the organism is a parasite living on its 

 host, but it is one of the few that gives more than it takes. 



The third effect of the decomposing organic matter is to modify 

 the physical properties of the soil. It possesses the glue-like properties 



