THE A'TJTRITIUN OF THE ROOT 53 



covering of plants, it does not lose so much of the natural 

 condensations by evaporation from the surface of the plants ; 

 it will therefore also contain, when it begins to be cultivated 

 again, a larger amount of water, i.e., the means of transport of 

 the soluble substances. Lastly, a certain amount of enriching 

 in nitrogen takes place through the wild leguminous plants. 



This beneficial effect of lying fallow is, however, only 

 noticeable in rich soils ; a sandy soil may, indeed, become 

 impoverished by lying fallow. For the soluble organic sub- 

 stances resulting from decomposition are not readily absorbed 

 by the sand, and may be washed down by strong rains into the 

 deeper layers of the soil, where they will only be accessible to 

 a few deep-rooting plants. Under such conditions, therefore, 

 the soil must always have a dense covering of plants. The 

 latter, however, will of course use up both water and nutritive 

 substances, and store up nothing for a future crop. A heavy 

 soil, too, may be impoverished under abnormal climatic con- 

 ditions by lying fallow. If, for instance, a continuous and 

 excessive rainfall has closed up the already very small inter- 

 stices of a clay soil for a considerable period, the process of 

 decomposition of organic substances may become altered, and 

 substances are formed which act injuriously on the root struc- 

 tures. We shall return to this subject in dealing with the 

 watering of pot-plants. 



Lying fallow, therefore, only has the desired result on a 

 heavy soil, and under normal or dry climatic conditions ; and 

 the present methods of cultivation have caused this means of 

 enriching the soil to be entirely given up, and now recourse 

 is had almost exclusively to the direct addition of nutritive 

 substances by manuping. 



(h.) Inorganic Manures. 



Originally stable manure was the only food material directly 

 added to the soil ; now-a-days, however, artificial manures 

 have come more to the front. These do not, it is true, con- 

 tain, like stable manure, all the food substances of plants, but 

 they furnish the soil with all the most essential and most 

 important ones \\\ a more concentrated form. In practice it 



