3 io SYSTEMS OF PERMANENT AGRICULTURE 



Belgium, 9.20 pounds, both of which are doubtless influenced by 

 the atmosphere from the cities with their numerous factories and 

 other sources of pollution. 



Professor Shutt reports 4.32 pounds of nitrogen per acre in one 

 year's precipitation at Ottawa, Canada, in 37.35 inches, of which 

 3.24 pounds were found in 24.05 inches of rain and 1.08 pounds in 

 13.3 inches of snow water (corresponding to about 133 inches of 

 snow), the average composition being based upon analyses of 46 

 samples of rain water and 32 samples of snow water. Of the nitro- 

 gen found in rain water, 61 per cent existed in free ammonia, 22 

 per cent in nitrate (and nitrite) form, and 17 per cent as organic 

 nitrogen, the corresponding percentages for snow water being 56, 

 34, and 10. 



Liebig also discussed very interestingly and, in the main, very 

 erroneously, the reasons for the value of crop rotation. In 1840 

 he wrote as follows: 



" Of all the views which have been adopted regarding the cause of the favor- 

 able effects of the alternations of crops, that proposed by M. Decandolle alone 

 deserves to be mentioned as resting on a firm basis. 



" Decandolle supposes that the roots of plants imbibe soluble matter of every 

 kind from the soil, and thus necessarily absorb a number of substances which 

 are not adapted to the purposes of nutrition, and must subsequently be expelled 

 by the roots, and returned to the soil as excrements. Now as excrements can- 

 not be assimilated by the plant which ejected them, the more of these matters 

 which the soil contains, the more unfertile must it be for plants of the same 

 species. These excrementitious matters may, however, still be capable of as- 

 similation by another kind of plants, which would thus remove from the soil, 

 and render it again fertile for the first. And if the plants last grown also expel 

 substances from their roots, which can be appropriated as food by the former, 

 they will improve the soil in two ways. 



"Now a great number of facts appear at first sight to give a high degree of 

 probability to this view. Every gardener knows that a fruit tree cannot be 

 made to grow on the same spot where another of the same species has stood; 

 at least not until after a lapse of several years. Before new vine stocks are 

 planted in a vineyard from which the old have been rooted out, other plants are 

 cultivated on the soil for several years. In connection with this it has been ob- 

 served, that several plants thrive best when growing beside one another; and, on 

 the contrary, that others mutually prevent each other's development. Whence 

 it was concluded, that the beneficial influence in the former case depended on a 

 mutual interchange of nutriment between the plants, and the injurious one in the 

 latter on a poisonous action of the excrements of each on the other respectively. 



