IRRIGATION. 139 



This may be so, but it is obviously purely conjectural. It opens 

 a passage for the access of the air to the roots of the plants. 

 This must be beneficial. It increases the temperature of the 

 soil ; but this must depend upon the relative condition between 

 the soil and the water applied, which must vary under different 

 circumstances. In the opinion of Sir Humphry Davy, &quot; In the 

 artificial watering of meadows, the beneficial effects depend 

 upon many different causes, some chemical, some mechanical.&quot; 

 This is certainly a safe opinion ; but its chemical effects are not 

 so easily determined. Let us hear the great authority. 



&quot; The atmosphere arid the soil offer the same kind of nourish 

 ment to the leaves and roots. The former contains a compara 

 tively inexhaustible supply of carbonic acid and ammonia ; the 

 latter, by means of its humus, generates constantly fresh carbonic 

 acid, whilst, during the winter, rain and snow introduce into the 

 soil a quantity of ammonia sufficient for the development of the 

 leaves and blossoms.&quot; 



&quot; The complete, or, it may be said, the absolute insolubility, 

 in cold water, of vegetable matter in progress of decay, (humus,) 

 appears, on closer consideration, to be a most wise arrangement 

 of nature. For if humus possessed even a smaller degree of 

 solubility than that ascribed to the substance called humic acid. 

 it must be dissolved by rain water. Thus the yearly irrigation 

 of meadows, which lasts for several weeks, would remove a 

 great part of it from the ground, and a heavy and columned 

 rain would impoverish a soil. But it is soluble only when com 

 bined with oxygen ; it can be taken up by water, therefore, 

 only as carbonic acid. 



u When kept in a dry place, humus may be preserved for cen 

 turies ; but when moistened with water, it converts the surround 

 ing oxygen into carbonic acid. As soon as the action of the 

 air ceases, that is, as soon as it is deprived of oxygen, the 

 humus suffers no further change. Its decay proceeds only when 

 plants grow in the soil containing it ; for they absorb by their 

 roots the carbonic acid as it is formed.&quot;* 



&quot; It is because the water of rivers and streams contains oxygen 

 in solution, that it effects the most complete and rapid putrefac 

 tion of the excrements contained in the soil, which it penetrates, 



* Liebig s Agricultural Chemistry, p. 127. 



