230 IRRIGATION. 



form more geine, more ammonia by putrefaction, and are 

 therefore preferable for composts. Oat straw contains more 

 potash, and buckwheat straw more magnesia than other 

 straws. 



The following table exhibits straws as arranged according 

 to their practical compost value, by Sprengel, and also their 

 proportion per cent, of organic matter, salts and nitrogen ; 

 the last as determined by Payen and Boussingault : 



old, and 

 ' 0.24 new. 



279. Having thus considered all the classes of manure, 

 and shown the possibility of enriching barren fields, without 

 the aid of animals, other subjects, intimately connected with 

 this discussion, may be here introduced. 



These are, the application of manure in the form of rain, 

 snow, and by overflowing streams, and the humble attempt 

 to imitate these natural processes, by irrigation. The effects 

 in these cases are alike. They are due to two distinct causes, 

 first, to the air of the water, and secondly, to the salts and 

 other materials dissolved by, or suspended in the water. 

 First, before it can be understood how irrigation acts, let it 

 be considered how pure water acts ; it is not said rain water, 

 for that acts in a double way, both by its purity and im- 

 purity. The more impure, the better manure is water. 

 The purer water is, the less is it fit for irrigation. 



