NITROGENOUS MANURES 269 



a dressing of this manure can sometimes be traced for 

 several years after it has been applied. This is probably 

 due to the accumulation of humus, which results from 

 the action of the manure. 



There may be some difference between a lasting manure 

 and one which produces lasting effects. In general, how- 

 ever, lasting effects are produced only by manures 

 which come slowly into action. This, of course, is not 

 an advantage but quite the reverse. The slower the action 

 of the manure the longer it will last, but the less valu- 

 able it will be. A manure which would last for ever 

 would be of no value at all, i.e., it would not be a manure. 



Other Nitrates. There is every reason to believe that 

 the nitrates of all non-poisonous bases would be equally 

 suitable for manurial purposes, but practically none of 

 them, except nitrate of soda, are ever used. They are 

 all more expensive. 



Potassium nitrate contains two constituents of manurial 

 value nitric acid and potash and where the latter sub- 

 stance is required, would, for this reason, doubtless give 

 a better result than an equivalent quantity of nitrate of 

 soda. Many of the prescriptions for garden manures 

 include a quantity of potassium nitrate, but apparently, 

 the constituents are introduced in that form merely to 

 simplify the recipes. The two ingredients can be ob- 

 tained more cheaply separately nitric acid in the form 

 of nitrate of soda, and potash in the form of sulphate of 

 potash, or other similar salt. The difference in cost of 

 transport is more than made up by the difference in 

 price. 



Nitrate of Lime. Calcium nitrate is formed by the nitri- 

 fication of organic matter in soils and composts. It could 

 easily be prepared from nitrate of soda, but it would be of 

 little advantage, for manurial purposes, to do so. The 

 change probably takes place naturally, in the soil, and the 



