154 THE CHEMICAL ANALYSIS OF SOILS [chap. 



nection it is interesting to note that the original mean- 

 ing of manure was to work by hand.* 



It becomes important, then, to attempt to discriminate 

 between the various forms in which the nitrogen, 

 potash, and phosphoric acid may be present in the soil, 

 according as they are soluble, or likely in a short time 

 to become sufficiently soluble to reach the crop. In the 

 case of nitrogen we know that of the various compounds 

 such as proteins and protein residues, amides, ammonia 

 salts, and nitrates which can be detected in the soil, only 

 the latter can enter the plant, but that, by processes of 

 fermentation, all of the other compounds will eventually 

 pass into the state of nitrate. Of the immediately soluble 

 nitrogen compounds — nitrates, nitrites, and ammonia — 

 a very small amount, varying from 5 to 200 lbs. per 

 acre, is ever present in the soil at any given time, 

 though it is constantly being renewed by fermentation 

 processes. 



Phosphoric acid also exists in the soil in many 

 distinct compounds: in combination with carbon, etc., 

 it is found in nuclein and lecithin, which in a more 

 or less humified condition are found among the plant 

 and animal residues : it also occurs as phosphate of 

 the sesquioxides of iron and alumina ; as tribasic, and 

 probably also as dibasic phosphate of lime. Of these 

 compounds the latter are undoubtedly the most soluble 

 in either pure water or the carbonic-acid-charged water of 

 the soil, but much must depend on the physical condition, 

 as well as on the chemical combination, in which the 

 material exists. For example, when using tribasic phos- 

 phate of lime as a manure, the softer phosphates, such as 

 steamed bone flour, are more effective than the chemically 

 similar but harder material in ground rock phosphate. 



* Cf. Defoe, Robinson Crusoe (17 19)— "The ground that I had 

 manured or dug up for them was not great." 



