174 THE CHEMICAL ANALYSIS OF SOILS [chap. 



garded as possessing most of its plant food in states 

 of combination that cannot be utilised by the plant, 

 and these forms slowly pass, by weathering and other 

 changes, into material which is available for the crop. 

 The plant food of the soil represents so much capital, 

 and, as in many another business, but a small proportion 

 of the capital is liquid at any given time : it is largely 

 the object of cultivation to effect such a turnover of the 

 capital as will liquidate some of it in a form available 

 for the nutrition of the crop. 



In the old systems of agriculture, before the land was 

 enclosed, the whole crop was grown out of capital, 

 nothing but labour was put into the soil : in which con- 

 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 two latter 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., 



* Cf. Defoe, Robinson Crusoe (1719) "The ground that I had 

 manured or dug up for them was not great." 



