282 THEORIES OF FERTILISER ACTION [chap. 



contain sufficient plant food for the nutriment of a 

 hundred or more full crops : the soil of the unmanured 

 plot on the Rothamsted wheatfield contained in 1893, 

 after fifty-four years' cropping without fertiliser, 2570 lb. 

 per acre of nitrogen, 2950 lb. of phosphoric acid, and 

 5700 lb. of potash. Of course much of this material is 

 in a highly insoluble condition, but though attempts 

 have been made by the use of weak acid solvents to 

 discriminate between the total plant food in the soil and 

 that portion of it which may be regarded as available for 

 the plant, no proper dividing line can be thus drawn. 

 The availability of a given constituent, say of phosphoric 

 acid, will depend upon the nature of the crop. A given 

 soil may contain sufficient easily soluble phosphoric 

 acid for the needs of the wheat plant and yet fail 

 to supply Swede turnips with what they require. 

 Again, the mechanical texture of the soil may be 

 such as to limit the root range of the plant, so that a 

 richer soil is necessary to produce as good results as are 

 obtained in a poorer soil of more open structure ; the 

 state of the micro-flora of the soil may also have much 

 to do with the amount of a given nutrient that can 

 reach the plant. 



Perhaps the best general point of view of the action 

 of fertilisers is obtained by extending the " law of the 

 minimum " originally enunciated by Liebig, according 

 to which the yield of a given crop will be limited by the 

 amount of the one particular constituent which may 

 happen to be deficient ; if the soil, for example, is 

 lacking in nitrogen, the yield will be proportional to the 

 supply of nitrogen in the fertiliser, and no excess of 

 other constituents will make up for the shortage of 

 nitrogen. To take an example from the Rothamsted 

 experiments. Table LXXXVII. shows the yield of 

 wheat grain and straw from the unmanured plot, 



