VIII.] DORMANT AND AVAILABLE PLANT FOOD 15* 



considered in the previous chapter, as to the changes 

 which the nitrogen compounds in the soil have to go 

 through before they can feed the crop. Proteins and 

 similar compounds of nitrogen are of no service to the 

 plant until they have been transformed by bacteria into 

 ammonia and nitrates ; hence the amount of nitrogen 

 available for the plant in the soil at any moment may 

 be but a small fraction of the whole. Just in the same 

 way, phosphoric acid and potash must be brought into 

 solution before they can enter the plant, and the rate 

 at which the compounds of these constituents in the 

 soil will become dissolved depends greatly on their 

 nature and physical condition. For rough practical 

 purposes we may divide the plant food in the soil into a 

 dormant and an available portion, though it would 

 probably be nearer the truth to consider the whole as 

 potentially available, yet differing greatly in its degree 

 of activity. It is indeed impossible to draw any absolute 

 line of distinction between the dormant and the avail- 

 able — only an absolute division can be made among the 

 nitrogen compounds, of which nitrates and ammonia 

 might be classed as available and the rest as dormant, 

 but even then some of the dormant materials might be 

 rendered available a few days or even hours later. 

 Again, among the compounds of phosphoric acid we 

 might regard dicalcium phosphate in the soil as avail- 

 able and iron phosphate as dormant, but really this only 

 means that the calcium phosphate would yield a solution 

 containing perhaps a hundred times as much phosphoric 

 acid as would the phosphate of iron. This shows how 

 vain it is to hope to separate the available from the 

 dormant plant food by means of a solvent which will 

 extract the one and leave the other undissolved ; in any 

 solvent, however weak, both dissolve up to a point, and 

 the differences are only of degree and not of kind. . The 



