I.] DORMANT AND AVAILABLE PLANT FOOD 23 



in a locked-up or dormant condition. The plant can 

 only obtain substances which have been previously dis- 

 solved in the water contained by soils in the field, hence 

 plant food in the soil is only available for the plant in 

 so far as it can pass into solution. 



Accepting, then, the fact that the soil contains a vast 

 store of all the elements necessary to its nutrition but 

 in forms of low availability, it remains to ascertain 

 which of the substances are normally likely to fall below 

 the current requirements of the crop. This is a ques- 

 tion that, can only be solved by field experiments, and 

 though the answer will vary with each crop and each 

 soil yet certain general principles at once become 

 evident and upon them the whole idea of a fertiliser is 

 based. For example, field experiments at once show 

 that certain elements indispensable to the plant, as seen 

 from water culture experiments, need not be supplied 

 to the crop in the field, since the soil is practically 

 always able to provide a sufficiency. Calcium, 

 magnesium, iron, sulphur, chlorine, and silicon fall into 

 this class ; to judge by field experiments alone there 

 are only three elements required for the nutrition of the 

 crop — nitrogen, phosphorus, and potassium — and this 

 means that soils can usually supply the elements 

 necessary to the plant in sufficient quantities, except in 

 these three cases. Fertilisers, then, are designed to 

 supply deficiencies in the soil, and for all practical 

 purposes are to be regarded as consisting of compounds 

 of nitrogen, phosphoric acid, and potash, either singly or 

 together. They may also contain magnesia, lime, or 

 sulphuric acid, but these, though equally necessary to 

 the plant, are not counted, since the unaided soil may 

 be trusted to furnish the crop with them. 



To summarise the position we have reached : a 

 fertiliser must contain one or more of the three sub- 



