3G4 The Arailabili/i/ of Mineral Phmt luxnl 



The Inadequacy of the Present Hypothesis. 



There ;ire a number of facts wliieh are difficult to reconcile with this 

 hypothesis. 



1. The relatio)! of the composition of the soil solution to the mineral 

 elements taken up, and the water transpired, by plants. In obvious and 

 natural accordance with the view that the soil solution is the nutrient 

 solution of the plant, many attempts have been made to express or dis- 

 place this solution from the soil, and by its analysis to ascertain the 

 essential chemical knowledjj;e of the fertility of the soil concerned. The 

 results of these experiments show that the ratio of certain mineral ele- 

 ments assimilated by plants to the water transpired is greater than the 

 ratio of these elements to the water in which they are dissolved in the soil. 

 Hall' has calculated that if the potash taken up by a clover C'rop is 

 assumed to have been dissolved in the water transpired, the calculated 

 solution is far more concentrated in respect of potash than solutions 

 obtained from the soil. Raniiinn- makes a similar observation but arrives 

 at a different conclusion. H;ill deduces that the dissolved material enters 

 the plant from the soil solution at a greater rate than the water. Ramann 

 is apparentlv unable to reconcile this view with the laws of diffusion and 

 concludes that the i)lant takes material from the soil other than that 

 which is in solution. 



2. The absorption of iron by plants. Iron is a necessary element for 

 plant growth. All plants take up a certain amount from the soil. The 

 amount necessary is relatively small, but it is difficult to see how even 

 this small amount can exist in sol id inn in soils containing a high percentage 

 of (thalk. 



3. The nvailabilil 1/ of phosphates. This is probably the greatest 

 difficulty of all. Tlic value of various phosphatic substances to plants is 

 not by any means in accordance with the present hypothesis. Basic slag 

 and certain mineral phosphates are sparingly soluble and are yet valual)le 

 phosphatic fertilizers. The evaluation of these substances entirely on a 

 solubility basis has not been an un(iualified success. Their usefulness as 

 phosphatic fertilizers is often comparable to that of a water soluble 

 phosphate. 



Tlie phosphates of iron and aluminium are in a curious position in the 

 literature. On t he one hand it is normally taught that these are insoluble 



» '/'At Soil. 19-20. 



- Lamlir. VirSlul. 1<)10, 88. 



