176 THE POT AS SIC FERTILISERS [chap. 



The apparent contradiction may be explained on 

 physiological grounds ; with the root crops ripeness 

 does not represent the completion of a migration process 

 of material previously stored up, such as takes place 

 from straw to grain, but marks the completion of the 

 work of the leaves in manufacturing carbohydrate and 

 passing it on to the root for storage. It has already 

 been shown (Table XLV.) that in the absence of 

 potash the leaves cannot carry on the assimilation 

 process properly, and probably they continue green 

 instead of ripening off because their function of storing 

 up material in the root has not been completed. 



From time to time field experiments have been 

 reported which show a reduced yield for the use of 

 potassic salts, and while in many cases the results 

 might be put down to experimental error, the cases 

 are too numerous to be entirely covered by such an 

 explanation. 



A clue to this apparent depressing effect of potash 

 is provided by the appearance of the soil on certain 

 of the experimental plots at Rothamsted, as on the 

 Barn field, where considerable amounts of potash salts 

 are applied every year. The behaviour of the soil, 

 which lies extremely wet and sticky after rain, and dries 

 with a hard glazed surface, shows that the clay particles 

 must have become thoroughly deflocculated, just as they 

 are on the plots receiving nitrate of soda (p. 55). 



This deflocculating effect of the potash salts, which 

 in themselves would flocculate clay particles, is due to 

 a prior interaction between the potassium salt and the 

 calcium carbonate in the soil, resulting in the formation 

 of a certain amount of potassium carbonate, the defloc- 

 culating powers of which have already been recognised. 



The destruction of tilth of the soil brought about in 

 this way may easily give rise to an irregular stand and so 



