39 



yellow and fall, indicating that the plant has finished its season's growth, 

 long before any such appearance is seen on the potash-starved plots along- 

 side, where a tuft of dark green and apparently growing leaves persists 

 until the plant is cut off by the frosts. Similar appearances, though in a 

 less pronounced degree, can be seen on ordinary crops in light soils, when- 

 ever a strip has been left to show the action of potash in the manure. 



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 straAv to grain, but marks the completion of the work of the leaves in 

 manufacturing carbohydrate and passing it on to the root for storage. 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 jBeld 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 numer- 

 ous 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 defiocculating 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 defiocculating 

 powers of which are recognized. 



The destruction of tilth of the soil brought about in this way may 

 easily give rise to an irregular stand and so account for an inferior plant 

 and a reduced yield on the plots receiving potash salts; the author has 

 observed a case on heavy land where the application of a rather excessive 

 amount of kainit so altered the texture of the soil, that the draught of 

 ploughs upon it was perceptibly increased, and the crop suffered to a 

 marked degree. 



Farmyard manure has frequently been blamed for carrying the seeds 

 of disease and of weeds which have passed through the animals making the 

 dung in an unchanged condition and thus contaminate the laud for other 



