784 ROTHAMSTED EXPERIMENTAL STATION. [DEC, 



and it is by no means easy to discover the proper mixture. Years 

 ago it was thought that the problem could be solved merely by 

 ascertaining the ash constituents of the plants and making up 

 a manure corresponding thereto; it is now known that no such 

 short cut is possible. A competent chemist could prepare no fewer 

 than 6,000 different brands of potato fertiliser, each useful 

 under certain conditions. Field trials alone enable one to 

 decide which of all these is the best in any given case. In 

 practice it would be impossible to test them all; it is also 

 unnecessary, since there are certain guiding principles which 

 give a very fair idea of the type of fertiliser needed. Pro- 

 longed field experiments are needed to establish these 

 principles, but in the end this is the shortest method of pro- 

 cedure, since without this knowledge we can do little but guess 

 the proper mixture to use. It is not possible to make a short 

 and simple summary of the results, but a discussion of them 

 from the farmers' point of view is given in the writer's 

 " Manuring for Higher Crop Production."* 



The proper amount to apply can be determined only by 

 experiment, and this has proved more difficult than was 

 expected. It was at first thought — the idea was started by 

 the famous German chemist Liebig — that the larger the dress- 

 ing of manure the larger would be the crop; or in other words, 

 that, up to a certain point, the crop yield was proportional to 

 the quantity of manure added. It was subsequently found, 

 however, that this was not the case. The next idea came much 

 later from another German chemist, Mitscherlich ; it is to the 

 effect that fertilisers exert their greatest action when used 

 in small quantities, and that they show less and less action as 

 the dressing increases. This, if true, would justify low farm- 

 ing as being the most likely to give maximum profits. The 

 Rotham^ted experiments indicate, however, that this is not 

 correct. The effect of small dressings is found to be less than 

 that of larger ones; the most profitable procedure therefore 

 is to use moderately large dressings rather than too small a 

 quantity of artificials per acre. A bold policy is probably best.t 



The question of the size of the dressing, however, is inti- 



* Cambridge University Press. 1917. 



t This result is at variance with one that has been obtained in the United 

 States of America. Prof. Warren, of Ithaca, informs the writer that farmers 

 there obtain good results, and he is inclined to think their best proportionate 

 results, from such small manurial dressings as 4 lb. nitrogen (equivalent to 

 20 lb. sulphate of ammonia), and 20 lb. of phosphoric acid (170 lb. 26 per cent, 

 superphosphate) per acre. The cause of these differences would form an 

 interesting subject of inquiry. 



