15 



Tlie experimental work falls under two headings : — 



1. The influence of fertilisers on the yield of crops under 



different conditions of soil and climate ; 



2. Their effect in altering- the composition or quality of the 



crop. 



The effect of fertilisers on crop yield is studied in three ways. 

 The most direct and accurate is the method of water cultures and 

 pot cultures used in the Botanical Department. Here the condi- 

 tions are so rigidly controlled that the factors, except the one 

 under investigation, are kept as nearly constant as possible. The 

 results are plotted on curves which, if they pass certain statistical 

 tests, can be used as a basis for physiological deductions. Ex- 

 periments of this kind have shown that the plant responds to two 

 kinds of added substances : the usual nitrogen, phosphorus and 

 potassium compounds required in rather large amounts ; and 

 certain substances not yet fully known, which are required in very 

 small amounts only. Agricultural chemists and farmers are 

 familiar with the use of the former, but not of the latter. 



Dr. Winifred Brenchley has already studied certain cases, 

 notably manganese, and this year Miss Warington showed that 

 broad beans and certain other leguminous plants die prematurely 

 unless they receive a small quantity of boric acid in addition to the 

 so-called ''complete" plant food. The results suggest that some of 

 the anomalies and unexpected failures in fertiliser experience may 

 be traceable to the absence of some of these substances required in 

 homeopathic doses only. But we must caution farmers that this 

 work is still a long way from practical application and they must 

 on no account be beguiled into buying "catalytic" or "radio- 

 active" fertilisers in the hope of getting something outside the 

 usual fertiliser constituents. We have tested several of these 

 supposed "radioactive" fertilisers, but failed to obtain any benefit 

 from them. 



This method of experiment is invaluable where the factors can 

 be controlled, but otherwise it breaks down. For this reason it 

 does not give entirely reliable guidance for field practice where the 

 weather conditions are entirely uncontrollable, and it completely 

 fails to show how weather conditions influence the efliciency of 

 the various fertilisers. A second method is therefore adopted. 

 The Rothamsted data, extending as they do over a long series of 

 years, can be subjected to modern methods of mathematical 

 analysis. The variation in crop yield from season to season is 

 traced to two types of causes : (a) annual, the variation in each 

 season being independent of the years before and after, e.g., 

 weather; (h) continuous acting, of which there are two forms, 

 steady, such as soil-deterioration, and variable, such as weed 

 infestation. Mr. Fisher has devised methods for finding out how 

 much of the variation is due to each of these causes, and has been 

 able to trace out the average effect of rain above or below the 

 average in amount in each month of the plant's life. 



Methods are being developed to find out how much the crop 

 yield is likely to be altered by deviations from the average 

 weather and other conditions, and important results may emerge. 

 There must always be a risk about crop yields whatever steps the 

 farmer may take. At present the risks are entirely speculative. 



