Methods of Working 9 



3. Soil cultures in pots. 



In this case the conditions of life are still more natural, as the 

 plant roots find themselves in their normal medium of soil. But the 

 investigator has now far less control, and bacterial and other actions 

 come into play, while the nutrients and poisons supplied may set up 

 interactions with the soil which it is impossible to fathom. This method 

 is useful in the laboratory as it is more convenient for handling and 

 gives more exact quantitative results than plot experiments. Also the 

 pots can be protected from many of the untoward experiences that are 

 likely to befall the crops in the open field. The conditions are some- 

 what more artificial, as the root systems are confined and the drainage 

 is not natural, but on the whole the results of pot experiments are very 

 closely allied to those obtained in the field by similar tests. 



4. Field experiments. 



These make a direct appeal to the practical man, but of the scientific 

 methods employed the field experiments are the least under control. 

 The plants are grown under the most natural conditions of cultivation 

 it is possible to obtain, and for that reason much value has been 

 attached to such tests. Certainly, so far as the final practical applica- 

 tion is concerned, open field experiments are the only ones which give 

 information of the kind required. But from the scientific point of 

 view one very great drawback exists in the lack of control that the 

 investigator has over the conditions of experiment. The seeds, applica- 

 tion of poison, &c., can all be regulated to a nicety, but the constitution 

 of the soil itself and the soil conditions of moisture, temperature 

 and aeration introduce factors which are highly variable. No one can 

 have any idea of the composition of the soil even in a single field, as it 

 may vary, sometimes very considerably, at every step. Further, no one 

 knows the complicated action that may or may not occur in the soil on 

 the addition of extraneous substances such as manures or poisons. 

 Altogether, one is working quite in the dark as to knowledge of what 

 is going on round the plant roots. It is impossible to attribute the 

 results obtained to the direct action of the poison applied. While the 

 influence may be direct, it may also happen that certain chemical and 

 physical interactions of soil and poison occur, and that the action on 

 the plant is secondary and not primary, so that a deleterious or bene- 

 ficial result is not necessarily due to the action of the toxic or stimulating 

 substance directly on the plant, but it may be an indirect effect induced 

 possibly by an increase or decrease in the available plant food, or to some 



