EXPERIMEiSTTAL WORK WITH RAW ROCK PHOSPHATE. 21 



of available plant food, but the determination of the total quantities 

 of the fertilizer elements (by fusion with sodium carbonate) is fre- 

 quently of considerable importance, particularly if supplemented by 

 a mineralogical examination. 



Some soils high in phosphoric acid seem little affected by applica- 

 tions of phosphatic fertilizers, while in other soils the lack of phos- 

 phoric acid appears to be the factor limiting their fertility. 



PEEVIOUS TEEATMENT OF FIELD. 



If a soil is to be tested to determine its responsiveness to a phos- 

 phatic fertilizer, it is obvious that it should not have been treated 

 with phosphates a short time before the experiment. Again, unless 

 the experiment is undertaken primarily to show how a badl}^ managed 

 soil may be restored to former fertility, care should be taken that the 

 soil is not in a " run-down " condition, due to improper handling and 

 cropping. Ordinarily a field should be selected on which the soil is 

 in a condition as nearly as possible like its original state. 



Since most of our cleared land has been cultivated and much of it 

 fertilized, it is not often possible to obtain a field which is immedi- 

 ately available for experimental purposes. It is often well, therefore, 

 to allow a field to lie fallow for a year or so before using it for plot 

 work in order that the effect of previous treatments may be reduced 

 to a minimum. In any event the history of the field for a few years 

 prior to the experiment should be recorded. 



CLIMATIC CONDITIONS. 



The temperature, rainfall, sunshine, wind, and to a large extent, 

 blight, and insect pests are factors beyond control in field work. 



Because of the early stimulation produced by a soluble fertilizer 

 a late frost may cause more damage to plots thus treated than to 

 those on which a less soluble fertilizer has been used. On the other 

 hand, early stimulation and the quick maturity of certain crops are 

 almost essential in some of our Northern States in order that they 

 may be harvested before the early frost. 



High winds or hailstorms may also cause more damage to the 

 better-developed crop than to that which is backward for lack of 

 fertilizer. If such a factor is not considered the final results may 

 lead to very erroneous conclusions. Excessive rains will sometimes 

 leach a soluble fertilizer out of a sandy soil so quickly that its full 

 effect will not be felt and the less soluble fertilizers will appear to 

 greater advantage. On the other hand, the early stimulation pro- 

 duced on a plot by a soluble fertilizer may later enable the plants 

 to resist a severe drought more effectully than those on a plot which 

 has been treated with a less soluble fertilizer. 



