18 BULLETIN 699, U. S. DEPAETMEXT OF AGBICULTUEE. 



ing of the soil to insure equality in drainage, and a thorough distri- 

 bution of the fertilizer material are essential in order to obtain 

 reliable data from pot tests. 



Another disadvantage in pot work lies in the fact that it is 

 difficult to grow many plants to maturity in pots because of the 

 limited space in which the root systems x ave to expand and forage. 



While i^ot tests with fertilizer materials are valuable for prelimi- 

 nary experiments and are also useful supplements to field work, the 

 results obtained therefrom can not be regarded as final proof of the 

 value of the fertilizer material under investigation. In fact, pot 

 tests do not necessarily serve as an index to the results which will 

 be obtained in the field. 



The amount of pot or greenhouse work on ground raw rock phos- 

 phate is comparatively small. What has been done is discussed 

 further on in the detailed descriptions of the work of the State 

 experiment stations. 



FIELD EXPERIMENTS. 



The final proof of the value of a fertilizer material must rest upon 

 field experiments, but field experiments, unless carefully conducted 

 with due regard to the numerous factors influencing crop yields, are 

 often worse than valiteless. 



Because of limited knowledge of these factors, the earlier agrono- 

 mists had a tendency to publish the results of field work conducted 

 for a year or two only and to draw conclusions from very meager 

 data. Unfortunately, even now the results of field experiments are 

 often published without at the same time reporting data or mention- 

 ing certain factors which would be a great aid in interpreting the 

 significance of the crop yields. Moreover, the experiments (particu- 

 larly raw rock phosphate tests) are conducted in such difi:'erent ways 

 and under such a variety of conditions that it is impossible to 

 reduce them to a uniform basis for the sake of comparison. The 

 desirability of having field investigators employ the same methods 

 and a minimum standard in both variety and fertilizer experiments 

 has been pointed out by Piper and Stevenson.^ 



In considering the results of field experiments with raw rock 

 phosphate, careful attention should be given to the following factors: 



1. Uniformity of experiment field. 



2. Topography and drainage conditions. 



3. Physical and chemical composition of the soil. 



4. Previous treatment of field. 

 6. Climatic conditions. 



6. Injuries from disease, insects, and animals. 



1 standardization of Field Experimental Methods in Agronomy. Proc. Am. Soc, Agron., 

 2, 70-76 U910>, 



