18 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
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 rave to expand and forage. 
While pot 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 Tone 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 valueless. 
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 different 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. 
5. Climatic conditions. 
6. Injuries from disease, insects, and animals. 
1 Standardization of Field Experimental Methods in Agronomy. Proc, Am. Soe. Agron., 
2, 70-76 (1910), 
