EXPERIMENTAL 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. 
PREVIOUS TREATMENT OF FIELD. 
If a soil is to be tested to determine its responsiveness to a phos- 
phatic fertilizer, 1t 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 badly 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 le 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. 
