EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 19 
7. Kind of crops grown and selection of seed. 
8. Rate of application and uniform distribution of phosphates. 
9. Methods of comparing raw rock with other phosphates. 
10. Effect of other fertilizers. 
11. Number and distribution of plots, 
12. Duration of experiment. 
UNIFORMITY OF EXPERIMENTAL FIELD. 
The greatest care should be exercised in selecting a field contain- 
ing soil of a uniform character throughout, for unless the soil on the 
various plots has the same productive power or the probable dif- 
ference in its natural fertility on these plots is determined, the yields 
obtained from the application of fertilizers may have little or no 
meaning. A soil survey of a field is of great importance, but such a 
survey unless supplemented by borings may not give sufficient in- 
formation, since an apparently uniform soil may so vary in depth 
from place to place as to cause wide differences in the productivity 
of the various plots. Hall and Russell? state that a simple yet sensi- 
tive method of determining the uniformity of a soil consists in as- 
certaining the percentage of moisture in samples collected from va- 
rious parts of a field at the same time to the same depth. An even 
more sensitive test, it is said, consists in determining the percentage 
of nitrates in such samples. 
Piper and Stevenson? state that it is difficult to say what is the 
ordinary error due to soil inequality. Hall and Russell* state that 
the Rothamsted records show that there is an error of 10 per cent on 
plots where the past treatment has been uniform and general con- 
ditions were favorable for experimental work. 
In conducting fertilizer experiments in the field attempts have 
been made to determine the so-called “natural fertility” of the 
plots subsequently treated with fertilizers. Offhand the most logical 
method appears to be to complete at least one rotation of the sys- 
tem to be employed during the experiment without the addition 
of any fertilizer whatever. The usefulness of this method of valuing 
the plots, however, appears very doubtful. In fact, it might well 
be asked, “Is there such a thing as natural fertility under field con- 
ditions when these conditions vary so greatly from year to year?” 
Lyon‘ has shown that when corn, oats, and wheat were grown for 
several years on the same fields no definite relation was shown be- 
tween the yields of the various plots from year to year. 
LS 
1Chem. News, 102, 180 (1910). 
2 Standardization of Field Experimental Methods. Proc. Am. Soc. Agron., 2, 70-76 
(1910). 
Error of Experiment in Agricultural Field Tests. Chem. News, 102, 80 (1910). 
4Experiments to Estimate Errors in Field Plat Tests. Jour. Am. Soc, Agron., 8, 
89-114 (1911). 
