28 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTUEE. 



DUBATION OP EXPEEIMENTS. 



Because of the numerous conditions just discussed, some of which 

 are beyond control and some of which vary in spite of the exercise 

 of the greatest care, it is obviously unwise to draw conclusions from 

 field experiments which have been conducted for a short time only. 

 Climatic conditions vary so greatly from year to year that in order 

 to gain an accurate knowledge of the value of a fertilizer material 

 the same crop should be grown several times on the same land. 

 Where proper systems of rotation are practiced it takes from 6 to 

 20 years to accomplish this end. In studying the effect of the less 

 soluble fertilizers, time plays a very important part. Not only does 

 the material often become more soluble, but it becomes more thor- 

 oughly distributed in the soil from year to year, and hence is more 

 readily available to the root systems of crops. 



RESULTS OF EXPERIMENTS. 

 ALABAMA. 



No field experiments with raw rock phosphate continuing beyond 

 three years have been conducted by the Alabama station. Two series 

 of cooperative one-year experiments, however, were carried on, one in 

 1891,^ consisting of 25, and the other in 1892,^ consisting of 35 experi- 

 ments. In these experiments equal amounts of acid phosphate and raw 

 rock phosphate were compared but they were applied at a rate (240 

 to 300 pounds per acre) considerably below that at which the latter 

 should prove effective. In these experiments the average yields of 

 the acid phosphate plots were appreciably greater than those of the 

 raw rock plots, though the latter showed considerable gains over 

 the average of the checks. 



A field experiment conducted for three years was reported by the 

 Alabama station in 1913.^ The general scheme and results of this 

 experiment were in accord with those just mentioned, but the yield 

 of the raw rock plot the third year was almost identical with that 

 of the acid phosphate plot. 



CONNECTICUT. 



The work of the Connecticut station with raw ground rock phos- 

 phate has been very limited, none of the experiments having been 

 conducted sufficiently long to warrant repetition in detail. An ex- 

 periment began in 1887 * and continued for three years ^ on the same 



»Ala. Agr. Expt. Sta., Bui. No. 23 (1891). 



2 Ala. Agr. Expt. Sta., Bui. No. 34 (1892). 



8 Ala. Agr. Expt. Sta., Bui. No. 173, p. 139 (1913). 



*Conn. Agr. Expt. Sta,, Ann. Rept. for 1888, pp. 110-117 (1889), 



«Conn. Agr. Expt. Sta., Ann. Rept. for 1889 (1890). 



