288 SYSTEMS OF PERMANENT AGRICULTURE 



increase of only 4.2 grams, which, added to the increase of 6.3 grams 

 due to clover alone, makes only 10.5 grams.' In other words, the 

 sum of the gains which they make when used separately was 2.8 

 grams less than the increase produced when the rock phosphate 

 and clover were turned under together. Somewhat similar results 

 are produced with clover and bone meal when used separately and 

 together; also with bone meal and potassium, and with rock 

 phosphate and potassium. Such marked combined action does not 

 appear, however, from other combinations, possibly because of 

 other limiting factors. As a general average, the rock phosphate 

 has made slightly better gains than the steamed bone meal. 



The pots used in these investigations are io|- inches in diameter, 

 consequently i gram per pot corresponds to i pound per square 

 rod, or to 160 pounds per acre. The actual yields in grams per pot 

 are given, but the results may also be computed to bushels per 

 acre. It should be remembered that pot cultures constitute an 

 intensive form of agriculture. They are carried on under almost 

 complete control, except in very warm weather, when too much 

 shade may be required to avoid too high temperature. The yields 

 obtained are usually two or three times as much as can be expected 

 in the field under ordinary weather conditions. They are not, 

 however, larger than could be obtained in the field under perfect 

 weather conditions. The largest yield reported in Table 51 is 25.3 

 grams per pot, or 67 bushels of wheat per acre. Pot culture yields 

 have been produced corresponding to 142 bushels of wheat, and 

 to 230 bushels of oats, per acre. 



Doctor Alfred M. Peter of the Kentucky Station has kindly 

 furnished the author the following data secured by him with the 

 cooperation of Mr. S. C. Jones of the Kentucky Geological Survey: 



KENTUCKY EXPERIMENTS: POT CULTURES 



