174 ANNITAT. rKPOt^T OF THH Off. D«c. 



1. Acid Florida Kock. — This was prepared by treating; a. Florida 

 phosphatic rock with sulphuric acid, thereby converting a large part 

 of the phosphate into an available form. At the beginning of the 

 first experiment this phosphate had the following composition: 20. GO 

 I)er cent, total phosphoric acid, of which IG.OO per cent, was avail 

 able (19.97 per cent, soluble, 1.93 per cent, citrate soluble). In the 

 later work it was found that the composition had changed some- 

 wliat, but the amount of available phosphate remained about the 

 same. 



2. Crude, finely ground Florida rock (floats), containing 32.88 per 

 cent, total phosphoric acid, none of which was soluble, with only 

 2.46 per cent, soluble in ammonium citrate. This was obtained from 

 the commercial ground rock by stirring it with w^ater, allowing the 

 coarse particles to subside and then pouring ofT the tuibid water. 

 The "floats" used in this experiment consisted of the sediment de- 

 posited from these washings. 



3. A phosphate of iron and alumina (Redonda). The first sample 

 used contained 49.77 per cent, phosphoric acid, a large part of which, 

 42.77 per cent, was soluble in ammonium citrate. The Redonda. 

 underwent such rapid changes in the intervals between the experi- 

 ments that it became necessary to prepare fresh quantities at each 

 successive planting. The analysis given above is fairly representa- 

 tive of all. 



Twenty grains of the floats, containing 6.58 grains total phosphoric 

 acid, were used for a single box. The other phosphates used were 

 first analyzed and such quantities used for each box that the total 

 quantity present was in each case the same, 6.58 grams. The actual 

 amount of available phosphoric acid thus supplied to each box by the 

 various phosphates were: By the acid rock, 5.39 grams; by the floats, 

 .49 grams; by the Redonda, 5.67 grams. 



DETAILS OF THE EXPERIMENT. 



The experiments were conducted in one of the green-houses, the 

 plants being grown in wooden boxes, fourteen inches square and 

 twelve inches deep. When filled to within one and one-half inches 

 of the top, the boxes contained 320 pounds of sand. The sand used 

 was taken from a knoll near the river, at a depth of three or four 

 feet, and was nearly free from organic matter. Traces of phosphoric 

 acid were present, but this was in the insoluble form, and the 

 quantity in each box was the same, its presence is not considered ob- 

 jectionable. The sand was carefully screened before being used, 

 and thoroughly mixed with the phosphates and other plant foods. 



In each period twelve boxes were used for each kind of plant. 

 In the first box the acid rock was used; in the second, the un 



