Apbil 23, 1915] 



SCIENCE 



617 



■oxide content have a relatively high feeding 

 power for the phosphorus in raw rock phos- 

 phate. For plants containing a relatively low 

 ■calcium oxide content the converse of the 

 ■ahove is true. A calcium oxide content of less 

 than one per cent, may be considered rela- 

 tively low. Corn, oats, rye, wheat and millet 

 belong in this class. A calcium oxide content 

 of somewhat more than one per cent, may 

 be considered relatively high. Peas, clover, 

 alfalfa, buckwheat and most of the species of 

 the cruciferse belong in this class. 



The explanation of the above relation is 

 made possible by means of the laws of mass 

 action and chemical equilibrium. The reac- 

 tion making the phosphorus in raw rock phos- 

 phate available to plants is one between car- 

 honic acid and the tricalcium phosphate in the 

 rock phosphate, which may be represented as 

 follows : 



CasCPO^j -1- 2H,C03±5 Ca^H,(P002 



-f CaH,(C03)2. 



As is well known if none of the products to 

 "the right of the reaction are removed from 

 solution, the reaction soon reaches a state of 

 equilibrium. If the di-calcium phosphate is 

 •continually removed but the calcium bi-car- 

 bonate only in part, then the reaction will 

 continue a little farther, but also soon comes 

 to a state of equilibrium due to the accumu- 

 lation of the calcium bi-carbonate. When this 

 point is reached, the further solution of the 

 phosphate is prevented. This is the condition 

 that obtains for such plants as are low in cal- 

 cium oxide and hence do not absorb the cal- 

 cium bicarbonate in the proportion to the 

 ■dicalcium phosphate as given in the reaction. 

 In such cases, the plants soon suffer for sol- 

 uble phosphates. If both of the products to 

 the right of the reaction are simultaneously 

 and continually removed in the proportion 

 given, then the reaction continues from left 

 to right and there results a continuous supply 

 of soluble phosphates along with soluble cal- 

 cium bicarbonate. This is the condition that 

 obtains, at least in part, with plants containing 

 a high calcium oxide content, and hence such 

 plants are strong feeders on raw rock phos- 

 phate. 



In accord with other investigators the writer 



has found that the use of ammonium nitrate 

 or sulfate as a source of nitrogen in quartz 

 plant culture work, greatly increases the avail- 

 ability of raw rock phosphate to plants which 

 are normally weak feeders on this material. 

 In the light of the present theory this is very 

 satisfactorily explained as follows: Calcium 

 bicarbonate being much more soluble in a 

 water solution of ammonium salts than in 

 water alone, it follows that the addition of 

 ammonium salts allows the preceding reaction 

 to continue from left to right to a much 

 greater extent than if water alone is present. 

 The addition of a salt in which the products 

 of the reaction are more soluble has the same 

 effect to a certain extent as is obtained by re- 

 moving the products of the reaction. 



With the theory^ here proposed it is possible 

 to predict from the calcium oxide content of a 

 plant whether or not that plant in quartz cul- 

 tures will be a strong or weak feeder on raw 

 rock phosphate. Under soil conditions there 

 are many subsidiary factors that influence the 

 availability of phosphates, and hence under 

 such conditions the relative growth of a plant 

 can not be taken rigidly as a true index of 

 its feeding power for the limiting element 

 which is supplied in an insoluble form. Seem- 

 ing deviations from the theory may result 

 under such conditions. It is possible that with 

 proper restrictions the theory can be applied 

 to the feeding power of plants in a broader 

 way, involving the use of other insoluble plant- 

 food materials besides rock phosphate, and the 

 general theorem could then be worded as fol- 

 lows : The feeding power of a plant for an in- 

 soluble substance depends primarily upon two 

 conditions, viz., (1) the solubility of that sub- 

 stance in carbonated water and, (2) whether 

 or not the plant removes from solution all the 

 products of the solubility reaction in the proper 

 proportion, so as to allow the solubility reac- 

 tion to continue indefinitely. 



With the theory here presented the writer 



2 Since writing this article the writer 's attention, 

 has been called to a recent publication in Zliur. 

 Opytn. Agron., 15 (1914), No. 1, 54, by F. V. 

 Chirikov, who from entirely independent work 

 from this, has come te practically the same conclu- 

 sion as the one set forth in this paper. 



