164 MASSACHUSETTS HORTICULTURAL SOCIETY. 



value when it consists of phosphate of lime that is truly "re- 

 verted," but there are other compounds which are dissolved by 

 ammonium citrate to which the term available should, in my judg- 

 ment, be applied with caution. Let me explain. 



We have recently erected at the Maine Experiment Station a 

 forcing house for the study of certain problems in plant nutrition. 

 For two years we have been investigating the foraging power of 

 a variety of plants for phosphoric acid as obtained from different 

 sources, and among other materials entering into our work is one 

 somewhat peculiar. It is made up of phosphates of iron and 

 aluminum, which in their natural condition were hydrates and 

 insoluble in the citrate solution, but which by the application of 

 heat are rendered anhydrous and soluble in ammonium citrate, 

 though not in water. In the parlance of fertilizer analysis this 

 phosphoric acid is therefore styled reverted and available. The 

 fact is, however (and this is a sad comment on the citrate method 

 for determining the real availability of phosphoric acid), the 

 phosphoric acid in this form does not prove to be highly available, 

 at least under the conditions of our experiments, except in the case 

 of turnips. In our experiments clover, peas, barley, corn, 

 potatoes and tomatoes have not been able, under forcing house 

 conditions, to feed from it readily, whereas, in the case of the 

 clover and peas at least, floats from Florida rock, very little of 

 the phosplioric acid of which is dissolved in the citrate solution, 

 have proven more useful. 



The important point in this connection is this : these anhydrous 

 phosphates of iron and aluminum are being used to increase the 

 per cent of so-called available phosphoric acid in some of our 

 mixed fertilizers. To this fact in its relation to the forcing house 

 and the market garden, I shall refer later. 



The potash salts imported from Germany are the almost 

 exclusive source of potash in our mixed fertilizers. We have 

 three of these compounds, the chloride or muriate, the sulphate, 

 and lately the carbonate, all of which are soluble in water, and 

 which, for general agricultural uses, we may regard as practically 

 equal in value, and probably so for the special uses of the horti- 

 culturist. Commercially the sulphate is more valuable than the 

 muriate. 



The commercial sources of nitrogen are greater in number and 

 more varied in character than is the case with either phosphoric 

 acid or potash. No plant-food problem, either commercially or 



