USE OF PHOSPHORUS IN DIFFERENT FORMS 243 



and slag, is that it is free from acidity and has no tendency to injure 

 the soil. This is a minor advantage, because, if acidity develops 

 from the continued use of acid phosphate (and it does), it can be 

 corrected at small expense by the addition of any form of lime. 



Another point, previously mentioned, of fundamental signifi- 

 cance is the simple fact that a form of phosphorus originally 

 present in all natural soil material is finely divided natural rock 

 phosphate, and through all agricultural history the principal 

 source of phosphorus in plant growth has been this same natural 

 phosphate. On most normal soils one of the chief benefits of farm 

 manure and green manures is undoubtedly due to their power to 

 liberate phosphorus from these insoluble natural phosphates of 

 the original soil. 



In considering culture experiments, whether field cultures or 

 pot cultures, three points should be kept in mind : 



(1) What are the limiting factors of plant growth under the 

 conditions of the experiment? 



(2) Does the applied fertilizer increase the crop yield by direct 

 or indirect action? 



(3) In case of insoluble fertilizers, are the conditions such that 

 the plant food applied will be made available to the crop? 



Thus, an experiment to determine the comparative agricultural 

 value of different forms of phosphorus cannot be expected to fur- 

 nish satisfactory evidence if conducted on a soil in which nitrogen 

 is the element that limits the crop yield; or, even though phos- 

 phorus is the first limiting element, the results cannot be conclu- 

 sive if the nitrogen limit is but little higher. For example, if the 

 conditions are such that the soil will furnish phosphorus for only 

 40 bushels of corn per acre, and sufficient nitrogen for only 45 bush- 

 els per acre, the yield cannot be increased above 45 bushels by the 

 addition of phosphorus alone, no matter what form is applied or 

 how much becomes available. In other words, one phosphate 

 fertilizer might supply phosphorus for only 5 bushels, and another 

 sufficient for 25 bushels, increase, but the results of the culture 

 experiment would show no such difference, because beyond the 

 45 bushels the yield is limited by a second entirely different factor. 



The second point is important with every form of experiment. 

 Thus, a student reported having found silver in an unknown solu- 



