EXPEEIMENTAL WORK WITH RAW ROCK PHOSPHATE. 17 



amounts of phosphoric acid and lime thus rendered water soluble 

 would be many times greater than required by these same crops. 



Nearl}^ all the experiments so far considered for determining the 

 effect of decaying organic matter upon rock phosphate have de- 

 pended on some weak solution (usually an acid) to measure the 

 availability of the phosphate thus treated. Truog's conclusion that 

 the conventional laboratory methods for measuring the availability 

 of phosphates are unsuitable under certain conditions seems quite 

 logical. Take, for instance, the fermentation of manure, which re- 

 sults in the formation of alkaline products, which predominate for 

 a time' at least over the acid products. There seems little reason 

 to suppose that changes in raw rock phosphate brought about by 

 such fermentation can be measured by the solubility of the phosphate 

 in acid solution. 



It is true that after the fermentation has proceeded further and 

 the alkaline products have been largely evolved or neutralized, <5i- 

 after fermentation has ceased and the altered phosphate has been 

 released by the death of the organisms which absorbed it into their 

 cells, one might expect that the solubility of the phosphate in water 

 saturated with carbon dioxide would be an index to its availability. 

 But while carbon dioxide increases quite appreciably the solubility 

 of the difficultly soluble phosphates, this effect is limited at any one 

 time by the amount of the gas which can enter into solution. During 

 the growing season, however, the constant absorption of the dis- 

 solved phosphate by plant roots allows the carbon dioxide to act 

 continually upon more phosphatic material. Decaying organic mat- 

 ter m contact with raw phosphate, tlierefore, should be furnishing 

 carbon dioxide sufficient to keep a fairly constant supply of phos- 

 phoric acid in the soil solution, and therefore the availability of the 

 phosphate should be considerably increased by its presence. 



POT AND GREENHOUSE WORK. 



The main advantage of pot work in testing out fertilizer materials 

 lies in the fact that the conditions under which the experiments are 

 conducted can be to a large extent controlled. 



Soil in 'the same mechanical condition and of the same chemical 

 composition can be obtained for a series of pots vnth little difficulty. 

 Uniform conditions of moisture and temperature can be maintained 

 throughout the growing period, and such disturbing factors as blight, 

 insect pests, and injuries from heavy rain, hail, or high winds can 

 be reduced to a minimum. 



On the other hand, the slightest variation in the conditions of the 

 experiment may produce such differences in the crops as to lead to 

 very erroneous conclusions. Careful selection of seed, uniform pack- 



56841°— Bull. 699—18 2 



