EXPERIMENTAL 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. 
Nearly 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, or 
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 in contact with raw phosphate, therefore, 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 with 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 
