46 
Transactions Texas Academy of Science. — 1908-1909. 
resultant of complex forces, among which the chemical nature of 
the nitrogenous compounds of the soil is only one. 
The phosphoric acid and potash of the soil are subjected to a less 
number of factors, and the plant food at the disposal of the plant 
probably depends to a greater extent upon the nature of the com- 
pounds present, but other factors undoubtedly enter in and affect 
the results. 
Another matter which must enter into consideration is the differ- 
ence in the power of different plants to withdraw plant food from 
the soil. The exact relation of various plants in this respect has not 
been worked out, even for a single form of plant food, but un- 
doubtedly there is a difference in assimilative power and a great 
difference. This must be taken into consideration in applying re- 
sults of chemical analysis. 
The action of fertilizers upon a given soil depends somewhat 
upon the season, on the physical character of the soil, the previous 
treatment of the soil, and upon the rotation of crops to be grown 
as well as on the chemical nature of the soil. 
Hence the chemical analysis of a soil cannot tell us exactly the 
best fertilizer to be used for a given soil and a given crop. 
The chemical analysis of a soil thus can give us only a limited 
amount of information, yet such information is very valuable. 
ACTIVE PHOSPHORIC ACID. 
Nitrogen and phosphoric acid are the two most important forms 
of plant food for the South. Phosphates are used in enormous quan- 
tities for fertilizing purposes and their use is no doubt due to a 
need of the soil Avhich it is profitable to supply. By active phosphoric 
acid is meant the phosphoric acid extracted from the soil by N-5 
nitric acid. Other solvents have been proposed and tested, but 
this one appears to be the most suitable. Chemical tests have been 
made by us to ascertain the nature of the information given us by 
this solvent. Full details are presented in Bulletin 126 of the Texas 
Experiment Station. 
By tests with mineral phosphates, it was found that fifth-normal 
nitric acid dissolves calcium phosphate completely, but dissolves 
mineral aluminum phosphates or basic ferric phosphates only to a 
slight extent. It thus distinguishes between these two classes of 
compounds in the soil. 
But the solvent may not distinguish between minerals having 
unequal values to plants. Soils may contain equal quantities of 
phosphates soluble in fifth-normal nitric acid, and yet give up un- 
