Soil Fertility and Phosphoric Acid. 
43 
phoric acid. The difference between the two shows the effects of the 
phosphoric acid. 
A pot experiment can be arranged so that the soil and the plant food 
are controlling factors in the growth of the plant. The results are 
secured comparatively quickly. For certain problems it is unequaled. 
How far the results of pot experiments can he applied to the field is 
an open question. In. the field the plant has a larger quantity of soil 
to draw from; soils may appear more deficient in pots than they do in 
the field. Further the depth of the soil is a factor. That the same soil 
six inches deep in one place and twelve inches deep in another affords 
different amounts of plant food is too obvious for discussion. In some 
cases the subsoils must be considered. Some subsoils may supply plant 
food, while others do not. It is merely my intention to indicate the 
complexities of the problem which confronts us. 
The last method for studying the plant food of the soil which I shall 
mention is by chemical analysis with weak solvents. This method ap- 
pears promising. Like pot experiments, the same difficulties would 
meet us in applying the results as with pot experiments. 
The Texas Experiment Station has been conducting pot experiments 
upon typical soils from various sections of the State. These experi- 
ments show the needs of the soil, in itself, for plant food. The results 
of a number of these experiments have been published in our Bulletin 
Ho. 99. As a rule, the soils of Texas are very much in need of phos- 
phoric acid. 
We are also studying the chemical methods for determining the needs 
of the soil 'for phosphoric acid, and we have had some success, in 
spite of the complexity of the problem. I wish to explain, briefly, how 
difficult this problem is. 
We have found, in the first place, that soils may contain compounds 
which are equally soluble but which are unequal in the readiness with 
which they give up their phosphoric acid to plants. 
We have found, in the second place, that soils may withdraw from 
solution a part of the phosphoric acid which weak acids have dissolved. 
Different soils have different absorbing powers. Two soils may contain 
equal quantities of phosphoric acid of equal value to the plant, and yet, 
on account of this peculiar absorbing power of the soil, the quantity 
of phosphoric acid which is dissolved from the two soils may be widely 
different. I have one soil which absorbs 95 per cent of the phosphoric 
acid put in the weak acid solution. 
We have found, in the third place, that the same compound of phos- 
phoric acid may have different value to the same plant in different soils. 
In spite of these difficulties, which we are subjecting to a thorough 
study, we are making progress. We have been, testing our soils for their 
