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solution of the question of fertility. However, to determine definitely, once for all, 
whether the fertility of the soil, using this word as I do in this connection to indi- 
cate the possibility of plant growth from the composition of the soil — I am using 
it in a limited sense to-day — to see it this was related to the physical properties of the 
soil or to the chemical properties, we made an extract of the soil according to a con- 
ventional method of analysis, that is, by using 1,000 grams of soil and 1,200 cubic 
centimeters of water, stirring for three minutes, allowing twenty minutes to settle, 
and filtering through a Pasteur filter, which removes the clay and incidentally the 
bacteria. Then we grew plants in the water culture so prepared and found that the 
plants grown in these solutions exhibited the same characteristics as the plants grown 
in the soils from which the solutions were derived, showing that we had transferred 
the limiting conditions of fertility from the soil into the solution that had been pre- 
pared. It was therefore evident that the limiting conditions of fertility did not exist 
in the physical properties of the soil. 
With that indication before us we again took up the investigation of the chemical 
constitution of the soil, directing our attention particularly to the molecular combi- 
nation of the salts — that is to say, whether the character of the salt itself had any 
effect upon the plant, as we helieved from our investigations in Bulletin 22 that the 
actual amounts of potash, lime, and phosphoric acid were not associated ordinarily 
with crop production. 
The first thing that we found was that it was possible to determine easily the 
functional activity of the plant in these different solutions, prepared from good and 
from poor soils — that is, from fertile and infertile soils — by growing them either in 
soil extracts or in the soil itself, by measuring the relative transpiration of the plant. 
Transpiration, as you will observe, is in a sense a measure of respiration — that is, 
it can be taken as a relative measure of that if you have plants under the same 
conditions of sunlight, heat, and ventilation. Transpiration is the evaporation of 
water from the leaves. It is not an essential thing in itself, but accidental and inci- 
dental to the respiration of the plant, just as the loss of water through the lungs is 
incidental to the breathing of a person. It can, however, be taken as a measure of 
the functional activity of the plant, and we have so used it. 
One of the first things we found was that plants grown in extracts of poor soils, or 
on the poor soils themselves, transpired much less water than plants grown in the 
good solutions or on the good soils. They were functionally less active. It takes 
about six to nine days to notice the difference, however. After that the plants in 
the poor solutions evidently suffered. They did not act as though there were a toxic 
substance in the soil or as though they were poisoned, because the daily rate of 
transpiration would then have gone up to a maximum and fallen again, as we have 
frequently observed. On the contrary, transpiration went right on, but the daily 
increment of solution used — that is, the daily plant food used — was greater always 
on the good soil than on the poor soil. This difference appeared usually from six to 
nine days after the seedlings were started, and the curves representing the transpi- 
ration continued to diverge in a very marked way, so that soils can be easily recog- 
nized by their behavior to the transpiration and respiration of the plant. Now 7 , it 
seemed important to find out the cause of this, and it was finally located in the effect 
of the salts on the roots. In the solutions that show T ed a very low transpiration, the 
tips of the roots — the growing part, the part that absorbs water — had thickened up 
and corked over. It was not able to absorb as it did before. On the contrary, roots 
growing in the solutions from the good soils remained perfectly clear, transparent, 
and healthy, and showed no tendency to this corking or thickening or hardening or 
whatever it may be. It appears that the salt itself, or the complex that may exist 
in the soil combination, or whatever it may be that exists in solution in the soil, may 
have a more or less irritating effect on the root, and if it is irritating the root corks 
over and shuts itself up, so to speak, so that it will get rid of this irritating substance. 
As soon as the root adjusts itself to this condition, in order to prevent wilting the 
leaves adjust themselves to the root, so that there is actually less transpiration than 
if this irritation had not been set up at the root. It is well known to all of you that 
it is possible to almost entirely prevent the transpiration of water and to starve a 
plant in a very strong culture solution. 
Then one of the important questions was whether the nature of the salt had much 
to do with this transpiration and with the effects on the roots. For this purpose we 
made up a number of solutions with potassium, calcium, magnesium, sodium, and 
ammonium, combined with different acids, to give nitrates, chlorids, phosphates, 
sulphates, and carbonates, and by adding these same bases in the amounts per million 
of water, in different combinations with the acids, as, for example, first as nitrate of 
potash, sulphate of lime, and so on, and then as nitrates of lime and sulphate of pot- 
ash, and going around changing the character of the salts containing the same 
amount of essential plant-food bases, we got very different results in the functional 
