344 



SCIENCE 



[Vol. XVII. No. 437 



I want now to show you that the size of these spaces upon 

 which the circulation of water depends may be varied at will 

 by the ordinary commercial fertilizers used by farmers. And 

 first let me show you the very simple but cirious (for hith- 

 erto unexplained) phenomena of fiocculation. 



Here is some muddy water in this beaker. The particles 

 of clay are so extremely small, and have so much surface in 

 proportion to their weisrht, that the ordinary convection cur- 

 rents in the liquid are sufficient to keep them in suspension 

 for an indefinite time. A trace of salt, kainit, or acid will 

 cause the clay to come together in light, loose flocks, like 

 curdled milk, and these flocks will quickly settle and leave 

 the water above perfectly clear. If only a trace of these 

 substances has been added, a few drops of ammonia will 

 neutralize this effect, and break up the flocks and push the 

 clay particles without the range of their mutual attrac- 

 tion, so that the liquid will not clear for days or weeks or 

 years. 



When this is watched under the microscope, the particles 

 in the turbid liquid when ammonia is present — if they are 

 very small and freely suspended in the liquid — do not or- 

 dinarily come very close together, or if they do they are 

 shoved aside by an elastic cushion. When the least excess 

 of acid, salt, or lime is added, however, they not only come 

 close together but segregate in large flocks, which float 

 around as though held by a rigid hand. If too much acid 

 has not been added, the further addition of ammonia will 

 push the particles apart again, but this cannot be kept up 

 indefinitely, for the accumulation of the salt formed causes 

 a permanent fiocculation, which we have not yet been able 

 to overcome. 



As I have said, the reason for this has not yet been satis- 

 factory explained, although it has formed the substance of 

 several memoirs to the National Academy of Sciences and of 

 a large bulletin of the United States Geological Survey. It 

 is a phenomenon of great economic importance, as it accounts 

 for the formation of flats and shoals at the mouth of rivers 

 where they empty their muddy water into the salt waters of 

 the ocean, for the curious periodic shoaling and deepening 

 of the channel at the mouth of the Mississippi River with low 

 and high water, and for the peculiar clearness of limestone 

 water. It is a phenomenon also of the utmost importance to 

 agriculture. 



I am glad to say that Dr. Kimball has taken an interest in 

 it, and has given valuable aid and suggestions, and I believe 

 we shall be able to work it out before long, as we already 

 have a very plausible and tentative explanation awaiting 

 experimental verification. 



I will try to show you that similar forces may act in tlie 

 soil, and produce very material and important modifications 

 in the arrangement of the soil grains — changing in a very 

 remarkable degree the relation of the soil to the circulation 

 of water. 



Here are three argand lamp chimneys eight inches long 

 and two inches in diameter, the upper two inches of the tube 

 being graduated on the side. Equal weights of the same soil 

 occupies six inches in depth of each tube. The soil is the 

 characteristic truck land of Anne Arundel County, — light, 

 loose, and loa my ; almost too light for wheat or grass, for 

 water circulates too freely in it for these crops. An inch in 

 depth of water passed through these saturated soils in just 

 about the same time (twenty-five minutes) : a few drops of a 

 solution of kainit was added to the water in this second 

 tube, and a few drops of ammonia to the water in this third 

 one. The effect of the kainit, as in the muddy liquid, is to 



pull the fine panicles of clay much closer to the grains of 

 sand and to make the soil more loamy and looser in texture. 

 The large spaces have become larger, and the small spaces 

 smaller, and the effect of this, as you saw with the tubes, is- 

 to very materially increase the rate with which water circu- 

 lates in the soil. 



Now I do not pretend to say that even under the intense 

 condition of my experiment this change is instantaneous, for 

 it is not. While the acid or salt, or kainit or lime, makes 

 it possible for the soil particles to come closer together, the 

 motive power which actually brings them together is proba- 

 bly the changing temperature and ciianging moisture con- 

 tent, so that in practice the change in the physical structure 

 of the soils will probably be very gradual, and be noticeable 

 only after several years of continuous application. 



Sir John Lawes has observed that the continued use of 

 nitrate of soda has made his soil more loamy and porous. 

 It is a matter of common experience that such changes occur 

 in stiff clay land from the continued use of acid phosphates 

 and lime, but no special significance has ever been attached 

 to it, as it has been considered incidental to other benefits 

 (hitherto unexplained, be it understood) derived from the 

 application. 



The effect of ammonia on the soil is even more remarka- 

 ble, as it is so instantaneous, and the effect even in this 

 short time is so marked. The ammonia loosens the hold of 

 the clay particles on the grains of sand, and the currents of 

 water in the narrow spaces seemingly are sufficient to detach 

 them, as the liquid, before clear, is now muddy. The fur- 

 ther movements can be watched under a microscope focused 

 against the side of the tube. The clay flocculates immedi- 

 ately, probably from the effect of the salts in the soil, and 

 these loose flocks, floating around, catch against the projecting- 

 sides of the grains of sand, and the spaces gradually fill up 

 with this light, loose material. 



The clay is more evenly distributed throughout the soil,, 

 and the circulation of water is very much retarded. While 

 before the ammonia was put in, the inch of water passed 

 through the soil in about twenty-five minutes, it will take it 

 now at least six or eight hours. 



From our own work it is probable that the organic matter 

 of stable manure and the alkaline carbonate of wood ashes 

 would have much this same effect, and I believe this is the 

 reason the agricultural value of these substances on certain 

 soils has always been out of all proportion to the amount of 

 plant food they contain. 



This interpretation of the results of the mechanical analy- 

 sis of soils gives a very clear explanation of the marked adap- 

 tability of certain plants for certain characters of soils under 

 the same climatic conditions. Truck, wheat, grass, and the 

 different grades of tobacco all succeed best on soils which 

 differ essentially in their pliysical properties. Not only so,, 

 but it is quite possible to calculate the relative rate with 

 which water*will circulate through these different types,, 

 and we have, therefore, a means of classifying soils by re- 

 ferring them to these types; and when the observed rate of 

 circulation differs from the rate calculated from the me- 

 chanical analysis, as it does in " worn out" lands, we have 

 the still more important information of the changes which 

 have occurred in the structure of the soil, and we have seen 

 that this may be varied at will by the ordinary fertilizing- 

 materials. I am satisfied that it is through some such care- 

 ful study of the soil further advance in agriculture will be 

 made and the most intelligent use of manures and fertilizers 

 be secured. 



