272 SOILS. 



small quantities by most culture plants; so that they rarely 

 need to be supplied in fertilizers. Nitric acid, however, is not 

 only one of the most important fertilizers, but also the most 

 expensive ; hence the passing-off of nitrates in drainage-water 

 is of such serious concern to the farmer, that the causes of its 

 occurrence, and the means of preventing such loss, should be 

 fully understood. This subject will, however, be more fully 

 considered farther on. 



The above Distinctions not Absolute. It should, however, 

 be also understood that while the above statements hold good in 

 a general way, yet the line drawn is by no means an absolute 

 one. For just as in the case of physical adsorption the long 

 passing-through of distilled water will gradually abstract the 

 substances condensed on the surface of the soil-grains, so an 

 overwhelming amount of a solution of any one kind will have 

 a tendency to substitute its own ingredients for those already 

 present in the soil, removing the latter to a greater or less 

 extent, even in the case of potash and phosphoric acid. 



As an example in point, may be cited the case of the natural minerals 

 Analcite and Leucite, which Lemberg was able to reciprocally trans- 

 form from their natural condition of soda- and potash-alumina silicates 

 merely by alternate treatment with solutions of potassium and sodium 

 chlorids respectively. (See chap. 3, p. 37). The same is true in the 

 case of the zeolitic matter of the soil. There is nevertheless a distinct 

 preference in the direction of the retention of potash as against soda ; 

 so that in the case of alkali soils, a large excess of potash is found to 

 be present in the zeolitic form, notwithstanding the presence of some- 

 times very large amounts of the chlorid, sulfate and carbonate of soda. 

 This preferable retention of potash is, of course, of material advantage 

 in the case of the use of soluble potash-fertilizers, as well as in prevent- 

 ing the waste of the potash of the soil itself. 



ABSORPTION, OR CONDENSATION, OF GASES BY SOILS. 



Like all bodies in a state of fine division, soils are capable of 

 absorbing a not inconsiderable amount of various gases. It 

 may be said that in general, other things being equal, the 

 amount thus condensed on the surface of the soil-grains is more 

 or less directly proportional to the facility with which the gas 



