492 



SCIENCE 



[N. S. Vol. XL. No. 1031 



compounds which react with litmus when the 

 test paper is brought into direct contact with 

 the solid particles and which enter into double 

 decomposition with any salt with which they 

 come in contact liberating the corresponding 

 soluble acid. This latter assumption is rather 

 absurd in the light of our modern ideas of 

 chemistry. The law of mass action is prob- 

 ably one of the most generally accepted laws 

 of chemistry and if we are to accept this law, 

 it is hard to conceive of any acids, as insoluble 

 as these humic acids must be, entering into a 

 double decomposition with a neutral salt such 

 as sodium chloride and setting free such a 

 strong acid as hydrochloric acid. Much ex- 

 perimental work has been done on these humic 

 acids, most of this work consisting of attempts 

 to isolate the acids in a pure form. Experi- 

 menters claim to have done this and have even 

 gone so far as to assign definite chemical 

 formulse to some half dozen of these acids. 

 However, no two experimenters seem able to 

 agree on these formulae. 



Since Van Bemmelen's work on colloids and 

 adsorption, a newer and certainly far more 

 reasonable theory has been put forward to ex- 

 plain the action of acid soils. It is well known 

 that the coagulation of a colloid by a solution 

 of a neutral salt is accompanied by the ad- 

 sorption of one or other of the ions. If the 

 ooUoid be electro-negative, it wiU adsorb the 

 positively charged ion of the salt setting free 

 a corresponding amount of acid. If it is 

 electro-positive, it will adsorb the negatively 

 charged ion setting free a corresponding 

 amount of the base. In the case of the soils, 

 there is present much negatively charged col- 

 loidal matter. If deficient in basic material, 

 this colloidal matter is present in a defloccu- 

 lated condition and is capable of adsorbing the 

 base from any neutral salt with which it comes 

 in contact. Thus if the soil particles are 

 brought into contact with blue litmus, it ad- 

 sorbs the base of the blue litmus salt leaving 

 the red acid dye on the paper. When shaken 

 with a solution of a neutral salt, the basic 

 portion of the salt is adsorbed leaving a corre- 

 sponding amount of acid in solution. If the 

 salt used be sodium chloride, the sodium 



hydroxide is adsorbed and hydrochloric acid 

 liberated. 



The acid soils may be divided into two 

 types: first, those found in sandy upland 

 regions, and second, those found in peat or 

 muck lands. The first type has been thor- 

 oughly investigated by the virriter'^ in the 

 chemical laboratory of the Michigan Agricul- 

 tural Experiment Station, and he has been 

 able to show that not only is the peculiar be- 

 havior of these soils not due to the presence of 

 any true organic acids, but that it is not due 

 to organic matter at all. It was found that 

 soils in which all the organic matter had been 

 destroyed, still retained their acid properties, 

 these properties being due to the presence of 

 coUoidal substances, probably hydrated sili- 

 cates of iron and aluminum. The second type 

 of soils have been investigated by Baumann 

 and Gully^ who have shown that in the peat 

 soils the acid proi)erties are due to the colloidal 

 matter of the cell covering of the hyalin 

 sphagnum cells. 



The remedy for soil acidity is well knovm. 

 If a soil be treated with lime (either calcium 

 carbonate or calcium hydroxide), the acid 

 properties are destroyed and the soil restored 

 to its former condition of fertility. Many 

 methods have been devised for the determina- 

 tion of the degree of acidity of the soil or, as 

 it is often called, the " lime requirement " 

 of the soil. Most of these methods are based 

 on the old humic acid theory in spite of the 

 fact that this theory has been so thoroughly 

 discredited of late. Such a method was re- 

 cently described by E. Trugg.^ The method 

 consists of treating the soil with calcium 

 chloride, zinc sulphide and water. The soil, 

 if acid, reacts with the zinc sulphide liberating 

 hydrogen sulphide which can be detected by 

 means of lead acetate paper. As to the use of 

 the calcium chloride, we wiU quote from the 

 article : 



The calcium chloride is added to make the test 

 more sensitive. It reacts with the comparatively 

 insoluble soil acids and forms a small amount of 



iJour. of Phys. Chem., 18, 355 (1914). 



2 Mitteilung der K. Boyr. Moorkulturanstalt, 

 1910, 31-156. 



sSciENCB, 50, 246 (1914). 



