August 14, 1914] 



SCIENCE 



247 



However, carbonic acid reacts acid to litmus, 

 and, contrary to general belief, the reddening 

 of litmus paper -wben put into carbonated 

 water for several minutes is i)ermanent even 

 on drying. In testing fresh soil it is therefore 

 necessary to keep all living plant roots away 

 from the paper, as they may turn it red, due to 

 the excretion of carbon dioxide. The soil 

 water may be highly enough charged with 

 carbon dioxide to affect the test. The moist 

 hand must also not come in contact with the 

 litmus paper, for that may redden the paper. 

 When a soil is only slightly acid the litmus 

 test is not sharp and positive and thus often 

 causes confusion. 



With a view of securing a more reliable test 

 the writer has evolved the following zinc sul- 

 fide method. It was found that acid soils when 

 boiled with zinc sulfide and water would liber- 

 ate hydrogen sulfide, which, as is well known, 

 can be detected very easily and positively with 

 lead acetate paper. With this as a basis, the 

 following method was worked out: 



Ten grams of soil are placed in a 300 c.c. 

 Erlenmeyer flask and to this is added 1 gr. 

 calcium chloride, 0.1 gr. of zinc sulfide, and 

 100 c.c. water. This is thoroughly shaken and 

 then heated over a flame. After the contents 

 have boiled one minute, a strip of moistened 

 lead acetate paper is placed over the mouth 

 of the flask and the boiling continued two 

 minutes more, when the paper is removed. If 

 the soil is acid the paper will be darkened on 

 the under side in proportion to the degree of 

 acidity. If it is non-acid, no darkening will 

 occur if the test has heen ■performed as just 

 outlined. 



The calcium chloride is added to make the 

 test more sensitive. It reacts with the com- 

 paratively insoluble soil acids and forms a 

 small amount of hydrochloric acid which 

 readily liberates hydrogen sulfide from zinc 

 sulfide. The mixture is boiled one minute 

 before putting the test paper in place in order 

 to expel most of the carbon dioxide and also 

 to more nearly bring all tests to the same con- 

 dition before applying the paper. This test 

 will positively detect smaller amounts of soil 

 acids than the litmus test. The range of 



colors, showing degree of acidity, is large, 

 being from white to black. 



At first thought it seems possible that on 

 boiling soils with water, some which had 

 undergone anaerobic fermentation might give 

 off appreciable amounts of hydrogen sulfide 

 and thus confuse the test. On careful con- 

 sideration this appears very improbable, for if 

 the soil is alkaline any hydrogen sulfide formed 

 in the process of fermentation will combine 

 with the excess of bases present and is thus 

 not given off in the test. Fresh peat and muck 

 soils, some of which had lately been inundated, 

 were tested and in no cases did the alkaline 

 ones give a coloration to the test paper. The 

 test has been applied to a considerable number 

 of soils and also other materials of known 

 reaction and as yet not a single objection to 

 the test has arisen. 



As a quantitative method, an effort is being 

 made to measure the degree of acidity by 

 titrating with standard iodine solution the 

 hydrogen sulfide which a soil will liberate. 

 Whether this will work with all soils has as 

 yet not been determined. By using this test 

 for the end point in the Veiteh lime water 

 method for acidity or lime requirements, the 

 present Veiteh method is considerably short- 

 ened and made far more accurate. 



The most important part of the test, how- 

 ever, is the fact that it can be made approxi- 

 mately quantitative, and still require only very 

 simple apparatus — such as can be carried 

 right into the field, and require no more than 

 ten to fifteen minutes for the determination. 

 This will make it of great value to the exten- 

 sion man, field agent, etc. In fact, the farmer 

 himself will be able to determine the lime re- 

 quirements of his soil, by following very 

 simple directions. The principle of this quan- 

 titative method depends upon the fact that for 

 any particular class of soils the degree of 

 acidity is closely proportional to the intensity 

 of color produced on the paper when the test 

 is conducted as previously outlined. The color 

 on the test paper needs only to be compared to 

 a standard color scale and from an accompany- 

 ing table the degree of acidity or lime require- 

 ments is read off directly. This standard color 



