4 LEGUME INOCULATION AND THE LITMUS REACTION OF SOILS. 



Just enough water (tested and found neutral) is added to saturate 

 the soil on the filter paper. Covers are placed on the petri dishes, 

 and they are allowed to stand for one hour. A check petri dish con- 

 taining only litmus paper and filter paper moistened with the same 

 water is allowed to stand under the same conditions.*^ The filter 

 paper, which separates the soil from the testing paper, gives a uni- 

 formity of hackground and evenness of contact, and it also allows 

 the litmus paper and the filter paper to be checked against each other 

 alone, and this procedure enables one to distinguish very delicate 

 reactions. Clean white blotting paper substituted for filter paper 

 gives similar results, but a longer time is required for the soil to act 

 upon the litmus paper. 



RELATION OF LITMUS REACTION AND LEGUME INOCULATION. 



The following typical example will show the relation between the 

 soil reaction to neutral litmus paper and the growth of certain 

 varieties of the nodule bacteria of legumes: 



On two tobacco fields near Upper Marlboro, Md., vetch was sown 

 in the fall of 1906, seed having been inoculated with bacteria in 

 liquid cultures prepared by the Bureau of Plant Industry. One of 

 these fields was bottom land, the other upland. In the spring it 

 was found that the vetch on the bottom land had made a very poor 

 growth and was sickly in appearance. An examination of the roots 

 showed an entire absence of nodules. The vetch on the upland 

 field made a fair growth and was plentifully supplied with root 

 nodules. Samples tested by the petri-dish method showed at once 

 that the soil from the bottom land reddened the litmus, while the 

 upland soil blued the litmus; the check, of course, remained un- 

 changed. 



Soil extracts from each field were prepared by leaching 100 grams 

 of soil with 200 cubic centimeters of distilled water, the soils being 



a A modified form of this litmus test (using materials commonly available) gives 

 results which agree satisfactorily with the petri-dish and filter-paper combination. 

 Litmus paper can be obtained at any drug store; 5 cents' worth will be sufficient 

 for making several tests. Neutral paper in strips is to be preferred; blue strips may 

 be used. The paper should be kept in a tightly stoppered bottle and should not be 

 touched by the hands. Use forceps or a pin to place the litmus strip in position. 

 Cut out disks of clean white blotting paper of the right size to fit tightly in the bottom 

 of an ordinary drinking glass, or tumbler. Place the litmus strip in the bottom of the 

 glass, the blotting paper over this, and then add the soil to be tested. Add suffi- 

 cient water to saturate the soil, and cover the glass with a saucer. At the same time 

 prepare another glass in the same way, without adding the soil. This "check" will 

 show whether the water or the blotting paper used has affected the litmus paper. 

 The test glasses should be allowed to stand at least two hours; the color of the litmus 

 paper against a clean white background can then be observed through the bottom of 

 the glass mthout disturbing the soil. 

 [Cir, 71] 



