38 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 



cases, very thin. The usual variation in the depth of this layer is 

 from 4 to 12 inches. A suitable quality of topsoil is produced some- 

 times by agencies other than cultivation, when the depth may be 

 considerably greater than 12 inches. 



The safest guide in identifying topsoil that will make a satisfactory 

 road surface is the service test, and, fortunately, it is nearly always 

 possible to make this test by simply observing the condition of a 

 section of road already constructed of the soil in question. The best 

 method to follow in making a selection is to pick out such sections 

 of earth road in the vicinity of the one to be surfaced as remain con- 

 tinuously in good condition. Then, if practicable, obtain the soil in- 

 tended for the new road from the fields adjacent to the best sections 

 of this good earth road. Otherwise employ specimens of the soil from 

 these fields to compare with other available soils and base the selec- 

 tion on such comparisons. When comparisons are not conclusive the 

 various kinds of available soil should be tried out on a small scale 

 before any considerable area of topsoil surface is constructed, because 

 there is at present no other reliable method for comparing the relative 

 excellence of different topsoils and it is very easy to be misled into a 

 wrong selection by incomplete laboratory tests. 



The following list of characteristics usually belonging to topsoil 

 of a satisfactory quality at least may prove helpful in selecting soils 

 upon which to make the service test referred to above : 



1. Some of the best topsoils so far discovered are of granitic ori- 

 gin ; that is, the soil has been formed by the decomposition of granite. 



2. Good topsoil usually has a very " gritty " texture, and when 

 rubbed between the fingers this characteristic should seem more pro- 

 nounced than any other. 



3. Samples of soil taken from a number of first-class topsoil roads 

 and subjected to mechanical analysis have shown an average sand 

 content of from 50 to 75 per cent and an average clay content of from 

 25 to 50 per cent. But it appears that satisfactory results have been 

 obtained most frequently where the sand content ranged between 65 

 and 70 per cent and the clay content between 30 and 35 per cent. 



4. The color of good topsoil is more frequently gray than other- 

 wise, but color is by no means an index to quality. Several cases 

 have been reported where excellent results were obtained with a soil 

 of dull-red color, while, again, a dark-brown soil has proved entirely 

 satisfactory. Usually the color of the soil gradually becomes lighter 

 after it is placed on the road, and some of the best gray soils have 

 become almost white after a few years' service. 



5. The soils designated ordinarily by farmers as " gray grit." 

 " bright tobacco soil," "upland soil," and " rotten granite " have 

 been found very frequently to possess the proper characteristics for 



