l->6 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



that of the soil proper. It is believed, however, to be desirable to 

 describe the soil in such manner as to denote separately the presem . 

 of a coarse matrix and a finer soil occupying its interstices. Thus 

 if ro cks or gravel formed more than 10 per cent of the mass we might 

 speak of the soil as a " rocky medium sand" or a "gravelly loam. " 

 In this event the fine gravel and finer material should be considered 

 constituting 100 per cent when using the following key: 



I LASSIFICATION OF SOILS ON MECHANICAL ANALYSIS. 



Soilfi containing -20 silt and clay: 



l oarse sand 25+ very coarse sand and coarse -and 



and less than 50 any other grade. 

 ,,,1 25 + very coarse sand, coarse and me- 

 dium sand, and less than 50 fine 

 sand. 



Fine sand 50+ fine sand, or —25 very coarse sand. 



coarse and medium .-and. 



Very line sand 50+ very fine sand. 



Soils containing 20 to 50 silt and clay: 



Sandy loam 25+ very coarse -and. coarse and 



medium sand. 



Fine sandy loam 50+ fine sand or —25 very coarse aand, 



coarse and medium sand. 



Sandy clay —20 silt. 



Soils containing 50+ silt and clay: 



Loam —20 clay, —50 silt. 



Silt loam -20 clay, 50+ silt. 



« lay loam 20 to 30 clay, —50 silt. 



Sihy clay loam 20 to 30 day. 50+ silt. 



< lay 30+ clay. 



THE DETERMINATION OF HUMUS 



The amount of humus in the soil, which plays an important part in 

 the water relations and may also be an important source of nutrient-, 

 may be determined in two general wa\ - 



1. By ignition, taking no account of the degree of decomposition 

 of the organic matter, and always involving seme error through the 

 evaporation of water which may exist in several forms in oven-dried 

 soils. 



"- ) . By extraction of the humified portion of the organic matter 

 with ammonia, and its subsequent ignition. 



It should be realized that these two methods produce entirely 

 different results and, in fact, they have distinct purposes. On the 

 one hand, the total organic matter is of interest because of it- bear- 

 ing on the water-holding properties of the soil, and m this connection 

 the total loss on ignition is probably as expressive a- any other meas 

 ure, though m soils containing large quantities "I" carbonates some 

 correction must be made for their breakdown. It is, however, a 



