126 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
I 
that of the soil proper. It is believed, however, to be desirable to 
describe the soil in such manner as to denote separately the presence 
of a coarse matrix and a finer soil occupying its interstices. Thus 
if rocks 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 
as constituting 100 per cent when using the following key : 
CLASSIFICATION OF SOILS ON MECHANICAL ANALYSIS. 
Soils containing — 20 silt and clay : 
Coarse sand 25+ very coarse sand and coarse sand 
and less than 50 any other grade. 
Sand 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 sand. 
Very fine sand 50+ very fine sand. 
Soils containing 20 to 50 silt and clay : 
\ Sandy loam 1 25+ very coarse sand, coarse and 
medium sand. 
Fine sandy loam 50+ fine sand or —25 very coarse 
sand, 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. 
Clay loam 20 to 30 clay, — 50 silt. 
Silty clay loam 20 to 30 clay, 50+ silt. 
Clay 30+ clay. 
THE DETERMINATION OF HUMTJS. 
The amount of humus in the soil, which plays an important part in 
the water relations and may also be an important source of nutrients, 
may be determined in two general ways : 
1. By ignition, taking no account of the degree of decomposition 
of the organic matter, and always involving some error through the 
evaporation of water which may exist in several forms in oven-dried 
soils. 
2. 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 its bear- 
ing on the water-holding properties of the soil, and in this connection 
the total loss on ignition is probably as expressive as any other meas- 
ure, though in soils containing large quantities of carbonates some 
correction must be made for their breakdown. It is, however, a 
