MANUAL OF AGRICULTURE. 265 



the strongest of soils, consists of pure clay, mixed with from 5 to 

 15 per cent, of siliceous sand; this can be separated from the 

 clay by boiling, or otherwise thoroughly incorporating the clay 

 with water, and then allowing the mixture to settle. The sand 

 settles first, and the liquid can be poured off just as the finely 

 divided clay begins to be precipitated at the bottom of the 

 vessel. 



A clay loam permits of from 15 to 30 per cent, of fine sand to 

 be separated from it in this manner. 



A loam, from 30 to 60 per cent. 



A sandy loam, from 60 to 90 per cent. 



A sandy soil contains no more than 10 per cent, of clay. 



A marly soil is one on which the proportion of lime contained 

 ranges from 5 to 20 per cent. 



A calcareous soil is one where the lime exceeds 20 per cent. 



Vegetahle moidcls have their range from the garden mould, con- 

 taining from 5 to 10 to the peaty soil in which the organic 

 matter may amount to 60 or 70 per cent. 



The last three classes of soils are also clayey, loamy, or sandy, 

 according to the description of the predominant element of the 

 admixture. This organic matter or hnmns springs, as we have 

 seen, from vegetable decay, When abundant, as in the case of 

 peaty soils, it is inactive, and inorganic matter is deficient in 

 quantity. But after drainage, and in regular cultivation, humus 

 rapidly decays, and gets used up till the subsoil is reached and 

 incorporated with the remaining humus, when there is found a 

 more normal soil. 



The amount of humus persistent in our cultivated soils is 

 rarely less than 5 per cent. Chemically, it is a mixture of 

 several acids, varied accordinof to its stages of decav, all com- 

 posed of carbon, hydrogen, and oxygen. It is not considered to 

 be a direct source of food to the plant, but it is of great im- 

 portance as a chemical agent for effecting changes in the soil. 

 As previously mentioned, it absorbs ammonia from the atmo- 

 sphere, and when free in the soil, and also soluble alkaline com- 

 pounds, thereby preventing their being washed out of the soil ; 

 and at the same time so retaining them, that they can easily be 

 liberated and absorbed by the plant roots. It likewise absorbs 

 oxygen irom the atmos])liere, and freeing it in the soil, maintains 

 a constant chemical action amongst the several constituents. 

 Clay i)0ssesses similar absorbent properties, and its presence is 

 therelbre of considerable vahie, when not so intrusive as to 

 im])e(le the exercise of pro])er cultivation. 



\Vith regard to the matters carried away in solution by tluit 

 portion of the rainfall which is drained out of the soil, Prof. 

 Anderson, in commenting on the analvses of dillerent drainage 

 water from various soils — the rainfall estimated at 2") mclies — 



