48 AGRICULTURE. 



compact. It hastens the decay of vegetable 

 matter. Limy soils are poor in potash and often 

 rich in phosphates (see " Lime," p. 95). 



IV. Humous Soils 



The decaying organic matter in soils is com- 

 posed of compounds of nitrogen, hydrogen, 

 oxygen, and carbon, and is called " humus." 

 Soils containing a large per cent, of this or- 

 ganic matter are designated as " humous 

 soils.' Humus orives a dark brown or black- 

 ish color to the soil. Leaf mould very largely- 

 consists of humus. Either a sandy or a clay 

 soil is improved by humus, not only on ac- 

 count of the additional plant-food, such as car- 

 bon dioxide, ammonia, and water, which is fur- 

 nished by its ultimate decomposition, but more 

 especially on account of the improvement of 

 the physical condition of the soil. 



Humus absorbs and retains moisture, and thus 

 improves a sandy soil. It improves a clay soil 

 by making it less compact and better aerated. 

 It improves the physical condition of worn-out 

 soils. 



Humous soils are often rich in nitrogen and 

 poor in mineral plant-food. A soil formed from 

 the addition of humus to a sand, clay, or calcare- 

 ous loam is called a clay or argillaceous loam, 

 or calcareous loam, according to the kind of soil 

 which forms the basis. 



