The Soil. 47 



the nitrogen contained in it, there is always found in its ash, 

 such plant food elements as phosphorus, potassium, iron and 

 sodium, together with silicon and aluminum. These ash consti- 

 tuents are thought to be of considerable importance because they 

 are apparently easily available to plants. 



The humus of soils is of the greatest agricultural importance. 

 It not only modifies its physical properties, but is the principal 

 storehouse for nitrogen. A soil rich in humus is rich in nitro- 

 gen ; a soil poor in humus is poor in nitrogen. The plant does 

 not use it directly, but its nitrogen must first be converted by 

 bacteria into water soluble forms, such as nitrates, before it is 

 available. By the decay of humus the proportion of carbon diox- 

 ide in the soil water is increased and thus the solvent powers of 

 the latter for plant food in the, mineral portion of the soil are 

 enhanced. 



Virgin soils are comparatively rich in humus, but continuous 

 cropping with no return to the soil of humus forming materials 

 may result in its being decreased from one-third to one-half in 

 a period of not more than fifteen years. The amount of humus 

 in soils is variable, dependent upon such factors as climate and 

 the previous soil treatment. In humid regions ordinary arable 

 soils vary in humus content from 2 to 5 per cent. Swampy, 

 peaty, and muck soils contain larger amounts. In a bog soil the 

 per cent of humus may be as high as 30 per cent. In arid re- 

 gions the amount of humus in the soil is normally less than found 

 in our humid regions, the amount rarely exceeding 1 per cent. 



These materials described above have great influence upon 

 both the physical and chemical properties of soils. The import- 

 ant physical properties of the constituents themselves are shown 

 in the table on page 48. 



The explanation of the terms fused in the table are as fol- 

 lows: Specific gravity is the weight of any volume of the 

 solid material compared with that of an equal volume of water. 

 Specific heat (equal weight) is the ratio of the amount of heat 

 necessary to raise the temperature of a certain quantity of a 



