398 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



cium sulphate, silicia, water, and a trace of organic matter. Thus clay 

 and native limestone will be seen to contain most of the elementary sub- 

 stances which enter into the composition of the ash of plants. 



VEGETABLE MATTER has accumulated in all cultivated soils, and in the 

 form of peat it sometimes composes the entire mass. It is to the pre- 

 sence of humus or vegetable matter that the rich brown colour of good 

 land is due, It may be described as a dark-brown, soft, porous, sub- 

 stance, seen in the greatest purity in the form of well-rotted wood or leaf- 

 mould. It is constantly in a state of decay or slow combustion, which is 

 never completely arrested until it is reduced to the condition of pure car- 

 bon. The earlier agricultural chemists attributed a greater importance 

 to this constituent of soils than is at present assigned to it. It was ob- 

 served that all garden soils and fertile loams were rich in vegetation mat- 

 ter, and the inference was drawn that it was the cause of fertility. The 

 late Baron Liebig, in his work on Agricultural Chemistry, published in 1840, 

 demolished this theory by showing that humus was not the cause, but 

 rather an inevitable consequence of richness. A rich soil, suitable for the 

 growth of plants, cannot fail to accumulate vegetable matter by the fall 

 of the leaf and the death of root fibres. A soil may be rich without hu- 

 mus, as is proved by the fertility of lava soils. The more a soil produces, 

 the greater will be its stock of humus, as for example, in the case of a 

 <jrop of mangel or swedes, or in the case of a hay or straw crop, the accu- 

 mulation of roots in the soil leaves it positively richer in humus than it 

 originally was, in spite of the many tons per acre of produce removed in 

 the root crops. Land adapted for the growth of timber will yield many 

 tons per acre of wood in the course of years, and yet the soil will be posi- 

 tively better stored with organic matter or humus at the end of the period 

 than it was at the commencement. This can only be explained on the 

 ground that the carbon, hydrogen, and nitrogen, which constitute humus, 

 are derived from the air and not from the soil. Although useful, it is 

 therefore seen to be less essential than any of the proximate constituents 

 yet noticed. 



Humus is not assimilated directly by flowering plants. It is valuable 

 as a perpetual source of carbon dioxide, and in a less degree as a source of 

 nitrogen. The gradual decay of humus maintains the interstitial atmos- 

 phere, rich in carbon dioxide, and impregnates the rain which penetrates 

 the soil with the same ingredient. It is in this manner that humus be- 

 comes useful in the nutrition of plants, and at the same time assists in 

 that slow digestion which liberates insoluble matter from the soil, and 

 renders it fit for the use of the plant. Humus is also highly valuable in 



