124 landscape Hrcbitecture 



all the necessary mineral matters, but it lacks the 

 something needed for plant growth which the garden 

 soil contains. This something is called humus, an 

 element rather difficult to define and still more diffi- 

 cult to describe in chemical terms. It is abundant 

 in fertile soil, but scarce or wanting in barren soil. 

 Though its chemical value is too complex to be 

 stated or even known, its origin is easy to understand. 

 "Humus is the remains of life of previous genera- 

 tions. When plants die, their roots, together with 

 their leaves, branches, and fruits, inevitably become 

 incorporated into the soil. Animals, too, leave upon 

 the ground a quantity of excrement and other dis- 

 charges; and plants likewise probably discharge 

 excretions into the soil. When animals die their 

 bodies, also, may become mixed with the earth. 

 Thus practically all kinds of organic matter from 

 animals and plants are being mixed continually with 

 mineral ingredients in the surface layers of the soil. 

 The micro-organisms in the soil feed upon these 

 dead materials, causing an extensive series of de- 

 compositions and recombinations. To this mass of 

 complex organic bodies \mdergoing decomposition 

 in the soil has been given the name humus. It will 

 be evident from this explanation of its origin that 

 humus cannot have a definite composition, and that 

 it will hardly be alike in any two soils. It wiU be 

 composed of different materials to start with, and 

 there will be a variety of different stages of decom- 

 position. We cannot hope to find any definite com- 



