Ch. V, 6] STRUCTURE OF SOILS 241 



from the water. It is in order to introduce air into such 

 soils that we drain them preparatory to growing crops. 



When air stands long in a soil, it loses part of its oxygen 

 and accumulates carbon dioxide from root respiration. Ac- 

 cordingly it is better for plants that this vitiated air should be 

 expelled at intervals, and replaced by a fresh supply. Such 

 a result accompanies soaking rains ; and the keeper of house 

 plants does well to imitate the method by giving the plants 

 an occasional thorough soaking, and allowing them to dry 



^out in large part between times. Such treatment is much 

 better than a frequent addition of small amounts, for the 

 latter method does not effect renewal of air. 



Humus. This comes fourth in abundance of ordinary soil 

 constituents. It comprises the dark-colored vegetable matter, 

 mostly the remains of decaying roots, which to the eye of an 

 expert is so characteristic a mark of a good soil. A mixture 

 of humus with sand and clay constitutes loam, the best of 

 garden soils. The proportion of humus in soils varies greatly, 

 from almost none through an optimum amount (represented 

 in our picture (Fig. 169), to a very great deal, as in muck, 

 which owes its black color thereto. Bogs consist almost 

 wholly of a kind of humus, called peat, which only partially 

 decays, and therefore accumulates. The value of humus in 

 a soil, from the plant point of view, is four-fold. It lightens, 

 or opens, a soil, thus increasing its aeration capacity; it 

 helps to retain moisture, being very absorbent; it adds 

 substances, by its decay, to the soil solution, some beneficial 

 and some harmful, though our knowledge of these matters is 

 scanty as yet ; and most important of all, it supports 

 numerous micro-organisms, which play a first role in soil 

 fertility. 



Dissolved Substances. In the soil water occur many 

 dissolved substances, and therefore it becomes a soil solu- 

 tion. Though profoundly important to plant life, the actual 

 quantity of such substances present is relatively small, even 

 the richest soil possessing only a small fraction of 1 per cent al- 



