208 THE BOOK OF NATURE STUDY 



the various groups of organisms at work in the soil ; some of 

 them convert such materials as farmyard manure, roots, leaves, 

 and the like into humus and simple compounds like the nitrates 

 upon which plants can feed. Others rob the higher plants of the 

 food that is in the soil and use it for their own nutriment, although 

 in the end it must be returned into circulation for the use of the 

 plant. One great group of bacteria, amongst which are those 

 forming the brown skin in flask 6, enrich the soil by gathering 

 nitrogen from the atmosphere, whilst others waste the nitrogenous 

 compounds present in the soil by converting them back to gas. 

 Others give rise to substances hurtful to the growth of plants ; 

 others, fortunately more rare, are capable of setting up disease in 

 human beings if they find entrance through a wound. The relative 

 predominance of one or other group depends upon many con- 

 ditions, upon the warmth of the soil, its degree of moisture and 

 aeration, its supply of this or that nutrient material, etc. ; in fact, 

 bacteria are influenced by much the same set of factors as are the 

 higher plants. Without bacteria in the soil the growth of our 

 crops would be practically impossible, and many of the common 

 operations of cultivation and management are unconsciously 

 directed towards effecting some change in the bacterial actions 

 in the soil that will eventually result in benefit to the plant. 



THE CHARACTERISTICS OF DIFFERENT SOILS 



It has already been seen how different kinds of rock give 

 r&e to sedentary soils of various types, which again in their turn, 

 after sorting by the river, result in one or two distinct alluvial 

 soils ; it is interesting and often valuable, from the farming and 

 gardening point of view, to study what kind of vegetation is 

 associated with each type of soil. We have seen how the water- 

 retaining powers of soil vary with the materials from which it is 

 made ; its water-retaining power will affect its temperature and a 

 dozen other factors, rendering it more suitable for one plant than 

 for another. Differences of chemical composition act in the same 

 way. In consequence, wherever plants have free play, as on un- 

 cultivated land or a meadow or amongst weeds of arable land, 

 the vegetation varies very greatly in passing from one type of 



