SOIL 77 
alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, 
on the other hand, in the usual absence of excessive amounts of solutes, 
absolutely controls the fate of the water that enters the ground, in addition 
to its influence upon the run-off. It determines the amount of gravitation 
water lost by percolation, as well as the water that can be raised by capil- 
larity. The resultant of these, the total soil water or holard, is hence an 
effect of structure, while the size and compactness of the particles are con- 
clusive factors in controlling the chresard. It must be recognized, how- 
ever, that these are all factors which enable us to interpret the amount of 
holard or chresard found in a particular soil. They have no direct impor- 
tant effect upon the plant, but influence it only in so far as they affect the 
water present. 
112. The value of soil surveys. The full appreciation of the preeminent 
‘value of water-content, particularly of the chresard, greatly simplifies the 
ecological study of soils. The ecologist’ is primarily concerned with soil 
water only in its relation to the plant, and while a fair knowledge of soil 
structure is essential to a proper understanding of this, he has little concern 
with the detailed study of the problems of soil physics. For the sake of a 
proper balance of values, he must avoid the tendency noted elsewhere of 
ignoring the claims of the plant, and of studying the soil simply as the seat 
of certain physical phenomena. Accordingly, it is felt that mechanical and 
chemical analyses, determinations of soluble salt-content, etc., have much 
less value than has been commonly supposed. The usual methods of soil 
survey, which pay little or no attention to water-content, and none at all to 
available water, are practically valueless for ecological research. This state- 
ment does not indicate a failure to appreciate the importance of the usual 
soil methods for many agricultural problems, such as the use of fertilizers, 
conservation of moisture, etc., though even here to focus the work upon 
water-content would give much more fundamental and serviceable results. 
For these reasons, slight attention will be paid to methods of mechanical and 
chemical analysis. In their stead is given a brief statement of the origin, 
structure, and character of soils with especial reference to water-content. 
113. The origin of soils. Rocks form soils in consequence of weathering, 
under the influence of physical and biotic factors. Weathering consists of 
two processes, disintegration, by which the rock is broken into component 
particles of various sizes, and decomposition, in which the rock or its frag- 
ments are resolved into minute particles in consequence of the chemical 
disaggregation of its minerals, or of some other chemical change. These 
processes are usually concomitant, although, as a rule, one is more evident 
than the other. The relation between them is dependent upon the character 
