220 



ANALYSIS OF THE ENVIRONMENT 



attributes of the soil and by the physical 

 forces that make for capillary attraction, 

 adsorption, vaporization, condensation, and 

 evaporation, and for transpiration from 

 plants. Capillary attraction is itself a func- 

 tion of pore size in the soil taken together 

 with the surface tension of water. The pull 

 of gravity also exerts a strong influence over 

 water, the passage of which in quantity 

 through the soil, as after a rain, drives out 

 the older, modified soil atmosphere and al- 

 lows an inflow of fresher air. Hence the 

 flow of water under compulsion of gravity 

 tends to increase the usually low oxygen 

 content and to lower the usually high par- 

 tial pressure of carbon dioxide in air-filled 

 soil spaces, thus promoting oxidation. 



In areas with poor drainage, soil may be- 

 come so water-logged as to drown out 

 many inhabitants seasonally or permanently 

 and allow the invasion of burrowing hydro- 

 coles, such as the crayfishes of temperate 

 latitudes. In dryer soils, the amount of 

 moisture that a given organism can remove, 

 rather than the total amount present, de- 

 termines whether the soil is too dry. The 

 proportion of soil moisture that remains af- 

 ter a plant has taken all the water it can 

 from the given soil and has wilted beyond 

 recovery is called the wilting coefficient, 

 and is expressed as the percentage of dry 

 weight of the soil. The wilting coeflBcient 

 varies widely with diflFerent plants and with 

 diflFerent soils; it is much higher in the 

 moisture-holding clays than in sand or 

 sandy loam. Similar values are important 

 for soft-bodied soil animals, but far less is 

 known about the basic water relations of 

 such organisms. 



The subject of wilting coefficients is dis- 

 cussed in plant ecology (Weaver and Cle- 

 ments, 1929), soil science (Russell, 1937), 

 and in plant physiology (E. C. Miller, 

 1938). The reasons for giving this subject 

 more space in plant physiology than in 

 plant ecology appear to be historical rather 

 than logical and are perhaps related to the 

 greater interest of the physiologists in pre- 

 cision measurements, a situation that hap- 

 pily is changing rapidly in some aspects of 

 ecology. 



Soil water exists in the following cate- 

 gories: (Bouyoucos, 1921): 



1. Freely moving gravitational water; often 

 ecologically unavailable or superavailable 



2. Water held in soil interstices and freezing 



at — 1.5° C; so-called free water; eco- 

 logically readily available 



3. Water adsorbed on soil particles and 

 freezing when supercooled to —4°; so- 

 called capillary adsorbed water; eco- 

 logically slightly available 



4. Water of hydration of soil colloids that 

 does not freeze; so-called bound water; 

 ecologically unavailable 



5. Water chemically combined as water of 

 crystallization; does not freeze and is 

 ecologically unavailable 



The indicated ecological relationships are 

 based primarily on observations on plants; 

 soil-ingesting animals may have somewhat 

 diflFerent relations with the more firmly held 

 water. The amount of water in the soil is 

 affected by such diverse factors as slope of 

 surface, nature of organic constituents, soil 

 texture, soil structure, and the amount and 

 type of precipitation. Snow is often impor- 

 tant, for it acts as a mulch and prevents 

 surface evaporation from the soil; if it 

 covers unfrozen ground and thaws slowly, 

 there is little run-off. Similarly, prolonged 

 gentle rains provide a much higher percent- 

 age of soil-penetrating moisture than does 

 an equal amount of water that falls as tor- 

 rential rain. Soils with somewhat sandy sur- 

 faces allow ready penetrations; the surface 

 then dries and breaks the upward capillary 

 flow. Such soils retain moisture better than 

 do heavier ones that do not readily form 

 a dust mulch. Water can thus be stored in 

 the soil for months, even over winter after 

 a good rainy season. This principle is basic 

 for dry farming in semiarid regions. 



Silty loam in good condition to support 

 growth of many plants has about half its 

 volume composed of pores, and the other 

 half is solid. Of the soUd substance, aboui^ 

 10 per cent is organic and 90 per cent in- 

 organic material. The pore space in such a 

 soil is approximately half occupied by air 

 and half by water (Lyon and Buckman. 

 1927). The proportion of organic matter 

 varies with different soils (see p. 224). 



SOIL CHEMISTRY 



In its passage through soil, subsoil, and 

 underlying superficial layers of the earth's 

 crust, water picks up a highly varied load 

 of dissolved chemicals, while giving up 

 some of those it may bring to the soil from 

 the atmosphere. The variety and quantit)- 

 of chemicals depend on the character of 

 the substrate through which the water per- 



