222 



ANALYSIS OF THE ENVIRONMENT 



arid climates. The salts include calcium car- 

 bonate, gypsum, and various sodium and 

 potassium salts. These soils are usually neu- 

 tral in reaction except when impregnated 

 with sodium or potassium carbonate; then 

 they become true alkaU soils. The salt ac- 

 cumulation is usually near the surface. 



Extremely saline soils such as occur near 

 the Dead Sea or Great Salt Lake are bare 

 of vegetation and almost free of animal hfe. 

 In such places spiders eke out an existence 

 on insects blown in from more fertile areas. 

 In the Great Basin in western North Amer- 

 ica, the upper soil contains some 2.5 per 

 cent of salts in salt flats, 0.8 per cent in 

 greasewood, and 0.04 per cent in sage- 

 brush communities (Weaver and Clements, 

 1929). Thus there is a gradual transition 

 from sahne to normal soils. 



Irrigation, especially if not accompanied 

 by subsurface drainage, often flushes salts 

 to the surface, where they are left by evap- 

 oration of the soil water. Even when this 

 does not happen to a marked extent, cal- 

 cium may be replaced by sodium with a 

 resulting dispersion of the soil particles, 

 leading, perhaps, to a tough, rubbery soil 

 mass with impaired tilth* and permeability. 

 These modifications produce a decided 

 change both for surface and in-soil biota 

 (Scofield, 1938). 



Only eight chemical elements are usually 

 present in soil solids in amounts exceeding 



1 per cent each. In their respective approx- 

 imate percentages, these are: oxygen, 46; 

 silicon, 28; aluminum, 8; iron, 5; calcium, 

 4; sodium, 3; potassium, 2.6; magnesium, 



2 (Emerson, 1930). As with protoplasm 

 itself and with sea salts, the bulk of the soil 

 is composed of common chemical elements. 

 It is the chemical constituents that make up 

 the remaining 1.4 per cent of the soil from 

 which we would normally expect to get 

 those present in Hmiting, minimal amounts, 

 and actually, in soil as in the sea (Chap. 

 14), available nitiogen and phosphorus 

 most commonly act as limiting factors. 



CONCEPT OF PRIOR PROBABILITY 



We come here full upon the concept of 

 prior probability. This matter has been out- 

 lined by Jefireys (1939) and Hutchinson 

 (1943, p. 342). Briefly stated, the concept 



" Tilth is a general term used by soil scien- 

 tists in describing the physical condition of a 

 soil in relation to plant growth, especially crops 

 (see Lyon and Buckman, 1927). 



of prior probabiHty, as apphed to the ecol- 

 ogy of chemical elements, states that one 

 well versed in the physical sciences, espe- 

 cially in geochemistry broadly conceived, 

 might predict with fair accuracy the prob- 

 able importance of any given element in 

 colloidal systems based on water; living 

 protoplasm is such a system. 



The bases for such predictions are largely 

 common sense considerations such as the 

 following: 



1. Rare elements would be less Hkely to 

 occur than would common ones. 



2. Highly insoluble elements would be 

 less hkely to be important than would the 

 more soluble ones. 



3. Elements largely confined to the 

 metallic core of the earth would be less 

 hkely to be present in quantity than would 

 those concentrated near the surface. 



Using such criteria, other things being 

 equal, it may be seen that hydrogen would 

 have a much higher prior probability than 

 aluminum, and aluminum than a rare ele- 

 ment like indium; such expectations are 

 realized. The concept of prior probability, 

 if applied on the basis of present knowl- 

 edge, would lead to some mistakes. Man- 

 ganese is more and strontium less significant 

 biologically speaking, than would be in- 

 dicated on this basis. Prior probability 

 would also underrate the importance of the 

 heavy metals. Despite these weaknesses, as 

 Hutchinson (1943) suggests, prior proba- 

 biHty may provide working hypotheses for 

 investigating the biological role of elements 

 not yet identified with biological systems. 



SOIL pH 



Soils are amphoteric buffers; they show 

 properties of both bases and acids. When 

 the buffering capacity is measured by deter- 

 mining the amount of alkali required to 

 effect a given change in pH, the descend- 

 ing scale of buffering capacity is: raw 

 humus > forest soil, A-horizon > loamy 

 sand > sand (Robinson, 1936). In the pres- 

 ence of a sufficient concentration of organic 

 acids, such as may be produced by decom- 

 position of organic compounds, the soil re- 

 action may become acid. The same result 

 follows the absence of sufficient calcium and 

 magnesium bases; in fact, since calcium 

 provides about 80 per cent of the exchange- 

 able bases in soils, a somewhat close rela- 

 tion frequently exists between pH and the 

 calcium content of the soil (Robinson, 



