Emil Truog 37 



neutral salts, particularly calcium and magnesium salts, and will be 

 absorbed as such by a growing plant; in addition, ample amounts of 

 calcium bicarbonate for absorption and use will be formed through the 

 action of carbonic acid on exchangeable calcium. 



It is well known that plants vary greatly in their preference and tol- 

 erance as regards the pH of the soil or other medium in which they are 

 growing. For example, the most favorable pH range of alfalfa in this 

 respect is considerably higher than that of timothy. If the contention 

 made in the preceding paragraph is valid, then, chemical analyses should 

 reveal a higher proportion of basic to acidic constituents in the alfalfa 

 than in the timothy. Accordingly, in Table II, the results of such 

 analyses for these two crops are presented. The percentages of con- 

 stituents present have been recalculated and presented in terms of 

 milliequivalents of cations and anions, respectively, so that the total 

 amounts of basic and acidic constituents present may be compared on 

 a chemical equivalent basis. This is necessary if the comparison is to 

 be valid for the purpose at hand. For example, the basic oxides A1 2 3 

 and CaO are expressed as milliequivalents of the cations A1+++ and 

 Ca++, and the acidic oxide P 2 O r , and N as milliequivalents of the anions 

 P0 4 - " and NO--. 



It will be noted that on a total milliequivalent basis, alfalfa contains 

 somewhat more acidic than basic constituents. However, it is ordinarily 

 expected that through the medium of symbiotic nitrogen fixation, alfalfa 

 should obtain at least two-thirds of its needed nitrogen from the air. 

 Nitrogen thus obtained brings no bases with it into the plant and, 

 hence, a subtraction representing two-thirds of this nitrogen is made 

 from the total of acidic constituents. After this is done, it will be noted 

 that the alfalfa plant must get from the soil nearly twice as much of 

 the basic as of the acidic soil constituents. Calculations indicated in the 

 table show that on the basis of these data, 41.6 per cent of the basic ele- 

 ments, particularly calcium, magnesium, and potassium, must be ab- 

 sorbed as the carbonate or bicarbonate, which, because of the weakness 

 of carbonic acid, provide the same end result as though these elements 

 were absorbed as free bases. The possibility of absorbing these elements 

 in adequate amounts as the carbonates is dependent on a high degree 

 of base saturation (low acidity) of the clay and humic acids. 



