Journal of Agricultural Research 
Vol. 26 , No. 3 
114 
solution of the same normality. When soils contained a large amount 
of calcium the solubility in the KC 1 solution was much less than in HC 1 , 
and the difference increased with increasing amounts of calcium. In 
several of the preceding experiments it was shown that very little or 
no calcium was adsorbed in the presence of KC 1 when the amount added 
in Ca(OH) 2 was small. This indicates that the calcium is changed to 
a soluble salt. Considerable amounts of calcium were adsorbed in the 
presence of KC1 when the added amount of Ca(OH) 2 was large. The 
power of KC 1 to change calcium in a soil to a soluble salt is limited. 
Calcium in soil is usually present as a silicate or a carbonate, a salt 
of a weak acid and strong base. When KC 1 is added to soil CaCl 2 and 
the potassium salts of the weak acids are formed, the reaction being 
often described as basic exchange (47). This is a reversible reaction, 
governed by the law of mass action. 
GENERAL DISCUSSION 
ORGANIC VERSUS INORGANIC ORIGIN OR ACIDITY 
There are a number of theories relative to soil acidity, soil reaction, 
and lime requirements. Some of these are discussed by Fisher (rj). 
The substance which determines whether the hydrogen ions or the hy¬ 
droxyl ions shall be in excess may be of organic or inorganic origin. When 
organic matter decays in soil organic acids may be formed. Such a 
source of acidity is common in muck and peat soils (29, p. 355). In 
soils principally of mineral origin, organic matter ap'parently does not 
play an important r 61 e in the production of soil acidity (jj, 25). That 
a considerable amount of an organic acid may be added to such mineral 
soils without increasing the hydrogen-ion concentration was shown in 
the experiment with oxalic acid. In fact, the addition of small amounts 
of the feebly ionized acid actually increased the hydroxyl-ion concentra¬ 
tion, probably because of the formation of salts of a weak acid and strong 
base. This explains the well-known fact that instead of increasing 
acidity the addition of organic matter may actually decrease the same. 
All of the soils used in this study were derived mostly from limestone, 
lime shale, and sandstone. They were therefore typical mineral soils. 
That organic matter does not determine the reaction of such soils is 
shown again by the fact that after ignition (Experiment 12) the P H values 
were not materially changed. The source of the excess hydrogen-ion 
concentration is in the mineral portion of the soil. Harris (77, 18 , 19) 
found many acid soils that were deficient in organic matter. Most 
acid soils are of mineral origin, and organic matter is only an indirect 
factor in the production of the acid condition. 
ACID CONDITION PRODUCED BY WEATHERING 
The cause of the acid condition of a mineral soil is due to chemical 
changes which are a part of the weathering process, or the process by 
which soils are formed from rocks and minerals. Acid soils are likely 
to be found where the annual rainfall exceeds evaporation, and alkaline 
soils when the opposite condition prevails. Continuous addition of 
NaNOg, (NH 4 ) 2 S 0 4 , or CaH 4 (P 0 4 ) 2 may also affect soil reaction (5, 10). 
When silicates, such as the feldspars, undergo weathering the chem¬ 
ical changes which take place may be represented by the equation: 
K 2 0 • A1 2 0 3 
6 Si0 2 -f- C0 2 + 2 H 2 0 = K 2 C0 3 -f- 4Si0 2 -b A1 2 0 3 
2 Si0 2 • H 2 0 
