684 
Journal of Agricultural Research 
Vol. XXVII. No. 9 
water except when the water contains carbon dioxid. When calcium 
carbonate, for example, passes into solution in water containing carbon 
dioxid, it is customary to refer to it as calcium bicarbonate and to symbol¬ 
ize it as Ca (HC 0 3 ) 2 . This salt, if it exists, is unknown except in solution. 
When a solution containing it is boiled, the carbon dioxid is driven off and 
calcium carbonate is precipitated. 
The carbonates of the alkali bases may be identified in and produced 
from nearly all soil solutions and drainage waters of irrigated lands. 
When the solution also contains carbon dioxid in sufficient quantity to 
render it neutral to an indicator such as phenolphthalein, these carbon¬ 
ates are said to exist as bicarbonates, for example NaHC 0 3 . It is possible 
to produce sodium carbonate or sodium bicarbonate by adding carbon 
dioxid to a solution of sodium hydrate. Both salts are common articles 
of commerce. 
As it occurs in the soil solution in irrigated land, sodium carbonate has 
been regarded as the cause of serious injury to the physical condition of 
the soil and as highly injurious to plant growth. Much investigation has 
been directed toward discovering the circumstances of its formation and 
means of removing or neutralizing it. The salt has been thought of and 
referred to as a carbonate. This designation has focused attention on 
the acid ion C0 3 and diverted attention from its basic companion sodium. 
The acid ion C0 3 is known as a weak ion. This is probably due to the fact 
that carbonic acid, H 2 C 0 3 , is inert as compared with such acids as hydro¬ 
chloric and sulphuric. Carbonic acid is formed when carbon dioxid dis¬ 
solves in water. The oxid of carbon is formed by the union of carbon and 
oxygen, as in burning charcoal. It may be released also by decomposition 
of the complex carbohydrates, such as plant tissues. It is assumed that 
most of the carbon dioxid that occurs in the soil solution is derived from 
the decomposition of organic matter in the soil. 
The compound known as sodium carbonate is said to be a combination 
of a weak acid with a strong base. In water solution it reacts like the 
alkaline hydroxid NaOH except that when neutralized with a strong 
acid such as hydrocholoric acid half the sodium in solution as sodium 
carbonate forms a so-called acid salt. The reactions involved may be 
symbolized as follows : 
(1) NaOH+HCl=NaCl+H 2 0 
(2) Na 2 C 0 3 +HCl=NaCl+NaHC 0 3 
and 
(3) NaHC 0 3 -f- HC 1 = NaCl -f H 2 CO s 
or 
(4) h 2 co 3 =h 2 o+co 3 . 
The reactions that occur in the soil when the soil solution contains 
sodium carbonate take place in the same way and to much the same 
extent with sodium hydrate. This fact permits the inference that it is 
the sodium and not the carbonate that causes the characteristic reactions. 
This inference appears to gain justification when considered in connection 
with the phenomena and the reactions involved in the exchange of bases 
in the soil. It has been repeatedly demonstrated, for instance, that when 
a soil is treated with a solution of sodium chlorid or sodium sulphate 
some of the sodium goes out of solution and an equivalent quantity of 
other bases come into solution. If the solution is then removed and re¬ 
placed with pure water, the soil and the water show the reactions char¬ 
acteristic of the effect of sodium. The soil is deflocculated and the 
solution becomes alkaline. In view of these facts it may be justifiable 
