tf OS Gs 
162 HOW CROPS FEED. 
Hine carbonate.of lime. .....,.. 0is+ cas iiesmwrenee wes oa ee eee 
* Heavy clay soil, (80 per cent clay)... ......5..e8senmeee . 41 
Garden mold, (7 per cent humus)... ......o. sean eee 52 
Pure Clay is. 5! os ad ce ann obs O's om oe pee 49 
Carbonate of magnesia (fine powder) «. ......-e.sassneene 82 
TITITUS “oii a op .0is 0 p.05es anys sipersice p= 5 © noo sel 120 
Davy found that one thousand parts of the soils named 
below, after having been dried at 212°, absorbed during 
one hour of exposure to the air, quantities of moisture as 
follows: 
Sterile soil of Bagshot heath. ..:........sseeekee aan 3 
Coarse Sand... 2. .scsc snes éwiew 02 a0 56 Os als 6 heen 8 
Wine sand). . 3.04260 ass vnee sags mae os 40lciss 2 le eee 1 
Soil from Mersey, Hssex. . .S. i.e... ain's ) eat 13 
Very fertile alluvium, Somersetshire. .....2.. .2-ee eee 16 
Extremely fertile soil of Ormistor, East Lothian......... 18 
An obvious practical result follows from the facts ex- 
pressed in the above tables, viz: that sandy soils which 
have little attractive force for watery vapor, and are there- 
fore dry and arid, may be meliorated in this respect by 
admixture with clay, or better with humus, as is done by 
dressing with vegetable composts and by green manuring. 
The first table gives us proof that gypsum does not exert 
any beneficial action in consequence of directly attracting 
moisture. Humus, or decaying vegetable matter, it will 
be seen, surpasses every other ingredient of the soil in 
absorbing vapor of water. Tlis is doubtless in some de- 
gree connected with its extraordinary porosity or amount 
of surface. How the extent of surface alone may act is 
made evident by comparing the absorbent power of car- 
bonate of lime in the two states of sand and of an im- 
palpable powder. The latter, it is seen, absorbed twelve 
times as much vapor of water as the former. Carbonate 
of magnesia stands next to humus, and it is worthy of 
note that it is a very light and fine powder. 
Finally, it is a matter of observation that “silica and 
lime in the form of coarse sand make the soil in which 
they predominate so dry and hot that vegetation perishes 
a8 i 
