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the carbon of different families of plants differs according to 
a fixed law, and that plants derive from, or exhaust the soil, 
not in proportion to the nutriment they contain, but in the 
inverse ratio to the surface of their leaves and green absorb- 
ing parts. It is certain that cacti, fungi, orchideae, musci, 
and even forest trees, are much more independent of the soil 
than any of the cultivated crops. 
One other remark respecting (fypsum, Liebig, Dr. Dau- 
beny, and Professor Johnston, suppose that the use of gyp- 
sum is to fix or to deprive ammonia of its volatility, (the 
Professor, however, gives it other uses) ; that a double 
decomposition takes place, and that sulphate of ammonia and 
carbonate of lime are formed. Now it is well known that 
gypsum is only beneficial to the clovers, grasses, and sain- 
foin, and is itself an essential ingredient of them : but if the 
effect were such as stated, it would have been found useful to 
all crops, and especially to those which contain nitrogen. But 
every solid foot of soil which contains 20 per cent of humic 
acid (and Sprengel states some contain 45 per ct.) is capable 
of fixing VI lb. of ammonia by its humic acid alone, — a much 
greater quantity than is ever applied by manure, &c., while 
the clay and iron absorb it, and have a tendency to combine 
with it. Yet we are told that its use is to convert ammonia 
into the sulphate, while Dr. Daubeny says it is probable that 
plants have no power to decompose the sulphate of ammonia, 
and if they had, free sulphuric acid would be most injurious 
to their structure ! The fact is, that straw and vegetable 
manure fix in the soil and economize the other valuable 
manures which contain ammonia, and hence another most 
important use of them. 
Nitrogen derived from ammonia or the nitrates, being then 
such a valuable substance to vegetation in general, the most 
desirable object is the best mode of its preservation ; for 
ammonia is so extremely volatile as to exhale at all known 
