lants. In the soil, ammonia is the most
important of all organic manures. In fact, the value of organic manure
may be estimated, either by the amount of ammonia which it will yield,
or by its power of absorbing ammonia from other sources.

The most important action of ammonia in the soil is the supply of
_nitrogen_ to plants; but it has other offices which are of consequence.
It assists in some of the chemical changes necessary to prepare the
matters in the soil for assimilation. Some argue that ammonia
_stimulates_ the roots of plants, and causes them to take up increased
quantities of inorganic matter. The discussion of this question would be
out of place here, and we will simply say, that it gives them such vigor
that they require increased amounts of ashy matter, and enables them to
take this from the soil.

[To how great a degree can the farmer control atmospheric
fertilizers?

What should be the condition of the soil?

What substances are good absorbents in the soil?

How may sandy soils be made retentive of ammonia?]

Although, in the course of nature, the atmospheric fertilizers are
plentifully supplied to the soil, without the immediate attention of the
farmer, it is not beyond his power to manage them in such a manner as
to arrest a greater quantity. The precautions necessary have been
repeatedly given in the preceding pages, but it may be well to name them
again in this chapter.

The condition of the soil is the main point to be considered. It must be
such as to absorb and retain ammonia--to allow water to pass _through_
it, and be discharged _below_ the point to which the roots of crops are
searching for food--and to admit of a free circulation of air.

The power of absorbing and retaining ammonia is not possessed by sand,
but it is a prominent property of clay, charcoal, and some other matters
named as absorbents. Hence, if the soil consists of nearly pure sand, it
will not make use of the ammonia brought to it from the atmosphere, but
will allow it to evaporate immediately after a shower. Soils in this
condition require additions of absorbent matters, to enable them to use
the ammonia received from the atmosphere. Soils already containing a
sufficient amount of clay or charcoal, are thus far prepared to receive
benefit from this source.

[Why does under-draining increase the absorptive power of the
soil?

How do plants obtain their carbonic acid?

How does carbonic acid affect caustic lime in