ffords the necessary
moisture to dissolve the elements of fertility already in the soil, but
also because it contains in itself, or brings with it from the
atmosphere, valuable fertilizing substances. In a learned article by Mr.
Caird, in the Cyclopedia of Agriculture, on the Rotation of Crops, he
says:

"The surprising effects of a fallow, even when unaided by any
manure, has received some explanation by the recent discovery of
Mr. Barral, that rain-water contains within itself, and conveys
into the soil, fertilizing substances of the utmost importance,
equivalent, in a fall of rain of 24 inches per annum, to the
quantity of ammonia contained in 2 cwt. of Peruvian guano, with 150
lbs. of nitrogeneous matter besides, all suited to the nutrition of
our crops."

About 42 inches of rain may be taken as a fair general average of the
rain-fall in the United States. If this supplies as much ammonia to the
soil as 3 cwt. of Peruvian guano to the acre, which is considered a
liberal manuring, and which is valuable principally for its ammonia, we
at once see the importance of retaining the rain-water long enough upon
our fields, at least, to rob it of its treasures. But rain-water has a
farther value than has yet been suggested:

"Rain-water always contains in solution, air, carbonic acid, and
ammonia. The two first ingredients are among the most powerful
disintegrators of a soil. The oxygen of the air, and the carbonic
acid being both in a highly condensed form, by being dissolved,
possess very powerful affinities for the ingredients of the soil.
The oxygen attacks and oxydizes the iron; the carbonic acid seizing
the lime and potash and other alkaline ingredients of the soil,
produces a further disintegration, and renders available the
locked-up ingredients of this magazine of nutriment. Before these
can be used by plants, they must be rendered soluble; and this is
only affected by the free and renewed access of rain and air. The
ready passage of both of these, therefore, enables the soil to
yield up its concealed nutriment."

We see, then, that the rains of heaven bring us not only water, but food
for our plants, and that, while we would remove by proper drainage the
surplus moisture, we should take care to first conduct it through the
soil far enough to fulfill its mission of fertility. We cannot suppose
that all rain-