_The only abundant source of carbonic acid in the soil, is decaying vegetable matter._

Hungry, leachy soils, from their deficiency of vegetable matter and of moisture, do not adequately yield their own native resources to the support of crops, because the conditions for converting their fixed into floating capital are wanting. Such soils dressed with peat or green manured, at once acquire the power of retaining water, and keep that water ever charged with carbonic acid: thus not only the extraneous manures which the farmer applies are fully economized; but the soil becomes more productive from its own stores of fertility which now begin to be unlocked and available.

Dr. Peters, of Saxony, has made some instructive experiments that are here in point. He filled several large gla.s.s jars, (2-1/2 feet high and 5-1/2 inches wide) with a rather poor loamy sand, containing considerable humus, and planted in each one, June 14, 1857, an equal number of seeds of oats and peas. Jar No. 2 had daily pa.s.sed into it through a tube, adapted to the bottom, about 3-1/4 pints of common air.

No. 3 received daily the same bulk of a mixture of air and carbonic acid gas, of which the latter amounted to one-fourth. No. 1 remained without any treatment of this kind, _i. e._: in just the condition of the soil in an open field, having no air in its pores, save that penetrating it from the atmosphere. On October 3, the plants were removed from the soil, and after drying at the boiling point of water, were weighed. The crops from the pots into which air and carbonic acid were daily forced, were about _twice as heavy_ as No. 1, which remained in the ordinary condition.

Examination of the soil further demonstrated, that in the last two soils, a considerably greater quant.i.ty of mineral and organic matters had become soluble in water, than in the soil that was not artificially aerated. The actual results are given in the table below in grammes, and refer to 6000 grammes of soil in each case:--

ACTION OF CARBONIC ACID ON THE SOIL.

-----------------------------------+-----------+--------+------------

_No. 1,

Without

_No. 2,

_No. 3, _Substances soluble in water, etc._

Artificial

Common

Air and

Supply of

Air

Carbonic

Air._

Added._

acid added._ -----------------------------------+-----------+--------+------------ Mineral matters

2.04

3.71

4.99 Potash

0.07

0.17

0.14 Soda

0.17

0.23

0.28 Organic matters

2.76

4.32

2.43

Weight of Crops

5.89

10.49

12.35 -----------------------------------+-----------+--------+------------

It will be seen from the above that air alone exercised nearly as much solvent effect as the mixture of air with one-fourth its weight of carbonic acid; this is doubtless, in part due to the fact that the air, upon entering the soil rich in humus, caused the abundant formation of carbonic acid, as will be presently shown must have been the case. It is, however, probable that organic acids (crenic and apocrenic,) and nitric acid were also produced (by oxidation,) and shared with carbonic the work of solution.

It is almost certain, that the acids of peat exert a powerful decomposing, and ultimately solvent effect on the minerals of the soil; but on this point we have no precise information, and must therefore be content merely to present the probability. This is sustained by the fact that the crenic, apocrenic and humic acids, though often partly uncombined, are never wholly so, but usually occur united in part to various bases, viz.: lime, magnesia, ammonia, potash, alumina and oxide of iron.

The crenic and apocrenic acids (that are formed by the oxidation of ulmic and humic acids,) have such decided acid characters,--crenic acid especially, which has a strongly sour taste--that we cannot well doubt their dissolving action.

IV.--_The influence of peat on the temperature_ of light soils dressed with it may often be of considerable practical importance. A light dry soil is subject to great variations of temperature, and rapidly follows the changes of the atmosphere from cold to hot, and from hot to cold. In the summer noon a sandy soil becomes so warm as to be hardly endurable to the feel, and again it is on such soils that the earliest frosts take effect. If a soil thus subject to extremes of temperature have a dressing of peat, it will on the one hand not become so warm in the hot day, and on the other hand it will not cool so rapidly, nor so much in the night; its temperature will be rendered more uniform, and on the whole, more conducive to the welfare of vegetation. This regulative effect on temperature is partly due to the stores of water held by peat. In a hot day this water is constantly evaporating, and this, as all know, is a cooling process. At night the peat absorbs vapor of water from the air, and condenses it within its pores, this condensation is again accompanied with the evolution of heat.

It appears to be a general, though not invariable fact, that dark colored soils, other things being equal, are constantly the warmest, or at any rate maintain the temperature most favorable to vegetation. It has been repeatedly observed that on light-colored soils plants mature more rapidly, if the earth be thinly covered with a coating of some black substance. Thus Lampadius, Professor in the School of Mines at Freiberg, a town situated in a mountainous part of Saxony, found that he could ripen melons, even in the coolest summers, by strewing a coating of coal-dust an inch deep over the surface of the soil. In some of the vineyards of the Rhine, the powder of a black slate is employed to hasten the ripening of the grape.

Girardin, an eminent French agriculturist, in a series of experiments on the cultivation of potatoes, found that the time of their ripening varied eight to fourteen days, according to the character of the soil.

He found, on the 25th of August, in a very dark soil, made so by the presence of much humus or decaying vegetable matter, twenty-six varieties ripe; in sandy soil but twenty, in clay nineteen, and in a white lime soil only sixteen.

It cannot be doubted then, that the effect of dressing a light sandy or gravelly soil with peat, or otherwise enriching it in vegetable matter, is to render it warmer, in the sense in which that word is usually applied to soils. The upward range of the thermometer is not, indeed, increased, but the uniform warmth so salutary to our most valued crops is thereby secured.

In the light soils stable-manure wastes too rapidly because, for one reason, at the extremes of high temperature, oxidation and decay proceed with great rapidity, and the volatile portions of the fertilizer are used up faster than the plant can appropriate them, so that not only are they wasted during the early periods of growth, but they are wanting at a later period when their absence may prove the failure of a crop.

B. The ingredients and qualities which make peat _a direct fertilizer_ next come under discussion. We shall notice:

_The organic matters including nitrogen (ammonia and nitric acid)_ (I):

_The inorganic or mineral ingredients_ (II):

_Peculiarities in the decay of Peat_ (III), _and_

_Inst.i.tute a comparison between peat and stable manure_ (IV).

I.--Under this division we have to consider:

1. _The organic matters as direct food to plants._

Thirty years ago, when Chemistry and Vegetable Physiology began to be applied to Agriculture, the opinion was firmly held among scientific men, that the organic parts of humus--by which we understand decayed vegetable matter, such as is found to a greater or less extent in all good soils, and _abounds_ in many fertile ones, such as const.i.tutes the leaf-mold of forests, such as is produced in the fermenting of stable manure, and that forms the princ.i.p.al part of swamp-muck and peat,--are the true nourishment of vegetation, at any rate of the higher orders of plants, those which supply food to man and to domestic animals.

In 1840, Liebig, in his celebrated treatise on the "Applications of Chemistry to Agriculture and Physiology," gave as his opinion that these organic bodies do not nourish vegetation except by the products of their decay. He a.s.serted that they cannot enter the plant directly, but that the water, carbonic acid and ammonia resulting from their decay, are the substances actually imbibed by plants, and from these alone is built up the organic or combustible part of vegetation.

To this day there is a division of opinion among scientific men on this subject, some adopting the views of Liebig, others maintaining that certain soluble organic matters, viz., crenic and apocrenic acids are proper food of plants.

On the one hand it has been abundantly demonstrated that these organic matters are not at all essential to the growth of agricultural plants, and can const.i.tute but a small part of the actual food of vegetation taken in the aggregate.

On the other hand, we are acquainted with no satisfactory evidence that the soluble organic matters of the soil and of peat, especially the crenates and apocrenates, are not actually appropriated by, and, so far as they go, are not directly serviceable as food to plants.

Be this as it may, practice has abundantly demonstrated the value of humus as an ingredient of the soil, and if not directly, yet indirectly, it furnishes the material out of which plants build up their parts.

2. _The organic matters of peat as indirect food to plants._ Very nearly one-half, by weight, of our common crops, when perfectly dry, consists of _carbon_. The substance which supplies this element to plants is the gas, carbonic acid. Plants derive this gas mostly from the atmosphere, absorbing it by means of their leaves. But the free atmosphere, at only a little s.p.a.ce above the soil, contains on the average but 1/2500 of its bulk of this gas, whereas plants flourish in air containing a larger quant.i.ty, and, in fact, their other wants being supplied, they grow better as the quant.i.ty is increased to 1/12 the bulk of the air. These considerations make sufficiently obvious how important it is that the soil have in itself a constant and abundant source of carbonic acid gas.

As before said, _organic matter, in a state of decay_, is the single material which the farmer can incorporate with his soil in order to make the latter a supply of this most indispensable form of plant-food.

When organic matters decay in the soil, their carbon ultimately a.s.sumes the form of Carbonic acid. This gas, constantly exhaling from the soil, is taken up by the foliage of the crops, and to some extent is absorbed likewise by their roots.

Boussingault & Lewy have examined the air inclosed in the interstices of various soils, and invariably found it much richer (10 to 400 times) than that of the atmosphere above. Here follow some of their results:

CARBONIC ACID IN SOILS.

-------------------------------------------------------------------------- Key: A - _Volumes of Carbonic acid in 100 of air in pores of Soil._ B - _Cubic feet of air in acre to depth of 14 inches._ C - _Cubic feet of Carbonic acid in acre to depth of 14 inches._ D - _Volumes of Carbonic acid to 100 of air above the soil._ E - _Cubic feet of air over one acre to height of 14 inches._ F - _Cubic feet of Carbonic acid over one acre to a height of 14 inches._

--------------------------------------------------------+-----+------+---- _Designation and Condition of Soil._

A

B

C --------------------------------------------------------+-----+------+---- Sandy subsoil of forest

0.24

4,326

14 Loamy " " "

0.82

3,458

28 Surface soil " "

0.86

5,768

56 Clayey soil of artichoke field

0.66

10,094

71 Soil of asparagus bed, unmanured for one year

0.79

10,948

86 " " " " newly manured

1.54

10,948

172 Sandy soil, six days after manuring, and three

days of rain.

2.21

11,536

257 " " ten " " " " "

" " "

9.74

11,536

1144 Compost of vegetable mold

3.64

20,608

772

_Carbonic Acid in Atmosphere_

D

E

F

-----+------+----

0.025

50,820

14 --------------------------------------------------------+-----+------+----

From the above it is seen that in soils containing little decomposing organic matters--as the forest sub-soils--the quant.i.ty of carbonic acid is no greater than that contained in an equal bulk of the atmosphere. It is greater in loamy and clayey soils; but is still small. In the artichoke field (probably light soil not lately manured), and even in an asparagus bed unmanured for one year, the amount of carbonic acid is not greatly larger. In newly manured fields, and especially in a vegetable compost, the quant.i.ty is vastly greater.

The organic matters which come from manures, or from the roots and other residues of crops, are the source of the carbonic acid of the soil.

These matters continually waste in yielding this gas, and must be supplied anew. Boussingault found that the rich soil of his kitchen garden (near Strasburg) which had been heavily manured from the barn-yard for many years, lost one-third of its carbon by exposure to the air for three months (July, August and September,) being daily watered. It originally contained 2.43 _per cent._ At the conclusion of the experiment it contained but 1.60 _per cent._, having lost 0.83 _per cent._

Peat and swamp-muck, when properly prepared, furnish carbonic acid in large quant.i.ties during their slow oxidation in the soil.

3. _The Nitrogen of Peat, including Ammonia and Nitric Acid._

The sources of the nitrogen of plants, and the real cause of the value of nitrogenous fertilizers, are topics that have excited more discussion than any other points in Agricultural Chemistry. This is the result of two circ.u.mstances. One is the obscurity in which some parts of the subject have rested; the other is the immense practical and commercial importance of this element, as a characteristic and essential ingredient of the most precious fertilizers. It is a rule that the most valuable manures, _commercially considered_, are those containing the most nitrogen. Peruvian guano, sulphate of ammonia, soda-saltpeter, fish and flesh manures, bones and urine, cost the farmer more money per ton than any other manures he buys or makes, superphosphate of lime excepted, and this does not find sale, for general purposes, unless it contains several _per cent._ of nitrogen. These are, in the highest sense, nitrogenous fertilizers, and, if deprived of their nitrogen, they would lose the greater share of their fertilizing power.

The importance of the nitrogen of manures depends upon the fact that those forms (compounds) of nitrogen which are capable of supplying it to vegetation are comparatively scarce.

It has long been known that peat contains a considerable quant.i.ty of nitrogen. The average amount in thirty specimens, a.n.a.lyzed under the author's direction, including peats and swamp mucks of all grades of quality, is equivalent to 1-1/2 _per cent._ of the air-dried substance, or more than thrice as much as exists in ordinary stable or yard manure.

In several peats the amount is as high as 2.4 _per cent._, and in one case 2.9 _per cent._ were found.

Of these thirty samples, one-half were largely mixed with soil, and contained from 15 to 60 _per cent._ of mineral matters.

Reducing them to an average of 15 _per cent._ of water and 5 _per cent._ of ash, they contain 2.1 _per cent._ of nitrogen, while the organic part, considered free from water and mineral substances, contains on the average 2.6 _per cent._ See table, page 90.

The five peats, a.n.a.lyzed by Websky and Chevandier, as cited on page 24, considered free from water and ash, contain an average of 1.8 _per cent._ of nitrogen.

We should not neglect to notice that peat is often comparatively poor in nitrogen. Of the specimens, examined in the Yale a.n.a.lytical Laboratory, several contained but half a _per cent._ or less. So in the a.n.a.lyses of Websky, one sample contained but 0.77 _per cent._ of the element in question.

As concerns the state of combination in which nitrogen exists in peat, there is a difference of opinion. Mulder regards it as chiefly occurring in the form of _ammonia_ (a compound of nitrogen and hydrogen), united to the organic acids from which it is very difficult to separate it.

Recent investigations indicate that in general, peat contains but a small proportion of ready-formed ammonia.

The great part of the nitrogen of peat exists in an insoluble and inert form: but, by the action of the atmosphere upon it, especially when mixed with and divided by the soil, it gradually becomes available to vegetation to as great an extent as the nitrogen of ordinary fertilizers.

It appears from late examinations that weathered peat may contain _nitric acid_ (compound of nitrogen with oxygen) in a proportion which, though small, is yet of great importance, agriculturally speaking. What a.n.a.lytical data we possess are subjoined.