Surface, six inches 2,407,900 Next " 2,444,200 Third " 2,480,500

"Or in round numbers, every six inches of soil weighed per acre 2 millions of pounds, which agrees tolerably well with the actual weight per acre of the preceding soil.

"The amount of phosphoric acid and nitrogen in each six-inch layer was determined separately, as before, when the following results were obtained: In Dried Soil.

First six Second Third six inches. six inches. inches.

Percentage of phosphoric acid .159 .166 .140 Nitrogen .189 .134 .089 Equal to ammonia .229 .162 .108

"An acre, according to these determinations, contains in the three separate sections:

First six Second Third six inches. six inches. inches.

lbs. lbs. lbs.

Phosphoric acid 3,975 4,150 3,500 Nitrogen 4,725 3,350 2,225 Equal to ammonia 5,725 4,050 2,700

"Here, again, as might naturally be expected, the proportion of nitrogen is largest in the surface, where all the decaying leaves dropped during the growth of the clover for seed are found, and wherein root-fibres are more abundant than in the lower strata. The first six inches of soil, it will be seen, contained in round numbers, 2 tons of nitrogen per acre, that is, considerably more than was found in the same section of the soil where the clover was mown twice for hay; showing plainly, that during the ripening of the clover seed, the surface is much enriched by the nitrogenous matter in the dropping leaves of the clover-plant.

"_Clover-roots_.--The roots from one square foot of this soil, freed as much as possible from adhering soil, were dried at 212, and when weighed and reduced to a fine powder, gave, on a.n.a.lysis, the following results:

Organic matter* 64.76 Mineral matter 35.24 ------- 100.00 ======= * Containing nitrogen 1.702 Equal to ammonia 2.066 Including clay and sand (insoluble silicious matter) 26.04

"A square foot of this soil produced 582 grains of dried clover-roots, consequently an acre yielded 3,622 lbs. of roots, or more than twice the weight of roots obtained from the soil of the same field where the clover was twice mown for hay.

"In round numbers, the 3,622 lbs. of clover-roots from the land mown once, and afterwards left for seed, contained 51 lbs. of nitrogen.

"The roots from the soil after clover-seed, it will be noticed, were not so clean as the preceding sample, nevertheless, they yielded more nitrogen. In 64.76 of organic matter, we have here 1.702 of nitrogen, whereas, in the case of the roots from the part of the field where the clover was twice mown for hay, we have in 81.33 parts, that is, much more organic matter, and 1.635, or rather less of nitrogen. It is evident, therefore, that the organic matter in the soil after clover-seed, occurs in a more advanced stage of decomposition, than found in the clover-roots from the part of the field twice mown. In the manure, in which the decay of such and similar organic remains proceeds, much of the non-nitrogenous, or carbonaceous matters, of which these remains chiefly, though not entirely, consist, is transformed into gaseous carbonic acid, and what remains behind, becomes richer in nitrogen and mineral matters. A parallel case, showing the dissipation of carbonaceous matter, and the increase in the percentage of nitrogen and mineral matter in what is left behind, is presented to us in fresh and rotten dung; in long or fresh dung, the percentage of organic matter, consisting chiefly of very imperfectly decomposed straw, being larger, and that of nitrogen and mineral matter smaller, than in well-rotted dung.

"The roots from the field after clover-seed, it will be borne in mind, were dug up in November, whilst those obtained from the land twice mown, were dug up in September; the former, therefore, may be expected to be in a more advanced state of decay than the latter, and richer in nitrogen.

"In an acre of soil, after clover-seed, we have:

Lbs.

Nitrogen in first six inches of soil 4,725 Nitrogen in roots 51 Nitrogen in second six inches of soil 3,350 ------- Total amount of nitrogen, per acre, in twelve inches of soil 8,126 =======

"Equal to ammonia, 9,867 lbs.: or, in round numbers, 3 tons and 12 cwts. of nitrogen per acre; equal to 4 tons 8 cwts. of ammonia.

"This is a very much larger amount of nitrogen than occurred in the other soil, and shows plainly that the total amount of nitrogen acc.u.mulates especially in the surface-soil, when clover is grown for seed; thus explaining intelligibly, as it appears to me, why wheat, as stated by many practical men, succeeds better on land where clover is grown for seed, than where it is mown for hay.

"All the three layers of the soil, after clover-seed, are richer in nitrogen than the same sections of the soil where the clover was twice mown, as will be seen by the following comparative statement of results:

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

I.

II.

Clover-Soil twice

Clover-Soil once mown

mown.

and then left for seed.

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

Upper

Second

Third

Upper

Next

Lowest

6 inches

6 inches

6 inches

6 inches

6 inches

6 inches --------------+--------+--------+--------+--------+--------+-------- Percentage of

nitrogen in

.168

.092

.064

.189

.134

.089 dried soil

Equal to

ammonia

.198

.112

.078

.229

.162

.108 --------------+--------+--------+--------+--------+--------+--------

"This difference in the amount of acc.u.mulated nitrogen in clover-land, appears still more strikingly on comparing the total amounts of nitrogen per acre in the different sections of the two portions of the 11-acre field.

Percentage of Nitrogen Per Acre.

First six Second Third six inches. six inches. inches.

Lbs. Lbs. Lbs.

I. In soil, clover twice } mown* } 3,350 1,875 1,325 II. In soil, clover once } mown and seeded } afterwards } 4,725 3,350 2,225 ===== ===== ===== Equal to ammonia: * I. Clover twice mown } 4,050 2,275 1,600 II. Clover seeded } 5,725 4,050 2,700

Lbs.

I. Nitrogen in roots of clover twice mown } 24 II. Nitrogen in clover, once mown, and grown } for seed afterwards } 51 I. Weight of dry roots per acre from Soil I } 1,493 II. Weight of dry roots per acre from Soil II } 3,622 Total amount of nitrogen in 1 acre, 12 inches } deep of Soil I* } 5,249 Total amount of nitrogen in 1 acre, 12 inches } deep of Soil II } 8,126 Excess of nitrogen in an acre of soil 12 } inches deep, calculated as ammonia in part } of field, mown once and then seeded } 3,592 --------- * Equal to ammonia } 6,374 Equal to ammonia } 9,867

"It will be seen that not only was the amount of large clover-roots greater in the part where clover was grown for seed, but that likewise the different layers of soil were in every instance richer in nitrogen after clover-seed, than after clover mown twice for hay.

"Reasons are given in the beginning of this paper which it is hoped will have convinced the reader, that the fertility of land is not so much measured by the amount of ash const.i.tuents of plants which it contains, as by the amount of nitrogen, which, together with an excess of such ash const.i.tuents, it contains in an available form. It has been shown likewise, that the removal from the soil of a large amount of mineral matter in a good clover-crop, in conformity with many direct field experiments, is not likely in any degree to affect the wheat-crop, and that the yield of wheat on soils under ordinary cultivation, according to the experience of many farmers, and the direct and numerous experiments of Messrs. Lawes and Gilbert, rises or falls, other circ.u.mstances being equal, with the supply of available nitrogenous food which is given to the wheat. This being the case, we can not doubt that the benefits arising from the growth of clover to the succeeding wheat, are mainly due to the fact that an immense amount of nitrogenous food acc.u.mulates in the soil during the growth of clover.

"This acc.u.mulation of nitrogenous plant-food, specially useful to cereal crops, is, as shown in the preceding experiments, much greater when clover is grown for seed, than when it is made into hay. This affords an intelligible explanation of a fact long observed by good practical men, although denied by others who decline to accept their experience as resting upon trustworthy evidence, because, as they say, land cannot become more fertile when a crop is grown upon it for seed, which is carried off, than when that crop is cut down and the produce consumed on the land. The chemical points brought forward in the course of this inquiry, show plainly that mere speculation as to what can take place in a soil, and what not, do not much advance the true theory of certain agricultural practices. It is only by carefully investigating subjects like the one under consideration, that positive proofs are given, showing the correctness of intelligent observers in the fields. Many years ago, I made a great many experiments relative to the chemistry of farm-yard manure, and then showed, amongst other particulars, that manure, spread at once on the land, need not there and then be plowed in, inasmuch as neither a broiling sun, nor a sweeping and drying wind will cause the slightest loss of ammonia; and that, therefore, the old-fashioned farmer who carts his manure on the land as soon as he can, and spreads it at once, but who plows it in at his convenience, acts in perfect accordance with correct chemical principles involved in the management of farm-yard manure. On the present occasion, my main object has been to show, not merely by reasoning on the subject, but by actual experiments, that the larger the amounts of nitrogen, potash, soda, lime, phosphoric acid, etc., which are removed from the land in a clover-crop, the better it is, nevertheless, made thereby for producing in the succeeding year an abundant crop of wheat, other circ.u.mstances being favorable to its growth.

"Indeed, no kind of manure can be compared in point of efficacy for wheat, to the manuring which the land gets in a really good crop of clover. The farmer who wishes to derive the full benefit from his clover-lay, should plow it up for wheat as soon as possible in the autumn, and leave it in a rough state as long as is admissible, in order that the air may find free access into the land, and the organic remains left in so much abundance in a good crop of clover be changed into plant-food; more especially, in other words, in order that the crude nitrogenous organic matter in the clover-roots and decaying leaves, may have time to become transformed into ammoniacal compounds, and these, in the course of time, into nitrates, which I am strongly inclined to think is the form in which nitrogen is a.s.similated, par excellence by cereal crops, and in which, at all events, it is more efficacious than in any other state of combination wherein it may be used as a fertilizer.

"When the clover-lay is plowed up early, the decay of the clover is sufficiently advanced by the time the young wheat-plant stands in need of readily available nitrogenous food, and this being uniformly distributed through the whole of the cultivated soil, is ready to benefit every single plant. This equal and abundant distribution of food, peculiarly valuable to cereals, is a great advantage, and speaks strongly in favor of clover as a preparatory crop for wheat.

"Nitrate of soda, an excellent spring top-dressing for wheat and cereals in general, in some seasons fails to produce as good an effect as in others. In very dry springs, the rainfall is not sufficient to wash it properly into the soil and to distribute it equally, and in very wet seasons it is apt to be washed either into the drains or into a stratum of the soil not accessible to the roots of the young wheat. As, therefore, the character of the approaching season can not usually be predicted, the application of nitrate of soda to wheat is always attended with more or less uncertainty.

"The case is different, when a good crop of clover-hay has been obtained from the land on which wheat is intended to be grown afterwards. An enormous quant.i.ty of nitrogenous organic matter, as we have seen, is left in the land after the removal of the clover-crop; and these remains gradually decay and furnish ammonia, which at first and during the colder months of the year, is retained by the well known absorbing properties which all good wheat-soils possess. In spring, when warmer weather sets in, and the wheat begins to make a push, these ammonia compounds in the soil are by degrees oxidized into nitrates; and as this change into food peculiarly favorable to young cereal plants, proceeds slowly but steadily, we have in the soil itself, after clover, a source from which nitrates are continuously produced; so that it does not much affect the final yield of wheat, whether heavy rains remove some or all of the nitrate present in the soil. The clover remains thus afford a more continuous source from which nitrates are produced, and greater certainty for a good crop of wheat than when recourse is had to nitrogenous top-dressings in the spring.

SUMMARY.

"The following are some of the chief points of interest which I have endeavored fully to develope in the preceding pages:

"1. A good crop of clover removes from the soil more potash, phosphoric acid, lime, and other mineral matters, which enter into the composition of the ashes of our cultivated crops, than any other crop usually grown in this country.

"2. There is fully three times as much nitrogen in a crop of clover as in the average produce of the grain and straw of wheat per acre.

"3. Notwithstanding the large amount of nitrogenous matter and of ash-const.i.tuents of plants, in the produce of an acre, clover is an excellent preparatory crop for wheat.

"4. During the growth of clover, a large amount of nitrogenous matter acc.u.mulates in the soil.

"5. This acc.u.mulation, which is greatest in the surface soil, is due to decaying leaves dropped during the growth of clover, and to an abundance of roots, containing, when dry, from one and three-fourths to two per cent of nitrogen.

"6. The clover-roots are stronger and more numerous, and more leaves fall on the ground when clover is grown for seed, than when it is mown for hay; in consequence, more nitrogen is left after clover-seed, than after hay, which accounts for wheat yielding a better crop after clover-seed than after hay.

"7. The development of roots being checked, when the produce, in a green condition, is fed off by sheep, in all probability, leaves still less nitrogenous matter in the soil than when clover is allowed to get riper and is mown for hay; thus, no doubt, accounting for the observation made by practical men, that, notwithstanding the return of the produce in the sheep excrements, wheat is generally stronger, and yields better, after clover mown for hay, than when the clover is fed off green by sheep.

"8. The nitrogenous matters in the clover remains, on their gradual decay, are finally transformed into nitrates, thus affording a continuous source of food on which cereal crops specially delight to grow.

"9. There is strong presumptive evidence that the nitrogen which exists in the air, in shape of ammonia and nitric acid, and descends, in these combinations, with the rain which falls on the ground, satisfies, under ordinary circ.u.mstances, the requirements of the clover-crop. This crop causes a large acc.u.mulation of nitrogenous matters, which are gradually changed in the soil into nitrates. The atmosphere thus furnishes nitrogenous food to the succeeding wheat indirectly, and, so to say, gratis.

"10. Clover not only provides abundance of nitrogenous food, but delivers this food in a readily available form (as nitrates), more gradually and continuously, and, consequently, with more certainty of a good result, than such food can be applied to the land in the shape of nitrogenous spring top-dressings."

"Thank you Charley," said the Doctor, "_that is the most remarkable paper I ever listened to_. I do not quite know what to think of it. We shall have to examine it carefully."

"The first three propositions in the Summary," said I, "are unquestionably true. Proposition No. 4, is equally true, but we must be careful what meaning we attach to the word 'acc.u.mulate.' The idea is, that clover gathers up the nitrogen in the soil. It does not _increase_ the absolute amount of nitrogen. It acc.u.mulates it--brings it together.