It is important to observe that when a salt is used, the base only is absorbed, and the acid escapes in combination with lime; even nitric acid, notwithstanding its importance as a food of plants, being in this predicament. From this it may be gathered that lime is not readily absorbed from solutions of its salts; indeed, it would appear that the only salt of that substance liable to absorption is the bicarbonate, from which it is taken to the extent of 1·4 per cent by the soil. The absorption of lime from this salt, and that of phosphoric acid, which takes place to a considerable extent, probably occurs, however, quite independently of the clay present in the soil, and is occasioned by its lime, which forms an insoluble compound with phosphoric acid, and by removing half the carbonic acid of the bicarbonate of lime converts it also into an insoluble state.
In addition to these mineral substances, organic matters are also removed from solution. This is conspicuously seen in the case of putrid urine, which not only loses its ammonia, but also its smell and colour, when allowed to percolate through soil; and an equally marked result was obtained with flax water, from which the organic matter was entirely abstracted.
The cause of this absorptive power is still very imperfectly known. Mr. Way having observed that sand has no such property, while clay, even when obtained from a considerable depth, always possesses it, supposed that the absorption was entirely due to that substance. A difficulty, however, presents itself in explaining how it should happen that while a pure clay absorbs only 0·2847 of ammonia, a loamy soil, of which one-half probably is sand, should absorb a larger quantity. The inference is, that the effect cannot be due to the clay as a whole, and Mr. Way has sought to explain it by supposing that there exist in the soil particular double silicates of alumina and lime. He has shown that felspar and the other minerals from which the soil is produced have no absorbent power, but that artificial compounds can be formed which act upon solutions of ammonia and potash in a manner very similar to the soil; but there is not the slightest evidence that these compounds exist in the soil, and in the year 1853[I] I pointed out the probability that clay is not the only agent at work, but that the organic matters take part in the process. So powerful indeed is the affinity of these substances for ammonia, that chemists are at one as to the difficulty of obtaining humic and other similar acids pure, owing to the obstinacy with which they retain it; and there cannot be a doubt that in many soils these substances are in this point of view of much importance. This is particularly the case in peat soils, which, though naturally barren, may be made to produce good crops by the application of sand or gravel; and as neither of these can cause any absorption of the valuable matters, we must attribute this effect to the organic matter. Referring to an earlier series of experiments made in 1850, I showed that, if a quantity of dry peat be taken and ammonia poured on it, its smell disappears; and this may be continued until upwards of 1·5 per cent of dry ammonia has been absorbed, and this quantity is retained by the peat.
In this case pure ammonia was used, but Way's experiments having shown that this alkali is not absorbed from its salts by organic matters, I expressed the opinion that humate of lime (which certainly exists in most soils) ought on chemical grounds to decompose the salts of ammonia and cause the retention of their base. The recent researches of Brustlein have shown that lime does cause the organic matters to absorb ammonia from its salts. He confirms the fact that pure ammonia is absorbed by peat, and shows that decayed wood has the same effect, although both are without action on solutions of its salts. A stiff clay, on the other hand, containing organic matters and much carbonate of lime, readily absorbed ammonia, both when pure and combined; but after extracting the lime by means of a dilute acid, it lost the power of taking it from its salts, although it retained the free alkali as completely as before. On the addition of a small quantity of lime, it again acquired the power of withdrawing ammonia from its compounds. These experiments may be explained, either on the supposition of the presence of humate of lime, or by supposing that the carbonate of lime first decomposed the salts of ammonia, and that the liberated alkali combined with the organic matter. It must be admitted, however, that it is very doubtful whether the ammonia and other substances are fixed in the soil by a true chemical combination. They are certainly retained by a very feeble attraction, for it appears from Brustlein's experiments that ammonia may be, to a considerable extent, removed by washing with abundance of water, and that if the soil which has absorbed ammonia be allowed to become dry in the air, it loses half its ammonia, and after four times moistening and drying, three-fourths have disappeared. These facts are certainly not incompatible with the presence of a true chemical compound, for the humate of ammonia is not absolutely insoluble, and many cases occur of actions taking place in the presence of water, which are entirely reversed when that fluid is removed; and it is quite possible that when humate of ammonia is dried in contact with carbonate of lime, it may be decomposed, and carbonate of ammonia escape. There are other circumstances, however, which render it, on the whole, most probable that the combination is not wholly chemical, but rather of a physical character, among which may be more especially mentioned the fact, that the quantity of the substances retained by the soil is dependent on the degree of dilution of the fluid from which they are taken; and that the quantity absorbed never exceeds a very small fraction of the weight of the soil.
The practical inferences to be drawn from these facts regarding the value of soils are of the highest importance. It is obvious that two soils having exactly the same chemical composition may differ widely in absorptive power, and that which possesses it most largely must have the highest agricultural value. The examination of different soils, in this point of view, is a subject of much importance, and deserves the best attention of both farmers and chemists, although little has as yet been done in regard to it, and the results which have been obtained are not of a very satisfactory character. Liebig states, that in his experiments, all the arable soils examined possessed the same absorptive power, whether they contained a large or a small proportion of lime or alumina. It can scarcely be expected, however, that this should be true in all cases, and there are many facts which seem to indicate that differences must exist. It is well known that there are some soils in which the manure is very rapidly exhausted, and it is more than probable that this effect is due to deficient absorptive power, which leaves the soluble matters at the mercy of the weather, and liable at any moment to be washed out by a heavy fall of rain.
The more strictly mechanical properties of the soil, such as its relations to heat and moisture, are not less important than its chemical composition. It is known that soils differ so greatly in these respects as sometimes materially to affect their productive capacity. Thus, for instance, two soils may be identical in composition, but one may be highly hygrometric, that is, may absorb moisture readily from the air, while the other may be very deficient in that property. Under ordinary circumstances no difference will be apparent in their produce, but in a dry season the crop upon the former may be in a flourishing condition, while that on the latter is languishing and enfeebled, merely from its inability to absorb from the air, and supply to the plant the quantity of water required for its growth. In the same way, a soil which absorbs much heat from the sun's rays surpasses another which has not that property; and though in many cases this effect is comparatively unimportant, in others it may make the difference between successful and unsuccessful cultivation in soils which lie in an unfavourable climate or exposure.
The investigation of the physical characters of soils has attracted little attention, and we owe all our present knowledge of the subject to a very elaborate series of researches on this subject, published by Schübler, nearly thirty years ago. He determined 1st, The specific gravity of the soils; 2d, The quantity of water which they are capable of imbibing; 3d, The rapidity with which they give off by evaporation the water they have imbibed; that is, their tendency to become dry; 4th, The extent to which they shrink in drying; 5th, Their hygrometric power; 6th, The extent to which they are heated by the sun's rays; 7th, The rapidity with which a heated soil cools down, which indicates its power of retaining heat; 8th, Their tenacity, or the resistance they offer to the passage of agricultural implements; 9th, Their power of absorbing oxygen from the air. Each of these experiments was performed on several different soils, and on their mechanical constituents. Schübler's experiments are undoubtedly important, and though the methods employed are some of them not altogether beyond cavil, they have apparently been performed with great care. It is nevertheless desirable that they should be repeated, for such facts ought not to rest on the authority of one experimenter, however skilful and conscientious, nor on a single series of soils, which may not give a fair representation of their general physical properties. In fact, Schübler appears to imagine that having once determined the extent to which the sand, clay, and other mechanical constituents of the soil possess these properties, we are in a condition to predicate the effect of their mixture in variable proportions, although this is by no means probable.
In examining these properties, Schübler selected for experiment, pure siliceous sand, calcareous sand (carbonate of lime in coarse grains), finely powdered carbonate of lime, pure clay, humus, and powdered gypsum. He used also a heavy clay consisting of 11 per cent of sand and 89 of pure clay, a somewhat stiff clay containing 24 per cent of sand and 76 of clay, a light clay with 40 per cent of sand and 60 of pure clay, a garden soil consisting of 52·4 per cent of clay, 36·5 of siliceous sand, 1·8 of calcareous sand, 2 per cent of finely divided carbonate of lime, and 7·2 of humus, and two arable soils, one from Hoffwyl, and one from a valley in the Jura, the former a somewhat stiff, the latter a light soil.
| Specific gravity. | Water absorbed by 100 parts per cent. | Of 100 parts of water absorbed there evaporate in four hours at 66° | Diminution in bulk during drying of 100 parts moist soil. | |
| Siliceous sand | 2·753 | 25 | 88·4 | 0·0 |
| Calcareous sand | 2·822 | 29 | 75·9 | 0·0 |
| Light clay | 2·701 | 40 | 52·0 | 6·0 |
| Stiff clay | 2·652 | 50 | 45·7 | 8·9 |
| Heavy clay | 2·603 | 61 | 34·9 | 11·4 |
| Pure clay | 2·591 | 70 | 31·3 | 18·3 |
| Carbonate of lime | 2·468 | 85 | 28·0 | 5·0 |
| Humus | 1·225 | 190 | 20·5 | 20·0 |
| Gypsum | 2·358 | 27 | 71·7 | 0·0 |
| Garden soil | 2·332 | 96 | 24·5 | 14·9 |
| Soil from Hoffwyl | 2·401 | 52 | 32·0 | 12·0 |
| Soil from Jura | 2·526 | 47 | 40·1 | 9·5 |
| Quantity of hygrometric water absorbed by 77·165 grains of the soil spread on a surface of 141·48 square inches. | Power of retaining heat. Calcareous sand, 100. | ||||
| 12 hours. | 24 hours. | 48 hours. | 72 hours. | ||
| Siliceous sand | 0 | 0 | 0 | 0 | 95·6 |
| Calcareous sand | 0·154 | 0·231 | 0·231 | 0·231 | 100·0 |
| Light clay | 1·617 | 2·002 | 2·156 | 2·156 | 76·9 |
| Stiff clay | 1·925 | 2·310 | 2·618 | 2·695 | 71·1 |
| Heavy clay | 2·310 | 2·772 | 3·080 | 3·157 | 68·4 |
| Pure clay | 2·849 | 3·234 | 3·696 | 3·773 | 66·7 |
| Carbonate of lime | 2·002 | 2·387 | 2·695 | 2·695 | 61·8 |
| Humus | 6·160 | 7·469 | 8·470 | 9·240 | 49·0 |
| Gypsum | 0·077 | 0·077 | 0·077 | 0·077 | 73·2 |
| Garden soil | 2·695 | 3·465 | 3·850 | 4·004 | 64·8 |
| Soil from Hoffwyl | 1·232 | 1·771 | 1·771 | 1·771 | 70·1 |
| Soil from Jura | 1·078 | 1·463 | 1·540 | 1·540 | 74·3 |