Improving the Soil by Paring and Burning.—It has long been familiarly known, that a decided improvement has been produced on some soils by burning. Its advantages have chiefly been observed on two sorts, heavy clays and peat soils, on both of which it has been practised to a great extent. The action of heat on the heavy clays appears to be of a twofold character, depending partly on the change effected in its physical properties, and partly on a chemical decomposition produced by the heat. The operation of burning is effected by mixing the clay with brushwood and vegetable refuse, and allowing it to smoulder in small heaps for some time. It is a process of some nicety, and its success is greatly dependent on the care which has been taken to keep the temperature as low as possible during the whole course of the burning.
Experience has shown that burning is by no means equally advantageous to all clays, but is most beneficial on those containing a considerable quantity of calcareous matter, and of silicates of potash. In such clays heat operates by causing the lime to decompose the alkaline silicates, and liberate a quantity of the potash which was previously in an unavailable state. Its effect may be best illustrated by the following analyses by Dr. Voelcker of a soil, and the red ash produced in burning it.
| Soil. | Red Ash. | |
| Water | 0·93 | 1·18 |
| Organic matter | 10·67 | 3·32 |
| Oxides of iron and alumina | 13·40 | 18·42 |
| Carbonate of lime | 23·90 | 8·83 |
| Sulphate of lime | trace | 1·15 |
| Carbonate of magnesia | 1·10 | " |
| Magnesia | " | 1·76 |
| Phosphoric acid | trace | 0·71 |
| Potash | 0·38 | 1·08 |
| Soda | 0·13 | " |
| Chloride of sodium | " | 1·03 |
| Insoluble matter, chiefly clay | 49·66 | 62·52 |
| —— | —— | |
| 100·17 | 100·00 |
In this instance the quantity of burned soil amounted to about fifteen tons per acre, and it is obvious that the quantity of potash which had been liberated from the insoluble clay and the phosphoric acid are equal to that contained in a considerable manuring. In order to obtain these results, it is necessary, as has been already observed, to keep the temperature as low as possible during the process of burning, direct experiment having shown that when this precaution is not observed another change occurs, whereby the potash, which at low temperatures becomes soluble, passes again into an insoluble state. A part of the beneficial effect is no doubt also due to the change produced in the physical characters of the clay by burning, which makes it lighter and more friable, and by mixture with the unburnt clay ameliorates the whole. This improvement in the physical characters of the clay also requires that it shall be burnt with as low a heat as possible; for if it rises too high, the clay coheres into hard masses which cannot again be reduced to powder, and the success of the operation of burning may always be judged of by the readiness with which it falls into a uniform friable powder.
The improvement of peat by burning has been practised to some extent in Scotland, though less frequently of late years than formerly; but it is still the principal method of reclaiming peat soils in many countries, and particularly in Finland, where large breadths of land have been brought into profitable cultivation by means of it. The modus operandi of burning peat is very simple; it acts by diminishing the superabundant quantity of humus or other organic matters, which, in the previous section we have seen to be so injurious to the fertility of the soil. It may act also in the same way as it does on clay, by making part of the inorganic constituents more really soluble, although it is not probable that its effect in this way can be very marked. Its chief action is certainly by destroying the organic matters, and by thus improving the physical character of the peat, and causing it to absorb and retain a smaller quantity of water than it naturally does. For this reason it is that it proves successful only on thin peat bogs, for if they be deep, the inorganic matters soon sink into the lower part, and the surface relapses into its old state of infertility. It is probably for this reason that the practice has been so much abandoned in Scotland, more especially as other and more economical modes of treating peat soils have come into use.
Warping.—This name has been given to a method of improving soils by causing the water of rivers to deposit the mud it carries in suspension upon them, and which has been largely practised in the low lying lands of Lincoln and Yorkshire, where it was introduced about a century ago. It is most beneficial on sandy or peaty soil, and by its means large tracts of worthless land have been brought under profitable cultivation. It requires that the land to be so treated shall be under the level of the river at full tide, and it is managed by providing a sluice through which the river water is allowed to flood the land at high tide, and again to escape at ebb, leaving a layer of mud generally about a tenth of an inch in thickness, which it brought along with it. By the repetition of this process, a layer of several feet in thickness, of an excellent soil, is accumulated on the surface. Herapath, who has carefully examined this subject chemically, has shown that in one experiment where the water used contained 233 grains of mud per gallon, 210 were deposited during the warping. The following analyses will show the general nature of the matters deposited, and the change which they produce on the soil:—No. 1 is the mud from the Humber in its natural state, No. 2 a specimen of warp of average quality artificially dried, No. 3 a sandy soil before warping, and No. 4, the same fifteen years after having received a coating of 11 inches of mud.
| 1 | 2 | 3 | 4 | |
| Water | 47·49 | ... | 1·06 | 2·00 |
| Organic matter | 5·94 | 6·93 | 2·20 | 7·61 |
| Chloride of Calcium | ... | ... | } ... | ... |
| Magnesium | } ... | 0·10 | } ... | ... |
| Sodium | } 1·66 | } 0·94 | } 0·14 | 0·16 |
| Potassium | } | } | } | |
| Sulphate of Soda | } ... | 0·31 | } ... | ... |
| Magnesia | } ... | 1·18 | } ... | ... |
| Lime | trace | 1·10 | trace | trace |
| Carbonate of Magnesia | 2·60 | 0·31 | trace | 0·29 |
| Lime | 3·59 | 8·18 | trace | 0·46 |
| Potash and Soda | 0·18 | 0·47 | trace | 0·17 |
| Magnesia | 1·69 | 2·60 | trace | 0·26 |
| Lime | 0·39 | 0·68 | trace | 0·14 |
| Peroxide of Iron | } 6·63 | 5·05 | 0·08 | 1·17 |
| Alumina | } | 8·18 | 0·39 | 0·41 |
| Phosphate of Iron | 0·58 | 1·04 | trace | 0·28 |
| Silica | ... | 9·05 | 0·14 | 2·77 |
| Sand and Stones | 29·15 | 55·87 | 95·91 | 84·97 |
| ———— | ———— | ———— | ———— | |
| 100·00 | 100·00 | 100·00 | 100·00 |
It is easy to understand the importance of the effects produced by adding to any soil large quantities of a mud containing upwards of one per cent of phosphate of iron; and in point of fact, Herapath has calculated that in one particular instance the quantity of phosphoric acid brought by warping upon an acre of land, exceeded seven tons per acre. As, moreover, the matters are all in a high state of division, they must exist in a condition peculiarly favourable to the plant. The overflow of the Nile is only an instance of warping on the large scale, with this difference, that it is repeated once only in every year, whereas, in this country, the operation is repeated at every tide until a deposit sometimes of several feet in thickness is obtained, after which it is stopped, and the soil brought under ordinary cultivation.
An operation which is, in some respects, the converse of warping, has been carried out on Blair-Drummond Moss, where the peat has been dislodged and carried off by the action of water, leaving the subjacent soil in a state fitted for cropping. Of course both this and warping are restricted to special localities, but they are most important means of ameliorating the soil when circumstances admit of their being carried out.