It will be seen that each distinct formation gives rise to a great variety of fertility, even where the basis remains the same; but it is of great importance to the farmer to ascertain the general nature of the rocks and strata on which his farm lies. In these soils which we have mentioned, no notice has been taken of organic matter, because this does not seem in any way connected with their formation. The primary strata are distinguished by having no traces of organic remains in their composition. It is in the tertiary strata, especially those which have been formed by the destruction of animal and vegetable substances, that organic matter becomes a peculiar object of attention; and it is doubtless from this reason alone that the alluvial soils formed by the deposit of a variety of earths in a state of great division, and mixed with a portion of organic matter, form by far the most productive lands. They will bear crop alter crop with little or no addition of manure. These soils are found along the course of rivers which traverse extensive plains, and which have such a current as to keep very fine earth suspended by a gentle, yet constant, agitation, but not sufficiently rapid to carry along with it coarse gravel or sand. Where-ever there is an obstruction to the current, and an eddy is formed, there the soil is deposited in the form of mud, and, gradually accumulating, lorms these alluvial soils which are so remarkable for their iertility. In these soils the impalpable matter greatly predominates ; but the intimate mixture of the earths with organic matter, in a state which has been called humus, prevents their consolidating into a stiff clay, and the gases which are continually evolved from the organic matter, keep the poies open, and give scope to the growth and nourishment of the root. Organic matter is no doubt essential to great fertility in a soil, but some soils require more of it than others. Humus, which is the form organic matter naturally comes to by slow decomposition in the earth, gives out certain elements which the roots can take up in their nascent state, and from which the obtain the carbon so abundant in all vegetable productions. But organic matter, in every stage of its spontaneous decomposition, keeps the pores of the soil open, and admits, even it it does not attract, air and moisture to the fibres of the roote. Professor Liebig, however, takes a different view of this subject. He says :— Land of the greatest fertility contains argillaceous earth and other disintegrated minerals with chalk and sand, in such a proportion as to give free aecess to air and moisture. The land in the vicinity of Mount Vesuvius may be considered as the type of a fertile soil, and its fertility is greater or less in different parts according to the proportion ot clay or sand which it contains. The soil which is formed by the disintegration of lava cannot possibly, on account of its origin, contain the smallest trace of vegetable matter; and yet it is well known that when the volcanic ashes have been exposed for some time to the influence of air and moisture, a soil is gradually formed in which all kinds of plants grow with the greatest luxuriance. This fertility is owing to the alkalies which are contained in the lava, and which, by exposure to the weather, are rendered capable of being absorbed by plants. Thousands of years have been necessai y to convert stones and rocks into the soil of arable land, and thousands of years more will be required for their perfect reduction—that is, for the complete exhaustion ot their alkalies. Air, water, and the change of temperature, prepare the different species of rocks for yielding to plants the alkalies which they contain. A soil which has been exposed for centuries to all the influences which affect the disintegration of rocks, but from which the alkalies have not been removed, will be able to afford the means of nourishment to those vegetables which require alkalies for their growth during many years ; but it must gradually become exhausted, unless those alkalies which have been removed are again replaced :—a period, therefore, will arrive, when it will be necessary to expose it from time to time to a further disintegration, in order to obtain a new supply ot soluble alkalies ; for, small as is the quantity of alkali which plants require, it is nevertheless quite indispensable for their perfect development. The first colonists of Virginia fouud a country, the soil of which was similar to that just mentioned; harvests of wheat and tobacco were obtained for a century from one and the aame field without the aid of manure; but now whole districts are converted into unfruitful pasture land, which, without manure produces neither wheat nor tobacco. From every acre of this land there were removed, in the space of one hundred years, 12,000 pounds of alkalies in leaves, grain, and straw. It became unfruitful, therefore, because it was deprived of every particle of alkali which had been reduced to a soluble state, and because that which was rendered soluble again in the space of one year, was not sufficient to satisfy the demands of the plants. It is the greatest possible mistake to suppose that the temporary diminution of fertility in a soil is owing to the loss of humus ; it is the mere consequence of the exhaustion ol the alkalies. Let us look at the condition of the country around Naples, which is famed for its fruitful corn land. The farms and villages are situated from eighteen to twenty-four miles distant from each other, and between them there are no roads, and consequently no transportation of manure. Now, grain has been cultivated on this land for thousands of years, without any part of that which is annually removed from the soil being artificially restored to it. How can any influence be ascribed to humus under such circumstances, when it is not even known whether humus was ever contained in the soil ? The method of culture in that district explains the permanent fertility. A field is cultivated once every three years, and is in the intervals allowed to serve as a sparing pasture for cattle. The soil experiences no change in the two years in which it lies fallow, further than that it is exposed to the influence of the weather, by which a fresh portion of the alkalies contained in it are again set free or rendered soluble. The animals fed on these fields yield nothing to these soils which they did not formerly possess. The weeds upon which they live spring trom the soil, and that which they return to it as excrement must always be less than that which they extract. The fields, therefore can have gained nothing from the mere feeding of cattle upon them ; on the contrary, the soil must have lost some of its constituents. Experience, has shown, in agriculture, that wheat should not be cultivated after wheat on the same soil, for it belongs, with tobacco, to the plant which exhaust a soil. But if the hnmus of a soil gives it the power of producing grain, how happens it that wheat does not thrive in many parts of Brazil, where the soils are particularly rich in this substance ? The cause is, that the strength of the stalk is due to silicate of potash, and that the grain requires phosphate of magnesia, neither of which substances a soil of humus can afford, since it does not contain them. The plant may, indeed, under such circumstances, become an herb, but will not bear fruit. Potash is not the only substance necessary for the existence of most plants ; indeed, the potash may be replaced in many cases by soda, lime, or magnesia. But other substances beside alkalies are required to sustain the life of plants. Phosphoric acid has been found in the ashes of all plants hitherto examined, and always in combination with alkalies or alkaline earths. Most seeds contain certain quantities of phosphates. In the seeds of different kinds of CDrn, particularly, there is abundance of phosphate ot magnesia. The soil in which plants grow furnishes them with phosphoric acid, and they in turn yield it to animals, to be used in the formation of their bones, and of those constituents of the brain which contain phosphorus. Much more phosphorus is thus afforded to the body than it requires when flesh, bread, fruit, and husks of grain are used for food ; and this excess is eliminated in the urine and solid excrements.