Garden soils usually consist of loam of some kind, the consequence of long cultivation.  Natural loams are the result of the decay and admixture of various earths, and they are mostly of a mellow texture, easily worked and highly productive.  They are, as a rule, the best of all soils, and their goodness is in part due to the fact that they contain a little of everything, with no great predominance of any one particular earth.  Cultivation also produces loam.  On a clay land we find a top crust of clayey loam, and on a lime or chalk land a top crust of calcareous loam.  Where cultivation has been long pursued the staple is broken and manures are put on, and the roots of plants assist in disintegration and decomposition.  Thus there is accumulation of humus and a decomposition of the rock proceeding together, and a loam of some sort is the result.  Hence the necessity of caution in respect of deep trenching, for if we bury the top soil and put in its place a crude material that has not before seen daylight, we may lose ten years in profitable cropping, because we must now begin to tame a savage soil that we have been at great pains to bring up, to cover a stratum of a good material prepared for us by the combined operations of Nature and Art during, perhaps, several centuries.  But deep and good garden soils may be safely trenched and freely knocked about, because not only does the process favour the deep rooting of the plants, but it favours also that disintegration which is one of the causes of fertility.  Every pebble is capable of imparting to the soil a solution—­infinitesimal, perhaps, but not the less real—­of silica, or lime, or potash, or phosphates, or perhaps of all these; but it must be exposed to light and air and moisture to enable it to part with a portion of its substance, and thus it is that mechanical tillage is of the first importance in all agricultural and horticultural operations.

The principal inorganic or mineral constituents of plants are potash, soda, lime, iron, phosphorus, sulphur, chlorine, and silica.  Clays and loams are generally rich in potash, sulphur, and phosphates, but deficient in soluble silica and lime.  Limestone and chalk are usually rich in lime and phosphates, but deficient in humus, silica, sulphur, and alkalies.  Sandy soils are rich in silica, but are generally poor in respect of phosphates and alkalies.  Therefore, on a clay or loam, farmyard manure is invaluable, because it contains ingredients that all crops appreciate, and also because it is helpful in breaking up the texture of the soil.  The occasional application of lime also is important for its almost magical effect on garden soil that has been liberally manured and heavily cropped for a long term of years.  Calcareous soils are greatly benefited by a free application to them of manure from the stable and cow-byre; but as a rule it would be like carrying coals to Newcastle to dress these soils with lime.  Clay may be put on with advantage; and nothing benefits a hot chalky