THE CHEMISTRY OF GARDEN CROPS 195 



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, for the staple is broken and manures are put on, and the 

 roots of plants assist in disintegrating and decomposing, and thus 

 there is accumulation of humus and a decomposition of the rock pro- 

 ceeding 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 dis- 

 integration 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, sulphur, chlorine, silica, and phosphates of lime 

 and iron. 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 of humus, 

 silica, sulphur, and alkalies. Sandy soils are rich in silica, and are 

 rarely deficient of lime, but are generally poor in respect of phos- 

 phates and alkalies. Therefore, on a clay or loam, farmyard manure is 

 invaluable, because it not only contains ingredients that all crops ap- 

 preciate, but is helpful in breaking up the texture of the soil. The 

 occasional application of lime also is of importance, and not seldom 

 this produces an almost magical effect on an old garden soil that has 

 been heavily cropped and liberally manured. Calcareous soils are 



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