226 SCIENCE OF GARDENING. Part II. 



temperature of the room. The soils in all these experiments were placed in small tin- 

 plate trays two inches square, and half an inch in depth ; and the temperature ascertained 

 by a delicate thermometer. Thus the temperature of the surface, when bare and exposed 

 to the rays of the sun, affords at least one indication of the degrees of its fertility ; and 

 the thermometer may be sometimes a useful instrument to the purchaser or improver of 

 lands." 



1065. . The moisture in the soil and sub-soil materially affects its temperature, and prevents, 

 as in the case of constantly saturated aquatic soils, their ever attaining to any great degree 

 either of heat or cold. The same observation will apply to moist peaty soils, or peat- 

 bogs. 



1066. Chemical agency of soils. Besides these uses of soils, which may be considered 

 mechanical, there is, Sir H. Davy observes, another agency between soils and or- 

 ganisable matters, which may be regarded as chemical in ks nature. The earths, and 

 even the earthy carbonates, have a certain degree of chemical attraction for many of the 

 principles of vegetable and animal substances. This is easily exemplified in the instance 

 of alumina and oil ; if an acid solution of alumina be mixed with a solution of soap, 

 which consists of oily matter and potassa, the oil and the alumina will unite and form a 

 white powder, which will sink to the bottom of the fluid. The extract from decomposing 

 vegetable matter, when boiled with pipe-clay or chalk, forms a combination by which the 

 vegetable matter is rendered more difficult of decomposition and of solution. Pure silica 

 and siliceous sands have little action of this kind ; and the soils which contain the most 

 alumina and carbonate of lime, are those which act with the greatest chemical energy in 

 preserving manures. Such soils merit the appellation, which is commonly given to them, of 

 rich soils ; for the vegetable nourishment is long preserved in them, unless taken up by 

 the organs of plants. Siliceous sands, on the contrary, deserve the term hungry, which 

 is commonly applied to them ; for the vegetable and animal matters they contain, not 

 being attracted by the earthy constituent parts of the soil, are more liable to be decom- 

 posed by the action of the atmosphere, or carried off from them by water. In most of the 

 black and brown rich vegetable moulds, the earths seem to be in combination with a pe- 

 culiar extractive matter, afforded during the decomposition of vegetables ; this is slowly 

 taken up or attracted from the earths by water, and appears to constitute a prime cause of 

 the fertility of the soil. 



1067. Thus all soils are useful to plants, as affording them a fixed abode and a range for 

 their roots to spread in search of food ; but some are much more so than others, as better 

 adapted by their constituent parts, climate, inclination of surface and subsoil attracting 

 and supplying food. 



Sect. V. Of the Improvement of Soils. 



1068. Soils may be rendered more Jit for answering the purposes of vegetation by pulveris- 

 ation, by consolidation, by exposure to the atmosphere, by an alteration of their constituent 

 parts, by changing their condition in respect to water, by changing their position in re- 

 spect to atmospherical influence, and by a change in the kinds of plants cultivated. All 

 these improvements are independently of the application of manures. 



Subsect. 1. Ptdverisation. 



1069. The mechanical division of the parts of soils is a very obvious improvement, and ap- 

 plicable to all in proportion to their adhesive texture. Even a free siliceous soil will, if 

 left untouched, become too compact for the proper admission of air, rain, and heat, and 

 for the free growth of the fibres ; and strong upland clays, not submitted to the plough 

 or the spade, will, in a few years, be found in the possession of fibrous-rooted perennial 

 grasses, which form a clothing on their surface, or strong tap-rooted trees, as the oak, 

 which force their way through the interior of the mass. Annuals and ramentaceous- 

 rooted herbaceous plants cannot penetrate into such soils. 



1070. The first object of pulverisation is to give scope to the roots of vegetables, for 

 without abundance of roots no plant will become vigorous, whatever may be the richness 

 of the soil in which it is placed. The fibres of the roots, as we have seen (740.), take 

 up the extract of the soil by intro-susception ; die quantity taken up, therefore, will not 

 depend alone on the quantity in the soil, but on the number of absorbing fibres. The 

 more the soil is pulverised, the more these fibres are increased, the more extract is ab- 

 sorbed, and the more vigorous does the plant become. Pulverisation, therefore, is not only 

 advantageous previous to planting or sowing, but also during the progress of vegetation, 

 when applied in the intervals between the plants. In this last case it operates also in the 

 way of pruning, and by cutting off or shortening the extending fibres, causes them to 

 branch out numerous others, by which the mouths or pores of the plants are greatly in- 

 creased, and such food as is in the soil has the better chance of being sought after, and 

 taken up by them. Tull and Du Hamel relate various experiments which decidedly 

 prove that, cceterk paribus, the multiplication of the fibres is as the inter-pulverisation ; 



