1838] 



FARMERS' REGISTER, 



621 



or carbonale of mafrnesia, are incapable of sup- 1 

 porting healthy vegetation. 



No soil i? fertile that rontnins as much as 19 

 parts out of 20 of any of the constituents that 

 have been nienlioned. 



It will be asked, are the pure earths in the soil 

 merely active as mechanical or indirect chemical 

 asentP, or do tliey actually aHord food to the 

 plant 1 This is an important question ; and not 

 difficult of solution. 



The earths consist, as I have before stated, of 

 metals, united to oxyiren ; and these metals have 

 not been decomposed ; there is consequently no 

 reason to suppose that the earths are convertible 

 into the elements of organized compounds, into 

 carbon, hydro<ren, and azote. 



Plants have been made to grow in given quan- 

 tities of earth. They consume very small por- 

 tions only, and what is lost may he accounted for 

 by the quantities found in their ashes ; that is to 

 pay. it has not been converted into any new pro- 

 ducts. 



The carbonic acid tmited to lime or magnesia, 

 if any stronger acid happens to be formed in the 

 soil durinsr the fermentation of vegetable matter, 

 which will disengage it from the earths, may be 

 decomposed : but the earths themselves cannot be 

 supposed convertible into other substances by any 

 process taking place in the soil. 



In all cases, the ashps of plants contain some of 

 the earths of the soil in which they grow ; but 

 these earths, as mav be seen from the table of the 

 ashes afforded by different plants given in the last 

 Lecture, never equal more than 5^ ^' ^^e weight 

 of the plant consumed. 



If they be considered as necessary to the vege- 

 table, it is as giving hardness and firmness to its 

 organization. Thus, it has been mentioned that 

 wheat, oats, and many of the hollow grasses, 

 have an epidermis principally of siliceous earth; 

 the use of which seems to be to strenathen them, 

 and defend them from the attacks of insects and 

 parasitical plants. 



Many soils are popularly distinguished as cold j 

 and the distinction, though at first view it may 

 appear to be founded on prejudice, is really just. 

 Some soils are much more heated by the rays 

 of the sun, all other circumstances being equal, 

 than others ; and soils brought to the satne degree 

 of heat cool in different times, i. e. some cool 

 much faster than others. 



This property has been very little attended to 

 in a philosophical point of view ; yet it is of the 

 hiirhest importance in agriculture. In ffeneral, 

 soils that consist principally of a stiff white clay 

 are difficultly heated ; and being usually very 

 moist, they retain their heat only for a short time. 

 Chalks are similar in one respect, that they are 

 difTicuUly heated ; but being drier they retain their 

 heat longer, less being consumed in causing the 

 evaporation of their moisture. 



A black soil, containing much soft vegetable 

 matter, is most heated by the sun and air ; and 

 the colored soils, and the soils containing much 

 carbonaceous matter, or ferruginous matter, ex- 

 posed under equal circumstances to the sun, ac- 

 quire a much higher temperature than pale-color- 

 ed soils. 



When soils are perfectly dry, those that most 

 readily become heated by the solar rays likewise 

 cool most rapidly, their power of losing heat by 



radiation being greatest ; but I have ascertained, 

 by experiment, that the darkest colored dry soil, 

 (ihat which contains abundance of animal or ve- 

 getable matter, substances which most facilitate 

 the diminution of temperature.) when heated 

 to the same degree, provided it be within the com- 

 mon limits of the eflect of solar heat, will cool 

 more slowly than a wet pale soil, entirely compos- 

 ed of earthy matter. 



I found '.hat a rich black mould, which contain- 

 ed nearly \ of the vegetable matter, had its tem- 

 perature increased in an hour from 65° to 88" by 

 exposure to sunshine ; whilst a chalk soil was 

 heated only to 69^ under the same circumstances. 

 But the mould removed into the shade, where the 

 temperature was 62°, lost, in half an hour, 15° ; 

 whereas the chalk, under the same circumstances, 

 had lost only 4°. 



A brown fertile soil and a cold barren clay were 

 each artificially heated to 88*^, having been pre- 

 viously dried : they were then exposed in a tem- 

 perature of 57° ; in half an hour the dark soil was 

 found to have lost 9^ of heat ; the clay had lost 

 only 6°. An equal portion of the clay containing 

 moisture, after beinir heated to 88", was exposed in 

 a temperature of 55° ; in less than a quarter of an 

 hour, it was found to have gained the 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 tem- 

 perature ascertained by a delicate thermometer. 



Nothing can be more evident than that the 

 genial heat of the soil, particularly in spring, must 

 be of" the highest importance to the rising plant. 

 And when the leaves are fully developed, the 

 ground is shaded, and any injurious influence, 

 which in the summer might be expected from too 

 great a heat, entirely prevented ; so that the tem- 

 perature of the surface, when bare and exposed to 

 the rays of the sun, affords at least one indication 

 of the degrees of its fi^rtility ; and the thermome- 

 ter may be sometimes a useful instrument to the 

 purchaser or improver of lands. 



There is a very simple test of the cooling or 

 radiating powers of soils, the formation of dew 

 upon them, or their relative increase of weight by 

 exposure to the air after being dried, in the day 

 or the night, in sunshine or in shade. The soil 

 that radiates most heat acquires the greatest in- 

 crease of weight ; and of course the radiating 

 powers of the soil are not only connected with its 

 temperature, but likewise ivith its relations to mois- 

 ture. 



The moisture in the soil influences its tempera- 

 ture ; and the manner in which it is distributed 

 through, or combined with, the earthy materials, 

 is of great importance in relation to the nutriment 

 of the plant. If water is too strongly attracted 

 by the earths, it will not be absorbed by the roots 

 of the plants ; if it is in too great quantity, or too 

 loosely united to them, it tends to injure or destroy 

 the fibrous parts of the roots. 



There are two slates in which water seems to 

 exist in the earths, and in animal and vegetable 

 substances: in the first state it is united by che- 

 mical, in the other by cohesive, attraction. 



If pure solution of ammonia or potassa be 

 poured into a solution of alum, alumina falls down 

 combined with water ; and the powder dried by 

 exposure to air will afford more than half its weight 

 of water by distillation ; in this instance the water 



