228 SCIENCE OF GARDENING. Pari IT. 



end, at least by the implements in common use ; but if it could be effected, it would be 

 injurious, because the difference of gravity between the organised matters and the earths, 

 has a constant tendency to separate them, and stirring a soil only by forks or pronged 

 implements, such as cultivators, would, in a short time, leave the surface of the soil too 

 light and spongy, and the lower part too compact and earthy. 



Subsect. 2. Of the Improvement of Soils by Compression. 



1078. Mechanical consolidation will improve some soils, such as spongy peats and light 

 dusty sands. It is but a limited source of improvement, but still it deserves to be 

 noticed. 



1079. The proper degree of adhesiveness is best given to loose soils by the addition of 

 earthy matters ; but mere rolling and treading are not to be altogether rejected. To be 

 benefited by rolling a soil must be dry, and the operation must not be carried too far. A 

 peat-bog drained and rolled, will sooner become covered with grasses than one equally 

 well drained and left alone. Drifting sands may be well rolled when wet, and by re- 

 peating the process after rains they will in time acquire a surface of grass or herbage. 

 Every agriculturist knows the advantages of rolling light soils after sowing, or even 

 treading them with sheep. Gardeners also tread in seeds on certain soils. 



Subsect. 3. Of the Improvement of Soils by Aeration or Fallowing. 



1080. Soils are benefited by the free admission of the weather to their interior parts. This is 

 generally considered as one of the advantages of fallowing, and its use in gardening is ex- 

 perienced in compost heaps, and in winter and summer ridging. The precise advantages, 

 however, of exposure to the air, independently of the concurrent influence of water, 

 heat, and the other effects mentioned as attendant on pulverisation, do not seem at present 

 to be correctly ascertained. It is allowed that carbonic acid gas may be absorbed by cal- 

 careous earths, and Dr. Thomson considers that the earths alone may thus probably 

 administer food to plants ; but Sir H. Davy seems to consider mere exposure to the at- 

 mosphere as of no benefit to soils whatever. " It has been supposed by some writers," he 

 says, " that certain principles necessary to fertility are derived from the atmosphere, which 

 are exhausted by a succession of crops, and that these are again supplied during the repose 

 of the land, and the exposure of the pulverised soil to the influence of the air ; but this in 

 truth is not the case. The earths commonly found in soils cannot be combined with more 

 oxygen ; none of them unite to azote ; and such of them as are capable of attracting car- 

 bonic acid, are always saturated with it in those soils on which the practice of fallowing is 

 adopted." 



1081. Aeration and repose, or summer fallow. " The vague ancient opinion of the use of 

 nitre, and of nitrous salts in vegetation," Sir H. Davy says, " seems to have been one of the 

 principal speculative reasons for the defence of summer fallows. Nitrous salts are produced 

 during the exposure of soils containing vegetable and animal remains, and in greatest 

 abundance in hot weather ; but it is probably by the combination of azote from these re- 

 mains with oxygen in the atmosphere that the acid is formed ; and at the expense of an 

 element, which otherwise would have formed ammonia ; the compounds of which are 

 much more efficacious than the nitrous compounds in assisting vegetation." It is proper 

 to observe that this reasoning is more speculative than experimental, and seems influenced, 

 in some degree, by the opinion adopted by the author, that fallows are of little use in 

 husbandry. One obvious advantage of aeration in summer, or a summer fallow, is, that 

 the soil may thus be heated by the sun to a degree which it never could be, if partially 

 covered with the foliage of even the widest-drilled crops. For this purpose, if the soil is 

 laid up in large lumps, it is evident it will receive more heat by exposing a greater sur- 

 face to the atmosphere, and it will retain this heat longer than can be expected, from the 

 circumstance of the lumps reflecting back the rays of heat radiated by each other. A 

 clayey soil, in this way, it is said {Farmers' Magazine, 1815), maybe heated to 120, 

 which may in some degree alter its absorbent powers as to water, and contribute materially 

 to the destruction of vegetable fibre, insects, and their eggs. By the aeration of lands in 

 winter, minute mechanical division is obtained by the freezing of the water in the soil ; 

 for, as water in the solid state occupies more space than when fluid, the particles of 

 earthy matters and of decomposing stones are thus rent asunder, and crumble down in 

 a fine mould. Rough stony soils will thus receive an accession to their finer soil every 

 winter. 



1082. Agricultural experience has fully proved that fallows are the only means by 

 which stiff clays in moist climates can be effectually cleared of weeds. Supposing there- 

 fore that no other advantage whatever was obtained, that no nutritive matter was 

 imbibed from the atmosphere, and the soil was neither chemically nor mechanically 

 benefited by aeration, this benefit alone the effectual eradication of weeds is suf- 

 ficient to justify the use of fallows on such soils. 



